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doc: Bring in FIT signature files

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Bring these files into the documentation.

Fix 'wtih' and 'it' typos and repeated 'could' while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
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Simon Glass 2023-06-23 13:22:06 +01:00 committed by Heinrich Schuchardt
parent 3c1e2c3261
commit ad29e08b79
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doc/uImage.FIT/beaglebone_vboot.txt
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Verified Boot on the Beaglebone Black
=====================================
Introduction
------------
Before reading this, please read verified-boot.txt and signature.txt. These
instructions are for mainline U-Boot from v2014.07 onwards.
There is quite a bit of documentation in this directory describing how
verified boot works in U-Boot. There is also a test which runs through the
entire process of signing an image and running U-Boot (sandbox) to check it.
However, it might be useful to also have an example on a real board.
Beaglebone Black is a fairly common board so seems to be a reasonable choice
for an example of how to enable verified boot using U-Boot.
First a note that may to help avoid confusion. U-Boot and Linux both use
device tree. They may use the same device tree source, but it is seldom useful
for them to use the exact same binary from the same place. More typically,
U-Boot has its device tree packaged wtih it, and the kernel's device tree is
packaged with the kernel. In particular this is important with verified boot,
since U-Boot's device tree must be immutable. If it can be changed then the
public keys can be changed and verified boot is useless. An attacker can
simply generate a new key and put his public key into U-Boot so that
everything verifies. On the other hand the kernel's device tree typically
changes when the kernel changes, so it is useful to package an updated device
tree with the kernel binary. U-Boot supports the latter with its flexible FIT
format (Flat Image Tree).
Overview
--------
The steps are roughly as follows:
1. Build U-Boot for the board, with the verified boot options enabled.
2. Obtain a suitable Linux kernel
3. Create a Image Tree Source file (ITS) file describing how you want the
kernel to be packaged, compressed and signed.
4. Create a key pair
5. Sign the kernel
6. Put the public key into U-Boot's image
7. Put U-Boot and the kernel onto the board
8. Try it
Step 1: Build U-Boot
--------------------
a. Set up the environment variable to point to your toolchain. You will need
this for U-Boot and also for the kernel if you build it. For example if you
installed a Linaro version manually it might be something like:
export CROSS_COMPILE=/opt/linaro/gcc-linaro-arm-linux-gnueabihf-4.8-2013.08_linux/bin/arm-linux-gnueabihf-
or if you just installed gcc-arm-linux-gnueabi then it might be
export CROSS_COMPILE=arm-linux-gnueabi-
b. Configure and build U-Boot with verified boot enabled:
export UBOOT=/path/to/u-boot
cd $UBOOT
# You can add -j10 if you have 10 CPUs to make it faster
make O=b/am335x_boneblack_vboot am335x_boneblack_vboot_config all
export UOUT=$UBOOT/b/am335x_boneblack_vboot
c. You will now have a U-Boot image:
file b/am335x_boneblack_vboot/u-boot-dtb.img
b/am335x_boneblack_vboot/u-boot-dtb.img: u-boot legacy uImage, U-Boot 2014.07-rc2-00065-g2f69f8, Firmware/ARM, Firmware Image (Not compressed), 395375 bytes, Sat May 31 16:19:04 2014, Load Address: 0x80800000, Entry Point: 0x00000000, Header CRC: 0x0ABD6ACA, Data CRC: 0x36DEF7E4
Step 2: Build Linux
--------------------
a. Find the kernel image ('Image') and device tree (.dtb) file you plan to
use. In our case it is am335x-boneblack.dtb and it is built with the kernel.
At the time of writing an SD Boot image can be obtained from here:
http://www.elinux.org/Beagleboard:Updating_The_Software#Image_For_Booting_From_microSD
You can write this to an SD card and then mount it to extract the kernel and
device tree files.
You can also build a kernel. Instructions for this are are here:
http://elinux.org/Building_BBB_Kernel
or you can use your favourite search engine. Following these instructions
produces a kernel Image and device tree files. For the record the steps were:
export KERNEL=/path/to/kernel
cd $KERNEL
git clone git://github.com/beagleboard/kernel.git .
git checkout v3.14
./patch.sh
cp configs/beaglebone kernel/arch/arm/configs/beaglebone_defconfig
cd kernel
make beaglebone_defconfig
make uImage dtbs # -j10 if you have 10 CPUs
export OKERNEL=$KERNEL/kernel/arch/arm/boot
c. You now have the 'Image' and 'am335x-boneblack.dtb' files needed to boot.
Step 3: Create the ITS
----------------------
Set up a directory for your work.
export WORK=/path/to/dir
cd $WORK
Put this into a file in that directory called sign.its:
/dts-v1/;
/ {
description = "Beaglebone black";
#address-cells = <1>;
images {
kernel {
data = /incbin/("Image.lzo");
type = "kernel";
arch = "arm";
os = "linux";
compression = "lzo";
load = <0x80008000>;
entry = <0x80008000>;
hash-1 {
algo = "sha1";
};
};
fdt-1 {
description = "beaglebone-black";
data = /incbin/("am335x-boneblack.dtb");
type = "flat_dt";
arch = "arm";
compression = "none";
hash-1 {
algo = "sha1";
};
};
};
configurations {
default = "conf-1";
conf-1 {
kernel = "kernel";
fdt = "fdt-1";
signature-1 {
algo = "sha1,rsa2048";
key-name-hint = "dev";
sign-images = "fdt", "kernel";
};
};
};
};
The explanation for this is all in the documentation you have already read.
But briefly it packages a kernel and device tree, and provides a single
configuration to be signed with a key named 'dev'. The kernel is compressed
with LZO to make it smaller.
Step 4: Create a key pair
-------------------------
See signature.txt for details on this step.
cd $WORK
mkdir keys
openssl genrsa -F4 -out keys/dev.key 2048
openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt
Note: keys/dev.key contains your private key and is very secret. If anyone
gets access to that file they can sign kernels with it. Keep it secure.
Step 5: Sign the kernel
-----------------------
We need to use mkimage (which was built when you built U-Boot) to package the
Linux kernel into a FIT (Flat Image Tree, a flexible file format that U-Boot
can load) using the ITS file you just created.
At the same time we must put the public key into U-Boot device tree, with the
'required' property, which tells U-Boot that this key must be verified for the
image to be valid. You will make this key available to U-Boot for booting in
step 6.
ln -s $OKERNEL/dts/am335x-boneblack.dtb
ln -s $OKERNEL/Image
ln -s $UOUT/u-boot-dtb.img
cp $UOUT/arch/arm/dts/am335x-boneblack.dtb am335x-boneblack-pubkey.dtb
lzop Image
$UOUT/tools/mkimage -f sign.its -K am335x-boneblack-pubkey.dtb -k keys -r image.fit
You should see something like this:
FIT description: Beaglebone black
Created: Sun Jun 1 12:50:30 2014
Image 0 (kernel)
Description: unavailable
Created: Sun Jun 1 12:50:30 2014
Type: Kernel Image
Compression: lzo compressed
Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB
Architecture: ARM
OS: Linux
Load Address: 0x80008000
Entry Point: 0x80008000
Hash algo: sha1
Hash value: c94364646427e10f423837e559898ef02c97b988
Image 1 (fdt-1)
Description: beaglebone-black
Created: Sun Jun 1 12:50:30 2014
Type: Flat Device Tree
Compression: uncompressed
Data Size: 31547 Bytes = 30.81 kB = 0.03 MB
Architecture: ARM
Hash algo: sha1
Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
Default Configuration: 'conf-1'
Configuration 0 (conf-1)
Description: unavailable
Kernel: kernel
FDT: fdt-1
Now am335x-boneblack-pubkey.dtb contains the public key and image.fit contains
the signed kernel. Jump to step 6 if you like, or continue reading to increase
your understanding.
You can also run fit_check_sign to check it:
$UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb
which results in:
Verifying Hash Integrity ... sha1,rsa2048:dev+
## Loading kernel from FIT Image at 7fc6ee469000 ...
Using 'conf-1' configuration
Verifying Hash Integrity ...
sha1,rsa2048:dev+
OK
Trying 'kernel' kernel subimage
Description: unavailable
Created: Sun Jun 1 12:50:30 2014
Type: Kernel Image
Compression: lzo compressed
Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB
Architecture: ARM
OS: Linux
Load Address: 0x80008000
Entry Point: 0x80008000
Hash algo: sha1
Hash value: c94364646427e10f423837e559898ef02c97b988
Verifying Hash Integrity ...
sha1+
OK
Unimplemented compression type 4
## Loading fdt from FIT Image at 7fc6ee469000 ...
Using 'conf-1' configuration
Trying 'fdt-1' fdt subimage
Description: beaglebone-black
Created: Sun Jun 1 12:50:30 2014
Type: Flat Device Tree
Compression: uncompressed
Data Size: 31547 Bytes = 30.81 kB = 0.03 MB
Architecture: ARM
Hash algo: sha1
Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
Verifying Hash Integrity ...
sha1+
OK
Loading Flat Device Tree ... OK
## Loading ramdisk from FIT Image at 7fc6ee469000 ...
Using 'conf-1' configuration
Could not find subimage node
Signature check OK
At the top, you see "sha1,rsa2048:dev+". This means that it checked an RSA key
of size 2048 bits using SHA1 as the hash algorithm. The key name checked was
'dev' and the '+' means that it verified. If it showed '-' that would be bad.
Once the configuration is verified it is then possible to rely on the hashes
in each image referenced by that configuration. So fit_check_sign goes on to
load each of the images. We have a kernel and an FDT but no ramkdisk. In each
case fit_check_sign checks the hash and prints sha1+ meaning that the SHA1
hash verified. This means that none of the images has been tampered with.
There is a test in test/vboot which uses U-Boot's sandbox build to verify that
the above flow works.
But it is fun to do this by hand, so you can load image.fit into a hex editor
like ghex, and change a byte in the kernel:
$UOUT/tools/fit_info -f image.fit -n /images/kernel -p data
NAME: kernel
LEN: 7790938
OFF: 168
This tells us that the kernel starts at byte offset 168 (decimal) in image.fit
and extends for about 7MB. Try changing a byte at 0x2000 (say) and run
fit_check_sign again. You should see something like:
Verifying Hash Integrity ... sha1,rsa2048:dev+
## Loading kernel from FIT Image at 7f5a39571000 ...
Using 'conf-1' configuration
Verifying Hash Integrity ...
sha1,rsa2048:dev+
OK
Trying 'kernel' kernel subimage
Description: unavailable
Created: Sun Jun 1 13:09:21 2014
Type: Kernel Image
Compression: lzo compressed
Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB
Architecture: ARM
OS: Linux
Load Address: 0x80008000
Entry Point: 0x80008000
Hash algo: sha1
Hash value: c94364646427e10f423837e559898ef02c97b988
Verifying Hash Integrity ...
sha1 error
Bad hash value for 'hash-1' hash node in 'kernel' image node
Bad Data Hash
## Loading fdt from FIT Image at 7f5a39571000 ...
Using 'conf-1' configuration
Trying 'fdt-1' fdt subimage
Description: beaglebone-black
Created: Sun Jun 1 13:09:21 2014
Type: Flat Device Tree
Compression: uncompressed
Data Size: 31547 Bytes = 30.81 kB = 0.03 MB
Architecture: ARM
Hash algo: sha1
Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
Verifying Hash Integrity ...
sha1+
OK
Loading Flat Device Tree ... OK
## Loading ramdisk from FIT Image at 7f5a39571000 ...
Using 'conf-1' configuration
Could not find subimage node
Signature check Bad (error 1)
It has detected the change in the kernel.
You can also be sneaky and try to switch images, using the libfdt utilities
that come with dtc (package name is device-tree-compiler but you will need a
recent version like 1.4:
dtc -v
Version: DTC 1.4.0
First we can check which nodes are actually hashed by the configuration:
fdtget -l image.fit /
images
configurations
fdtget -l image.fit /configurations
conf-1
fdtget -l image.fit /configurations/conf-1
signature-1
fdtget -p image.fit /configurations/conf-1/signature-1
hashed-strings
hashed-nodes
timestamp
signer-version
signer-name
value
algo
key-name-hint
sign-images
fdtget image.fit /configurations/conf-1/signature-1 hashed-nodes
/ /configurations/conf-1 /images/fdt-1 /images/fdt-1/hash /images/kernel /images/kernel/hash-1
This gives us a bit of a look into the signature that mkimage added. Note you
can also use fdtdump to list the entire device tree.
Say we want to change the kernel that this configuration uses
(/images/kernel). We could just put a new kernel in the image, but we will
need to change the hash to match. Let's simulate that by changing a byte of
the hash:
fdtget -tx image.fit /images/kernel/hash-1 value
c9436464 6427e10f 423837e5 59898ef0 2c97b988
fdtput -tx image.fit /images/kernel/hash-1 value c9436464 6427e10f 423837e5 59898ef0 2c97b981
Now check it again:
$UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb
Verifying Hash Integrity ... sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13
rsa_verify_with_keynode: RSA failed to verify: -13
-
Failed to verify required signature 'key-dev'
Signature check Bad (error 1)
This time we don't even get as far as checking the images, since the
configuration signature doesn't match. We can't change any hashes without the
signature check noticing. The configuration is essentially locked. U-Boot has
a public key for which it requires a match, and will not permit the use of any
configuration that does not match that public key. The only way the
configuration will match is if it was signed by the matching private key.
It would also be possible to add a new signature node that does match your new
configuration. But that won't work since you are not allowed to change the
configuration in any way. Try it with a fresh (valid) image if you like by
running the mkimage link again. Then:
fdtput -p image.fit /configurations/conf-1/signature-1 value fred
$UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb
Verifying Hash Integrity ... -
sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13
rsa_verify_with_keynode: RSA failed to verify: -13
-
Failed to verify required signature 'key-dev'
Signature check Bad (error 1)
Of course it would be possible to add an entirely new configuration and boot
with that, but it still needs to be signed, so it won't help.
6. Put the public key into U-Boot's image
-----------------------------------------
Having confirmed that the signature is doing its job, let's try it out in
U-Boot on the board. U-Boot needs access to the public key corresponding to
the private key that you signed with so that it can verify any kernels that
you sign.
cd $UBOOT
make O=b/am335x_boneblack_vboot EXT_DTB=${WORK}/am335x-boneblack-pubkey.dtb
Here we are overriding the normal device tree file with our one, which
contains the public key.
Now you have a special U-Boot image with the public key. It can verify can
kernel that you sign with the private key as in step 5.
If you like you can take a look at the public key information that mkimage
added to U-Boot's device tree:
fdtget -p am335x-boneblack-pubkey.dtb /signature/key-dev
required
algo
rsa,r-squared
rsa,modulus
rsa,n0-inverse
rsa,num-bits
key-name-hint
This has information about the key and some pre-processed values which U-Boot
can use to verify against it. These values are obtained from the public key
certificate by mkimage, but require quite a bit of code to generate. To save
code space in U-Boot, the information is extracted and written in raw form for
U-Boot to easily use. The same mechanism is used in Google's Chrome OS.
Notice the 'required' property. This marks the key as required - U-Boot will
not boot any image that does not verify against this key.
7. Put U-Boot and the kernel onto the board
-------------------------------------------
The method here varies depending on how you are booting. For this example we
are booting from an micro-SD card with two partitions, one for U-Boot and one
for Linux. Put it into your machine and write U-Boot and the kernel to it.
Here the card is /dev/sde:
cd $WORK
export UDEV=/dev/sde1 # Change thes two lines to the correct device
export KDEV=/dev/sde2
sudo mount $UDEV /mnt/tmp && sudo cp $UOUT/u-boot-dtb.img /mnt/tmp/u-boot.img && sleep 1 && sudo umount $UDEV
sudo mount $KDEV /mnt/tmp && sudo cp $WORK/image.fit /mnt/tmp/boot/image.fit && sleep 1 && sudo umount $KDEV
8. Try it
---------
Boot the board using the commands below:
setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait
ext2load mmc 0:2 82000000 /boot/image.fit
bootm 82000000
You should then see something like this:
U-Boot# setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait
U-Boot# ext2load mmc 0:2 82000000 /boot/image.fit
7824930 bytes read in 589 ms (12.7 MiB/s)
U-Boot# bootm 82000000
## Loading kernel from FIT Image at 82000000 ...
Using 'conf-1' configuration
Verifying Hash Integrity ... sha1,rsa2048:dev+ OK
Trying 'kernel' kernel subimage
Description: unavailable
Created: 2014-06-01 19:32:54 UTC
Type: Kernel Image
Compression: lzo compressed
Data Start: 0x820000a8
Data Size: 7790938 Bytes = 7.4 MiB
Architecture: ARM
OS: Linux
Load Address: 0x80008000
Entry Point: 0x80008000
Hash algo: sha1
Hash value: c94364646427e10f423837e559898ef02c97b988
Verifying Hash Integrity ... sha1+ OK
## Loading fdt from FIT Image at 82000000 ...
Using 'conf-1' configuration
Trying 'fdt-1' fdt subimage
Description: beaglebone-black
Created: 2014-06-01 19:32:54 UTC
Type: Flat Device Tree
Compression: uncompressed
Data Start: 0x8276e2ec
Data Size: 31547 Bytes = 30.8 KiB
Architecture: ARM
Hash algo: sha1
Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
Verifying Hash Integrity ... sha1+ OK
Booting using the fdt blob at 0x8276e2ec
Uncompressing Kernel Image ... OK
Loading Device Tree to 8fff5000, end 8ffffb3a ... OK
Starting kernel ...
[ 0.582377] omap_init_mbox: hwmod doesn't have valid attrs
[ 2.589651] musb-hdrc musb-hdrc.0.auto: Failed to request rx1.
[ 2.595830] musb-hdrc musb-hdrc.0.auto: musb_init_controller failed with status -517
[ 2.606470] musb-hdrc musb-hdrc.1.auto: Failed to request rx1.
[ 2.612723] musb-hdrc musb-hdrc.1.auto: musb_init_controller failed with status -517
[ 2.940808] drivers/rtc/hctosys.c: unable to open rtc device (rtc0)
[ 7.248889] libphy: PHY 4a101000.mdio:01 not found
[ 7.253995] net eth0: phy 4a101000.mdio:01 not found on slave 1
systemd-fsck[83]: Angstrom: clean, 50607/218160 files, 306348/872448 blocks
.---O---.
| | .-. o o
| | |-----.-----.-----.| | .----..-----.-----.
| | | __ | ---'| '--.| .-'| | |
| | | | | |--- || --'| | | ' | | | |
'---'---'--'--'--. |-----''----''--' '-----'-'-'-'
-' |
'---'
The Angstrom Distribution beaglebone ttyO0
Angstrom v2012.12 - Kernel 3.14.1+
beaglebone login:
At this point your kernel has been verified and you can be sure that it is one
that you signed. As an exercise, try changing image.fit as in step 5 and see
what happens.
Further Improvements
--------------------
Several of the steps here can be easily automated. In particular it would be
capital if signing and packaging a kernel were easy, perhaps a simple make
target in the kernel.
Some mention of how to use multiple .dtb files in a FIT might be useful.
U-Boot's verified boot mechanism has not had a robust and independent security
review. Such a review should look at the implementation and its resistance to
attacks.
Perhaps the verified boot feature could could be integrated into the Amstrom
distribution.
Simon Glass
sjg@chromium.org
2-June-14

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U-Boot FIT Signature Verification
=================================
Introduction
------------
FIT supports hashing of images so that these hashes can be checked on
loading. This protects against corruption of the image. However it does not
prevent the substitution of one image for another.
The signature feature allows the hash to be signed with a private key such
that it can be verified using a public key later. Provided that the private
key is kept secret and the public key is stored in a non-volatile place,
any image can be verified in this way.
See verified-boot.txt for more general information on verified boot.
Concepts
--------
Some familiarity with public key cryptography is assumed in this section.
The procedure for signing is as follows:
- hash an image in the FIT
- sign the hash with a private key to produce a signature
- store the resulting signature in the FIT
The procedure for verification is:
- read the FIT
- obtain the public key
- extract the signature from the FIT
- hash the image from the FIT
- verify (with the public key) that the extracted signature matches the
hash
The signing is generally performed by mkimage, as part of making a firmware
image for the device. The verification is normally done in U-Boot on the
device.
Algorithms
----------
In principle any suitable algorithm can be used to sign and verify a hash.
U-Boot supports a few hashing and verification algorithms. See below for
details.
While it is acceptable to bring in large cryptographic libraries such as
openssl on the host side (e.g. mkimage), it is not desirable for U-Boot.
For the run-time verification side, it is important to keep code and data
size as small as possible.
For this reason the RSA image verification uses pre-processed public keys
which can be used with a very small amount of code - just some extraction
of data from the FDT and exponentiation mod n. Code size impact is a little
under 5KB on Tegra Seaboard, for example.
It is relatively straightforward to add new algorithms if required. If
another RSA variant is needed, then it can be added with the
U_BOOT_CRYPTO_ALGO() macro. If another algorithm is needed (such as DSA) then
it can be placed in a directory alongside lib/rsa/, and its functions added
using U_BOOT_CRYPTO_ALGO().
Creating an RSA key pair and certificate
----------------------------------------
To create a new public/private key pair, size 2048 bits:
$ openssl genpkey -algorithm RSA -out keys/dev.key \
-pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537
To create a certificate for this containing the public key:
$ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt
If you like you can look at the public key also:
$ openssl rsa -in keys/dev.key -pubout
Device Tree Bindings
--------------------
The following properties are required in the FIT's signature node(s) to
allow the signer to operate. These should be added to the .its file.
Signature nodes sit at the same level as hash nodes and are called
signature-1, signature-2, etc.
- algo: Algorithm name (e.g. "sha1,rsa2048")
- key-name-hint: Name of key to use for signing. The keys will normally be in
a single directory (parameter -k to mkimage). For a given key <name>, its
private key is stored in <name>.key and the certificate is stored in
<name>.crt.
When the image is signed, the following properties are added (mandatory):
- value: The signature data (e.g. 256 bytes for 2048-bit RSA)
When the image is signed, the following properties are optional:
- timestamp: Time when image was signed (standard Unix time_t format)
- signer-name: Name of the signer (e.g. "mkimage")
- signer-version: Version string of the signer (e.g. "2013.01")
- comment: Additional information about the signer or image
- padding: The padding algorithm, it may be pkcs-1.5 or pss,
if no value is provided we assume pkcs-1.5
For config bindings (see Signed Configurations below), the following
additional properties are optional:
- sign-images: A list of images to sign, each being a property of the conf
node that contains then. The default is "kernel,fdt" which means that these
two images will be looked up in the config and signed if present.
For config bindings, these properties are added by the signer:
- hashed-nodes: A list of nodes which were hashed by the signer. Each is
a string - the full path to node. A typical value might be:
hashed-nodes = "/", "/configurations/conf-1", "/images/kernel",
"/images/kernel/hash-1", "/images/fdt-1",
"/images/fdt-1/hash-1";
- hashed-strings: The start and size of the string region of the FIT that
was hashed
Example: See sign-images.its for an example image tree source file and
sign-configs.its for config signing.
Public Key Storage
------------------
In order to verify an image that has been signed with a public key we need to
have a trusted public key. This cannot be stored in the signed image, since
it would be easy to alter. For this implementation we choose to store the
public key in U-Boot's control FDT (using CONFIG_OF_CONTROL).
Public keys should be stored as sub-nodes in a /signature node. Required
properties are:
- algo: Algorithm name (e.g. "sha1,rsa2048" or "sha256,ecdsa256")
Optional properties are:
- key-name-hint: Name of key used for signing. This is only a hint since it
is possible for the name to be changed. Verification can proceed by checking
all available signing keys until one matches.
- required: If present this indicates that the key must be verified for the
image / configuration to be considered valid. Only required keys are
normally verified by the FIT image booting algorithm. Valid values are
"image" to force verification of all images, and "conf" to force verification
of the selected configuration (which then relies on hashes in the images to
verify those).
Each signing algorithm has its own additional properties.
For RSA the following are mandatory:
- rsa,num-bits: Number of key bits (e.g. 2048)
- rsa,modulus: Modulus (N) as a big-endian multi-word integer
- rsa,exponent: Public exponent (E) as a 64 bit unsigned integer
- rsa,r-squared: (2^num-bits)^2 as a big-endian multi-word integer
- rsa,n0-inverse: -1 / modulus[0] mod 2^32
For ECDSA the following are mandatory:
- ecdsa,curve: Name of ECDSA curve (e.g. "prime256v1")
- ecdsa,x-point: Public key X coordinate as a big-endian multi-word integer
- ecdsa,y-point: Public key Y coordinate as a big-endian multi-word integer
These parameters can be added to a binary device tree using parameter -K of the
mkimage command::
tools/mkimage -f fit.its -K control.dtb -k keys -r image.fit
Here is an example of a generated device tree node::
signature {
key-dev {
required = "conf";
algo = "sha256,rsa2048";
rsa,r-squared = <0xb76d1acf 0xa1763ca5 0xeb2f126
0x742edc80 0xd3f42177 0x9741d9d9
0x35bb476e 0xff41c718 0xd3801430
0xf22537cb 0xa7e79960 0xae32a043
0x7da1427a 0x341d6492 0x3c2762f5
0xaac04726 0x5b262d96 0xf984e86d
0xb99443c7 0x17080c33 0x940f6892
0xd57a95d1 0x6ea7b691 0xc5038fa8
0x6bb48a6e 0x73f1b1ea 0x37160841
0xe05715ce 0xa7c45bbd 0x690d82d5
0x99c2454c 0x6ff117b3 0xd830683b
0x3f81c9cf 0x1ca38a91 0x0c3392e4
0xd817c625 0x7b8e9a24 0x175b89ea
0xad79f3dc 0x4d50d7b4 0x9d4e90f8
0xad9e2939 0xc165d6a4 0x0ada7e1b
0xfb1bf495 0xfc3131c2 0xb8c6e604
0xc2761124 0xf63de4a6 0x0e9565f9
0xc8e53761 0x7e7a37a5 0xe99dcdae
0x9aff7e1e 0xbd44b13d 0x6b0e6aa4
0x038907e4 0x8e0d6850 0xef51bc20
0xf73c94af 0x88bea7b1 0xcbbb1b30
0xd024b7f3>;
rsa,modulus = <0xc0711d6cb 0x9e86db7f 0x45986dbe
0x023f1e8c9 0xe1a4c4d0 0x8a0dfdc9
0x023ba0c48 0x06815f6a 0x5caa0654
0x07078c4b7 0x3d154853 0x40729023
0x0b007c8fe 0x5a3647e5 0x23b41e20
0x024720591 0x66915305 0x0e0b29b0
0x0de2ad30d 0x8589430f 0xb1590325
0x0fb9f5d5e 0x9eba752a 0xd88e6de9
0x056b3dcc6 0x9a6b8e61 0x6784f61f
0x000f39c21 0x5eec6b33 0xd78e4f78
0x0921a305f 0xaa2cc27e 0x1ca917af
0x06e1134f4 0xd48cac77 0x4e914d07
0x0f707aa5a 0x0d141f41 0x84677f1d
0x0ad47a049 0x028aedb6 0xd5536fcf
0x03fef1e4f 0x133a03d2 0xfd7a750a
0x0f9159732 0xd207812e 0x6a807375
0x06434230d 0xc8e22dad 0x9f29b3d6
0x07c44ac2b 0xfa2aad88 0xe2429504
0x041febd41 0x85d0d142 0x7b194d65
0x06e5d55ea 0x41116961 0xf3181dde
0x068bf5fbc 0x3dd82047 0x00ee647e
0x0d7a44ab3>;
rsa,exponent = <0x00 0x10001>;
rsa,n0-inverse = <0xb3928b85>;
rsa,num-bits = <0x800>;
key-name-hint = "dev";
};
};
Signed Configurations
---------------------
While signing images is useful, it does not provide complete protection
against several types of attack. For example, it it possible to create a
FIT with the same signed images, but with the configuration changed such
that a different one is selected (mix and match attack). It is also possible
to substitute a signed image from an older FIT version into a newer FIT
(roll-back attack).
As an example, consider this FIT:
/ {
images {
kernel-1 {
data = <data for kernel1>
signature-1 {
algo = "sha1,rsa2048";
value = <...kernel signature 1...>
};
};
kernel-2 {
data = <data for kernel2>
signature-1 {
algo = "sha1,rsa2048";
value = <...kernel signature 2...>
};
};
fdt-1 {
data = <data for fdt1>;
signature-1 {
algo = "sha1,rsa2048";
value = <...fdt signature 1...>
};
};
fdt-2 {
data = <data for fdt2>;
signature-1 {
algo = "sha1,rsa2048";
value = <...fdt signature 2...>
};
};
};
configurations {
default = "conf-1";
conf-1 {
kernel = "kernel-1";
fdt = "fdt-1";
};
conf-2 {
kernel = "kernel-2";
fdt = "fdt-2";
};
};
};
Since both kernels are signed it is easy for an attacker to add a new
configuration 3 with kernel 1 and fdt 2:
configurations {
default = "conf-1";
conf-1 {
kernel = "kernel-1";
fdt = "fdt-1";
};
conf-2 {
kernel = "kernel-2";
fdt = "fdt-2";
};
conf-3 {
kernel = "kernel-1";
fdt = "fdt-2";
};
};
With signed images, nothing protects against this. Whether it gains an
advantage for the attacker is debatable, but it is not secure.
To solve this problem, we support signed configurations. In this case it
is the configurations that are signed, not the image. Each image has its
own hash, and we include the hash in the configuration signature.
So the above example is adjusted to look like this:
/ {
images {
kernel-1 {
data = <data for kernel1>
hash-1 {
algo = "sha1";
value = <...kernel hash 1...>
};
};
kernel-2 {
data = <data for kernel2>
hash-1 {
algo = "sha1";
value = <...kernel hash 2...>
};
};
fdt-1 {
data = <data for fdt1>;
hash-1 {
algo = "sha1";
value = <...fdt hash 1...>
};
};
fdt-2 {
data = <data for fdt2>;
hash-1 {
algo = "sha1";
value = <...fdt hash 2...>
};
};
};
configurations {
default = "conf-1";
conf-1 {
kernel = "kernel-1";
fdt = "fdt-1";
signature-1 {
algo = "sha1,rsa2048";
value = <...conf 1 signature...>;
};
};
conf-2 {
kernel = "kernel-2";
fdt = "fdt-2";
signature-1 {
algo = "sha1,rsa2048";
value = <...conf 1 signature...>;
};
};
};
};
You can see that we have added hashes for all images (since they are no
longer signed), and a signature to each configuration. In the above example,
mkimage will sign configurations/conf-1, the kernel and fdt that are
pointed to by the configuration (/images/kernel-1, /images/kernel-1/hash-1,
/images/fdt-1, /images/fdt-1/hash-1) and the root structure of the image
(so that it isn't possible to add or remove root nodes). The signature is
written into /configurations/conf-1/signature-1/value. It can easily be
verified later even if the FIT has been signed with other keys in the
meantime.
Details
-------
The signature node contains a property ('hashed-nodes') which lists all the
nodes that the signature was made over. The image is walked in order and each
tag processed as follows:
- DTB_BEGIN_NODE: The tag and the following name are included in the signature
if the node or its parent are present in 'hashed-nodes'
- DTB_END_NODE: The tag is included in the signature if the node or its parent
are present in 'hashed-nodes'
- DTB_PROPERTY: The tag, the length word, the offset in the string table, and
the data are all included if the current node is present in 'hashed-nodes'
and the property name is not 'data'.
- DTB_END: The tag is always included in the signature.
- DTB_NOP: The tag is included in the signature if the current node is present
in 'hashed-nodes'
In addition, the signature contains a property 'hashed-strings' which contains
the offset and length in the string table of the strings that are to be
included in the signature (this is done last).
IMPORTANT: To verify the signature outside u-boot, it is vital to not only
calculate the hash of the image and verify the signature with that, but also to
calculate the hashes of the kernel, fdt, and ramdisk images and check those
match the hash values in the corresponding 'hash*' subnodes.
Verification
------------
FITs are verified when loaded. After the configuration is selected a list
of required images is produced. If there are 'required' public keys, then
each image must be verified against those keys. This means that every image
that might be used by the target needs to be signed with 'required' keys.
This happens automatically as part of a bootm command when FITs are used.
For Signed Configurations, the default verification behavior can be changed by
the following optional property in /signature node in U-Boot's control FDT.
- required-mode: Valid values are "any" to allow verified boot to succeed if
the selected configuration is signed by any of the 'required' keys, and "all"
to allow verified boot to succeed if the selected configuration is signed by
all of the 'required' keys.
This property can be added to a binary device tree using fdtput as shown in
below examples::
fdtput -t s control.dtb /signature required-mode any
fdtput -t s control.dtb /signature required-mode all
Enabling FIT Verification
-------------------------
In addition to the options to enable FIT itself, the following CONFIGs must
be enabled:
CONFIG_FIT_SIGNATURE - enable signing and verification in FITs
CONFIG_RSA - enable RSA algorithm for signing
CONFIG_ECDSA - enable ECDSA algorithm for signing
WARNING: When relying on signed FIT images with required signature check
the legacy image format is default disabled by not defining
CONFIG_LEGACY_IMAGE_FORMAT
Testing
-------
An easy way to test signing and verification is to use the test script
provided in test/vboot/vboot_test.sh. This uses sandbox (a special version
of U-Boot which runs under Linux) to show the operation of a 'bootm'
command loading and verifying images.
A sample run is show below:
$ make O=sandbox sandbox_config
$ make O=sandbox
$ O=sandbox ./test/vboot/vboot_test.sh
Simple Verified Boot Test
=========================
Please see doc/uImage.FIT/verified-boot.txt for more information
/home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000
Build keys
do sha1 test
Build FIT with signed images
Test Verified Boot Run: unsigned signatures:: OK
Sign images
Test Verified Boot Run: signed images: OK
Build FIT with signed configuration
Test Verified Boot Run: unsigned config: OK
Sign images
Test Verified Boot Run: signed config: OK
check signed config on the host
Signature check OK
OK
Test Verified Boot Run: signed config: OK
Test Verified Boot Run: signed config with bad hash: OK
do sha256 test
Build FIT with signed images
Test Verified Boot Run: unsigned signatures:: OK
Sign images
Test Verified Boot Run: signed images: OK
Build FIT with signed configuration
Test Verified Boot Run: unsigned config: OK
Sign images
Test Verified Boot Run: signed config: OK
check signed config on the host
Signature check OK
OK
Test Verified Boot Run: signed config: OK
Test Verified Boot Run: signed config with bad hash: OK
Test passed
Software signing: keydir vs keyfile
-----------------------------------
In the simplest case, signing is done by giving mkimage the 'keyfile'. This is
the path to a file containing the signing key.
The alternative is to pass the 'keydir' argument. In this case the filename of
the key is derived from the 'keydir' and the "key-name-hint" property in the
FIT. In this case the "key-name-hint" property is mandatory, and the key must
exist in "<keydir>/<key-name-hint>.<ext>" Here the extension "ext" is
specific to the signing algorithm.
Hardware Signing with PKCS#11 or with HSM
-----------------------------------------
Securely managing private signing keys can challenging, especially when the
keys are stored on the file system of a computer that is connected to the
Internet. If an attacker is able to steal the key, they can sign malicious FIT
images which will appear genuine to your devices.
An alternative solution is to keep your signing key securely stored on hardware
device like a smartcard, USB token or Hardware Security Module (HSM) and have
them perform the signing. PKCS#11 is standard for interfacing with these crypto
device.
Requirements:
Smartcard/USB token/HSM which can work with some openssl engine
openssl
For pkcs11 engine usage:
libp11 (provides pkcs11 engine)
p11-kit (recommended to simplify setup)
opensc (for smartcards and smartcard like USB devices)
gnutls (recommended for key generation, p11tool)
For generic HSMs respective openssl engine must be installed and locateable by
openssl. This may require setting up LD_LIBRARY_PATH if engine is not installed
to openssl's default search paths.
PKCS11 engine support forms "key id" based on "keydir" and with
"key-name-hint". "key-name-hint" is used as "object" name (if not defined in
keydir). "keydir" (if defined) is used to define (prefix for) which PKCS11 source
is being used for lookup up for the key.
PKCS11 engine key ids:
"pkcs11:<keydir>;object=<key-name-hint>;type=<public|private>"
or, if keydir contains "object="
"pkcs11:<keydir>;type=<public|private>"
or
"pkcs11:object=<key-name-hint>;type=<public|private>",
Generic HSM engine support forms "key id" based on "keydir" and with
"key-name-hint". If "keydir" is specified for mkimage it is used as a prefix in
"key id" and is appended with "key-name-hint".
Generic engine key ids:
"<keydir><key-name-hint>"
or
"<key-name-hint>"
In order to set the pin in the HSM, an environment variable "MKIMAGE_SIGN_PIN"
can be specified.
The following examples use the Nitrokey Pro using pkcs11 engine. Instructions
for other devices may vary.
Notes on pkcs11 engine setup:
Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc.
/usr/share/p11-kit/modules/opensc.module should be present on your system.
Generating Keys On the Nitrokey:
$ gpg --card-edit
Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00
Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Version ..........: 2.1
Manufacturer .....: ZeitControl
Serial number ....: xxxxxxxx
Name of cardholder: [not set]
Language prefs ...: de
Sex ..............: unspecified
URL of public key : [not set]
Login data .......: [not set]
Signature PIN ....: forced
Key attributes ...: rsa2048 rsa2048 rsa2048
Max. PIN lengths .: 32 32 32
PIN retry counter : 3 0 3
Signature counter : 0
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]
General key info..: [none]
gpg/card> generate
Make off-card backup of encryption key? (Y/n) n
Please note that the factory settings of the PINs are
PIN = '123456' Admin PIN = '12345678'
You should change them using the command --change-pin
What keysize do you want for the Signature key? (2048) 4096
The card will now be re-configured to generate a key of 4096 bits
Note: There is no guarantee that the card supports the requested size.
If the key generation does not succeed, please check the
documentation of your card to see what sizes are allowed.
What keysize do you want for the Encryption key? (2048) 4096
The card will now be re-configured to generate a key of 4096 bits
What keysize do you want for the Authentication key? (2048) 4096
The card will now be re-configured to generate a key of 4096 bits
Please specify how long the key should be valid.
0 = key does not expire
<n> = key expires in n days
<n>w = key expires in n weeks
<n>m = key expires in n months
<n>y = key expires in n years
Key is valid for? (0)
Key does not expire at all
Is this correct? (y/N) y
GnuPG needs to construct a user ID to identify your key.
Real name: John Doe
Email address: john.doe@email.com
Comment:
You selected this USER-ID:
"John Doe <john.doe@email.com>"
Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
Using p11tool to get the token URL:
Depending on system configuration, gpg-agent may need to be killed first.
$ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens
Token 0:
URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29
Label: OpenPGP card (User PIN (sig))
Type: Hardware token
Manufacturer: ZeitControl
Model: PKCS#15 emulated
Serial: 000xxxxxxxxx
Module: (null)
Token 1:
URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29
Label: OpenPGP card (User PIN)
Type: Hardware token
Manufacturer: ZeitControl
Model: PKCS#15 emulated
Serial: 000xxxxxxxxx
Module: (null)
Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below.
Use the URL of the token to list the private keys:
$ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \
"pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29"
Token 'OpenPGP card (User PIN (sig))' with URL 'pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29' requires user PIN
Enter PIN:
Object 0:
URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29;id=%01;object=Signature%20key;type=private
Type: Private key
Label: Signature key
Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE;
ID: 01
Use the label, in this case "Signature key" as the key-name-hint in your FIT.
Create the fitImage:
$ ./tools/mkimage -f fit-image.its fitImage
Sign the fitImage with the hardware key:
$ ./tools/mkimage -F -k \
"model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \
-K u-boot.dtb -N pkcs11 -r fitImage
Future Work
-----------
- Roll-back protection using a TPM is done using the tpm command. This can
be scripted, but we might consider a default way of doing this, built into
bootm.
Possible Future Work
--------------------
- More sandbox tests for failure modes
- Passwords for keys/certificates
- Perhaps implement OAEP
- Enhance bootm to permit scripted signature verification (so that a script
can verify an image but not actually boot it)
Simon Glass
sjg@chromium.org
1-1-13

2
doc/usage/cmd/source.rst
View File

@ -22,7 +22,7 @@ Two formats for script files exist:
* Flat Image Tree (FIT)
The benefit of the FIT images is that they can be signed and verifed as
decribed in :download:`signature.txt <../../uImage.FIT/signature.txt>`.
described in :doc:`../fit/signature`.
Both formats can be created with the mkimage tool.

612
doc/usage/fit/beaglebone_vboot.rst Normal file
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@ -0,0 +1,612 @@
.. SPDX-License-Identifier: GPL-2.0+
Verified Boot on the Beaglebone Black
=====================================
Introduction
------------
Before reading this, please read :doc:`verified-boot` and :doc:`signature`.
These instructions are for mainline U-Boot from v2014.07 onwards.
There is quite a bit of documentation in this directory describing how
verified boot works in U-Boot. There is also a test which runs through the
entire process of signing an image and running U-Boot (sandbox) to check it.
However, it might be useful to also have an example on a real board.
Beaglebone Black is a fairly common board so seems to be a reasonable choice
for an example of how to enable verified boot using U-Boot.
First a note that may to help avoid confusion. U-Boot and Linux both use
device tree. They may use the same device tree source, but it is seldom useful
for them to use the exact same binary from the same place. More typically,
U-Boot has its device tree packaged with it, and the kernel's device tree is
packaged with the kernel. In particular this is important with verified boot,
since U-Boot's device tree must be immutable. If it can be changed then the
public keys can be changed and verified boot is useless. An attacker can
simply generate a new key and put his public key into U-Boot so that
everything verifies. On the other hand the kernel's device tree typically
changes when the kernel changes, so it is useful to package an updated device
tree with the kernel binary. U-Boot supports the latter with its flexible FIT
format (Flat Image Tree).
Overview
--------
The steps are roughly as follows:
#. Build U-Boot for the board, with the verified boot options enabled.
#. Obtain a suitable Linux kernel
#. Create a Image Tree Source file (ITS) file describing how you want the
kernel to be packaged, compressed and signed.
#. Create a key pair
#. Sign the kernel
#. Put the public key into U-Boot's image
#. Put U-Boot and the kernel onto the board
#. Try it
Step 1: Build U-Boot
--------------------
a. Set up the environment variable to point to your toolchain. You will need
this for U-Boot and also for the kernel if you build it. For example if you
installed a Linaro version manually it might be something like::
export CROSS_COMPILE=/opt/linaro/gcc-linaro-arm-linux-gnueabihf-4.8-2013.08_linux/bin/arm-linux-gnueabihf-
or if you just installed gcc-arm-linux-gnueabi then it might be::
export CROSS_COMPILE=arm-linux-gnueabi-
b. Configure and build U-Boot with verified boot enabled::
export UBOOT=/path/to/u-boot
cd $UBOOT
# You can add -j10 if you have 10 CPUs to make it faster
make O=b/am335x_boneblack_vboot am335x_boneblack_vboot_config all
export UOUT=$UBOOT/b/am335x_boneblack_vboot
c. You will now have a U-Boot image::
file b/am335x_boneblack_vboot/u-boot-dtb.img
b/am335x_boneblack_vboot/u-boot-dtb.img: u-boot legacy uImage,
U-Boot 2014.07-rc2-00065-g2f69f8, Firmware/ARM, Firmware Image
(Not compressed), 395375 bytes, Sat May 31 16:19:04 2014,
Load Address: 0x80800000, Entry Point: 0x00000000,
Header CRC: 0x0ABD6ACA, Data CRC: 0x36DEF7E4
Step 2: Build Linux
--------------------
a. Find the kernel image ('Image') and device tree (.dtb) file you plan to
use. In our case it is am335x-boneblack.dtb and it is built with the kernel.
At the time of writing an SD Boot image can be obtained from here::
http://www.elinux.org/Beagleboard:Updating_The_Software#Image_For_Booting_From_microSD
You can write this to an SD card and then mount it to extract the kernel and
device tree files.
You can also build a kernel. Instructions for this are are here::
http://elinux.org/Building_BBB_Kernel
or you can use your favourite search engine. Following these instructions
produces a kernel Image and device tree files. For the record the steps
were::
export KERNEL=/path/to/kernel
cd $KERNEL
git clone git://github.com/beagleboard/kernel.git .
git checkout v3.14
./patch.sh
cp configs/beaglebone kernel/arch/arm/configs/beaglebone_defconfig
cd kernel
make beaglebone_defconfig
make uImage dtbs # -j10 if you have 10 CPUs
export OKERNEL=$KERNEL/kernel/arch/arm/boot
b. You now have the 'Image' and 'am335x-boneblack.dtb' files needed to boot.
Step 3: Create the ITS
----------------------
Set up a directory for your work::
export WORK=/path/to/dir
cd $WORK
Put this into a file in that directory called sign.its::
/dts-v1/;
/ {
description = "Beaglebone black";
#address-cells = <1>;
images {
kernel {
data = /incbin/("Image.lzo");
type = "kernel";
arch = "arm";
os = "linux";
compression = "lzo";
load = <0x80008000>;
entry = <0x80008000>;
hash-1 {
algo = "sha1";
};
};
fdt-1 {
description = "beaglebone-black";
data = /incbin/("am335x-boneblack.dtb");
type = "flat_dt";
arch = "arm";
compression = "none";
hash-1 {
algo = "sha1";
};
};
};
configurations {
default = "conf-1";
conf-1 {
kernel = "kernel";
fdt = "fdt-1";
signature-1 {
algo = "sha1,rsa2048";
key-name-hint = "dev";
sign-images = "fdt", "kernel";
};
};
};
};
The explanation for this is all in the documentation you have already read.
But briefly it packages a kernel and device tree, and provides a single
configuration to be signed with a key named 'dev'. The kernel is compressed
with LZO to make it smaller.
Step 4: Create a key pair
-------------------------
See :doc:`signature` for details on this step::
cd $WORK
mkdir keys
openssl genrsa -F4 -out keys/dev.key 2048
openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt
Note: keys/dev.key contains your private key and is very secret. If anyone
gets access to that file they can sign kernels with it. Keep it secure.
Step 5: Sign the kernel
-----------------------
We need to use mkimage (which was built when you built U-Boot) to package the
Linux kernel into a FIT (Flat Image Tree, a flexible file format that U-Boot
can load) using the ITS file you just created.
At the same time we must put the public key into U-Boot device tree, with the
'required' property, which tells U-Boot that this key must be verified for the
image to be valid. You will make this key available to U-Boot for booting in
step 6::
ln -s $OKERNEL/dts/am335x-boneblack.dtb
ln -s $OKERNEL/Image
ln -s $UOUT/u-boot-dtb.img
cp $UOUT/arch/arm/dts/am335x-boneblack.dtb am335x-boneblack-pubkey.dtb
lzop Image
$UOUT/tools/mkimage -f sign.its -K am335x-boneblack-pubkey.dtb -k keys -r image.fit
You should see something like this::
FIT description: Beaglebone black
Created: Sun Jun 1 12:50:30 2014
Image 0 (kernel)
Description: unavailable
Created: Sun Jun 1 12:50:30 2014
Type: Kernel Image
Compression: lzo compressed
Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB
Architecture: ARM
OS: Linux
Load Address: 0x80008000
Entry Point: 0x80008000
Hash algo: sha1
Hash value: c94364646427e10f423837e559898ef02c97b988
Image 1 (fdt-1)
Description: beaglebone-black
Created: Sun Jun 1 12:50:30 2014
Type: Flat Device Tree
Compression: uncompressed
Data Size: 31547 Bytes = 30.81 kB = 0.03 MB
Architecture: ARM
Hash algo: sha1
Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
Default Configuration: 'conf-1'
Configuration 0 (conf-1)
Description: unavailable
Kernel: kernel
FDT: fdt-1
Now am335x-boneblack-pubkey.dtb contains the public key and image.fit contains
the signed kernel. Jump to step 6 if you like, or continue reading to increase
your understanding.
You can also run fit_check_sign to check it::
$UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb
which results in::
Verifying Hash Integrity ... sha1,rsa2048:dev+
## Loading kernel from FIT Image at 7fc6ee469000 ...
Using 'conf-1' configuration
Verifying Hash Integrity ...
sha1,rsa2048:dev+
OK
Trying 'kernel' kernel subimage
Description: unavailable
Created: Sun Jun 1 12:50:30 2014
Type: Kernel Image
Compression: lzo compressed
Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB
Architecture: ARM
OS: Linux
Load Address: 0x80008000
Entry Point: 0x80008000
Hash algo: sha1
Hash value: c94364646427e10f423837e559898ef02c97b988
Verifying Hash Integrity ...
sha1+
OK
Unimplemented compression type 4
## Loading fdt from FIT Image at 7fc6ee469000 ...
Using 'conf-1' configuration
Trying 'fdt-1' fdt subimage
Description: beaglebone-black
Created: Sun Jun 1 12:50:30 2014
Type: Flat Device Tree
Compression: uncompressed
Data Size: 31547 Bytes = 30.81 kB = 0.03 MB
Architecture: ARM
Hash algo: sha1
Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
Verifying Hash Integrity ...
sha1+
OK
Loading Flat Device Tree ... OK
## Loading ramdisk from FIT Image at 7fc6ee469000 ...
Using 'conf-1' configuration
Could not find subimage node
Signature check OK
At the top, you see "sha1,rsa2048:dev+". This means that it checked an RSA key
of size 2048 bits using SHA1 as the hash algorithm. The key name checked was
'dev' and the '+' means that it verified. If it showed '-' that would be bad.
Once the configuration is verified it is then possible to rely on the hashes
in each image referenced by that configuration. So fit_check_sign goes on to
load each of the images. We have a kernel and an FDT but no ramkdisk. In each
case fit_check_sign checks the hash and prints sha1+ meaning that the SHA1
hash verified. This means that none of the images has been tampered with.
There is a test in test/vboot which uses U-Boot's sandbox build to verify that
the above flow works.
But it is fun to do this by hand, so you can load image.fit into a hex editor
like ghex, and change a byte in the kernel::
$UOUT/tools/fit_info -f image.fit -n /images/kernel -p data
NAME: kernel
LEN: 7790938
OFF: 168
This tells us that the kernel starts at byte offset 168 (decimal) in image.fit
and extends for about 7MB. Try changing a byte at 0x2000 (say) and run
fit_check_sign again. You should see something like::
Verifying Hash Integrity ... sha1,rsa2048:dev+
## Loading kernel from FIT Image at 7f5a39571000 ...
Using 'conf-1' configuration
Verifying Hash Integrity ...
sha1,rsa2048:dev+
OK
Trying 'kernel' kernel subimage
Description: unavailable
Created: Sun Jun 1 13:09:21 2014
Type: Kernel Image
Compression: lzo compressed
Data Size: 7790938 Bytes = 7608.34 kB = 7.43 MB
Architecture: ARM
OS: Linux
Load Address: 0x80008000
Entry Point: 0x80008000
Hash algo: sha1
Hash value: c94364646427e10f423837e559898ef02c97b988
Verifying Hash Integrity ...
sha1 error
Bad hash value for 'hash-1' hash node in 'kernel' image node
Bad Data Hash
## Loading fdt from FIT Image at 7f5a39571000 ...
Using 'conf-1' configuration
Trying 'fdt-1' fdt subimage
Description: beaglebone-black
Created: Sun Jun 1 13:09:21 2014
Type: Flat Device Tree
Compression: uncompressed
Data Size: 31547 Bytes = 30.81 kB = 0.03 MB
Architecture: ARM
Hash algo: sha1
Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
Verifying Hash Integrity ...
sha1+
OK
Loading Flat Device Tree ... OK
## Loading ramdisk from FIT Image at 7f5a39571000 ...
Using 'conf-1' configuration
Could not find subimage node
Signature check Bad (error 1)
It has detected the change in the kernel.
You can also be sneaky and try to switch images, using the libfdt utilities
that come with dtc (package name is device-tree-compiler but you will need a
recent version like 1.4::
dtc -v
Version: DTC 1.4.0
First we can check which nodes are actually hashed by the configuration::
$ fdtget -l image.fit /
images
configurations
$ fdtget -l image.fit /configurations
conf-1
fdtget -l image.fit /configurations/conf-1
signature-1
$ fdtget -p image.fit /configurations/conf-1/signature-1
hashed-strings
hashed-nodes
timestamp
signer-version
signer-name
value
algo
key-name-hint
sign-images
$ fdtget image.fit /configurations/conf-1/signature-1 hashed-nodes
/ /configurations/conf-1 /images/fdt-1 /images/fdt-1/hash /images/kernel /images/kernel/hash-1
This gives us a bit of a look into the signature that mkimage added. Note you
can also use fdtdump to list the entire device tree.
Say we want to change the kernel that this configuration uses
(/images/kernel). We could just put a new kernel in the image, but we will
need to change the hash to match. Let's simulate that by changing a byte of
the hash::
fdtget -tx image.fit /images/kernel/hash-1 value
c9436464 6427e10f 423837e5 59898ef0 2c97b988
fdtput -tx image.fit /images/kernel/hash-1 value c9436464 6427e10f 423837e5 59898ef0 2c97b981
Now check it again::
$UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb
Verifying Hash Integrity ... sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13
rsa_verify_with_keynode: RSA failed to verify: -13
-
Failed to verify required signature 'key-dev'
Signature check Bad (error 1)
This time we don't even get as far as checking the images, since the
configuration signature doesn't match. We can't change any hashes without the
signature check noticing. The configuration is essentially locked. U-Boot has
a public key for which it requires a match, and will not permit the use of any
configuration that does not match that public key. The only way the
configuration will match is if it was signed by the matching private key.
It would also be possible to add a new signature node that does match your new
configuration. But that won't work since you are not allowed to change the
configuration in any way. Try it with a fresh (valid) image if you like by
running the mkimage link again. Then::
fdtput -p image.fit /configurations/conf-1/signature-1 value fred
$UOUT/tools/fit_check_sign -f image.fit -k am335x-boneblack-pubkey.dtb
Verifying Hash Integrity ... -
sha1,rsa2048:devrsa_verify_with_keynode: RSA failed to verify: -13
rsa_verify_with_keynode: RSA failed to verify: -13
-
Failed to verify required signature 'key-dev'
Signature check Bad (error 1)
Of course it would be possible to add an entirely new configuration and boot
with that, but it still needs to be signed, so it won't help.
6. Put the public key into U-Boot's image
-----------------------------------------
Having confirmed that the signature is doing its job, let's try it out in
U-Boot on the board. U-Boot needs access to the public key corresponding to
the private key that you signed with so that it can verify any kernels that
you sign::
cd $UBOOT
make O=b/am335x_boneblack_vboot EXT_DTB=${WORK}/am335x-boneblack-pubkey.dtb
Here we are overriding the normal device tree file with our one, which
contains the public key.
Now you have a special U-Boot image with the public key. It can verify can
kernel that you sign with the private key as in step 5.
If you like you can take a look at the public key information that mkimage
added to U-Boot's device tree::
fdtget -p am335x-boneblack-pubkey.dtb /signature/key-dev
required
algo
rsa,r-squared
rsa,modulus
rsa,n0-inverse
rsa,num-bits
key-name-hint
This has information about the key and some pre-processed values which U-Boot
can use to verify against it. These values are obtained from the public key
certificate by mkimage, but require quite a bit of code to generate. To save
code space in U-Boot, the information is extracted and written in raw form for
U-Boot to easily use. The same mechanism is used in Google's Chrome OS.
Notice the 'required' property. This marks the key as required - U-Boot will
not boot any image that does not verify against this key.
7. Put U-Boot and the kernel onto the board
-------------------------------------------
The method here varies depending on how you are booting. For this example we
are booting from an micro-SD card with two partitions, one for U-Boot and one
for Linux. Put it into your machine and write U-Boot and the kernel to it.
Here the card is /dev/sde::
cd $WORK
export UDEV=/dev/sde1 # Change thes two lines to the correct device
export KDEV=/dev/sde2
sudo mount $UDEV /mnt/tmp && sudo cp $UOUT/u-boot-dtb.img /mnt/tmp/u-boot.img && sleep 1 && sudo umount $UDEV
sudo mount $KDEV /mnt/tmp && sudo cp $WORK/image.fit /mnt/tmp/boot/image.fit && sleep 1 && sudo umount $KDEV
8. Try it
---------
Boot the board using the commands below::
setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait
ext2load mmc 0:2 82000000 /boot/image.fit
bootm 82000000
You should then see something like this::
U-Boot# setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait
U-Boot# ext2load mmc 0:2 82000000 /boot/image.fit
7824930 bytes read in 589 ms (12.7 MiB/s)
U-Boot# bootm 82000000
## Loading kernel from FIT Image at 82000000 ...
Using 'conf-1' configuration
Verifying Hash Integrity ... sha1,rsa2048:dev+ OK
Trying 'kernel' kernel subimage
Description: unavailable
Created: 2014-06-01 19:32:54 UTC
Type: Kernel Image
Compression: lzo compressed
Data Start: 0x820000a8
Data Size: 7790938 Bytes = 7.4 MiB
Architecture: ARM
OS: Linux
Load Address: 0x80008000
Entry Point: 0x80008000
Hash algo: sha1
Hash value: c94364646427e10f423837e559898ef02c97b988
Verifying Hash Integrity ... sha1+ OK
## Loading fdt from FIT Image at 82000000 ...
Using 'conf-1' configuration
Trying 'fdt-1' fdt subimage
Description: beaglebone-black
Created: 2014-06-01 19:32:54 UTC
Type: Flat Device Tree
Compression: uncompressed
Data Start: 0x8276e2ec
Data Size: 31547 Bytes = 30.8 KiB
Architecture: ARM
Hash algo: sha1
Hash value: cb09202f889d824f23b8e4404b781be5ad38a68d
Verifying Hash Integrity ... sha1+ OK
Booting using the fdt blob at 0x8276e2ec
Uncompressing Kernel Image ... OK
Loading Device Tree to 8fff5000, end 8ffffb3a ... OK
Starting kernel ...
[ 0.582377] omap_init_mbox: hwmod doesn't have valid attrs
[ 2.589651] musb-hdrc musb-hdrc.0.auto: Failed to request rx1.
[ 2.595830] musb-hdrc musb-hdrc.0.auto: musb_init_controller failed with status -517
[ 2.606470] musb-hdrc musb-hdrc.1.auto: Failed to request rx1.
[ 2.612723] musb-hdrc musb-hdrc.1.auto: musb_init_controller failed with status -517
[ 2.940808] drivers/rtc/hctosys.c: unable to open rtc device (rtc0)
[ 7.248889] libphy: PHY 4a101000.mdio:01 not found
[ 7.253995] net eth0: phy 4a101000.mdio:01 not found on slave 1
systemd-fsck[83]: Angstrom: clean, 50607/218160 files, 306348/872448 blocks
.---O---.
| | .-. o o
| | |-----.-----.-----.| | .----..-----.-----.
| | | __ | ---'| '--.| .-'| | |
| | | | | |--- || --'| | | ' | | | |
'---'---'--'--'--. |-----''----''--' '-----'-'-'-'
-' |
'---'
The Angstrom Distribution beaglebone ttyO0
Angstrom v2012.12 - Kernel 3.14.1+
beaglebone login:
At this point your kernel has been verified and you can be sure that it is one
that you signed. As an exercise, try changing image.fit as in step 5 and see
what happens.
Further Improvements
--------------------
Several of the steps here can be easily automated. In particular it would be
capital if signing and packaging a kernel were easy, perhaps a simple make
target in the kernel.
Some mention of how to use multiple .dtb files in a FIT might be useful.
U-Boot's verified boot mechanism has not had a robust and independent security
review. Such a review should look at the implementation and its resistance to
attacks.
Perhaps the verified boot feature could be integrated into the Amstrom
distribution.
.. sectionauthor:: Simon Glass <sjg@chromium.org>, 2-June-14

3
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@ -12,3 +12,6 @@ doc/uImage.FIT
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x86-fit-boot
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verified-boot
beaglebone_vboot

760
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.. SPDX-License-Identifier: GPL-2.0+
U-Boot FIT Signature Verification
=================================
Introduction
------------
FIT supports hashing of images so that these hashes can be checked on
loading. This protects against corruption of the image. However it does not
prevent the substitution of one image for another.
The signature feature allows the hash to be signed with a private key such
that it can be verified using a public key later. Provided that the private
key is kept secret and the public key is stored in a non-volatile place,
any image can be verified in this way.
See verified-boot.txt for more general information on verified boot.
Concepts
--------
Some familiarity with public key cryptography is assumed in this section.
The procedure for signing is as follows:
- hash an image in the FIT
- sign the hash with a private key to produce a signature
- store the resulting signature in the FIT
The procedure for verification is:
- read the FIT
- obtain the public key
- extract the signature from the FIT
- hash the image from the FIT
- verify (with the public key) that the extracted signature matches the
hash
The signing is generally performed by mkimage, as part of making a firmware
image for the device. The verification is normally done in U-Boot on the
device.
Algorithms
----------
In principle any suitable algorithm can be used to sign and verify a hash.
U-Boot supports a few hashing and verification algorithms. See below for
details.
While it is acceptable to bring in large cryptographic libraries such as
openssl on the host side (e.g. mkimage), it is not desirable for U-Boot.
For the run-time verification side, it is important to keep code and data
size as small as possible.
For this reason the RSA image verification uses pre-processed public keys
which can be used with a very small amount of code - just some extraction
of data from the FDT and exponentiation mod n. Code size impact is a little
under 5KB on Tegra Seaboard, for example.
It is relatively straightforward to add new algorithms if required. If
another RSA variant is needed, then it can be added with the
U_BOOT_CRYPTO_ALGO() macro. If another algorithm is needed (such as DSA) then
it can be placed in a directory alongside lib/rsa/, and its functions added
using U_BOOT_CRYPTO_ALGO().
Creating an RSA key pair and certificate
----------------------------------------
To create a new public/private key pair, size 2048 bits::
$ openssl genpkey -algorithm RSA -out keys/dev.key \
-pkeyopt rsa_keygen_bits:2048 -pkeyopt rsa_keygen_pubexp:65537
To create a certificate for this containing the public key::
$ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt
If you like you can look at the public key also::
$ openssl rsa -in keys/dev.key -pubout
Device Tree Bindings
--------------------
The following properties are required in the FIT's signature node(s) to
allow the signer to operate. These should be added to the .its file.
Signature nodes sit at the same level as hash nodes and are called
signature-1, signature-2, etc.
algo
Algorithm name (e.g. "sha1,rsa2048")
key-name-hint
Name of key to use for signing. The keys will normally be in
a single directory (parameter -k to mkimage). For a given key <name>, its
private key is stored in <name>.key and the certificate is stored in
<name>.crt.
When the image is signed, the following properties are added (mandatory):
value
The signature data (e.g. 256 bytes for 2048-bit RSA)
When the image is signed, the following properties are optional:
timestamp
Time when image was signed (standard Unix time_t format)
signer-name
Name of the signer (e.g. "mkimage")
signer-version
Version string of the signer (e.g. "2013.01")
comment
Additional information about the signer or image
padding
The padding algorithm, it may be pkcs-1.5 or pss,
if no value is provided we assume pkcs-1.5
For config bindings (see Signed Configurations below), the following
additional properties are optional:
sign-images
A list of images to sign, each being a property of the conf
node that contains then. The default is "kernel,fdt" which means that these
two images will be looked up in the config and signed if present.
For config bindings, these properties are added by the signer:
hashed-nodes
A list of nodes which were hashed by the signer. Each is
a string - the full path to node. A typical value might be::
hashed-nodes = "/", "/configurations/conf-1", "/images/kernel",
"/images/kernel/hash-1", "/images/fdt-1",
"/images/fdt-1/hash-1";
hashed-strings
The start and size of the string region of the FIT that was hashed
Example: See :doc:`sign-images` for an example image tree source file and
sign-configs.its for config signing.
Public Key Storage
------------------
In order to verify an image that has been signed with a public key we need to
have a trusted public key. This cannot be stored in the signed image, since
it would be easy to alter. For this implementation we choose to store the
public key in U-Boot's control FDT (using CONFIG_OF_CONTROL).
Public keys should be stored as sub-nodes in a /signature node. Required
properties are:
algo
Algorithm name (e.g. "sha1,rsa2048" or "sha256,ecdsa256")
Optional properties are:
key-name-hint
Name of key used for signing. This is only a hint since it
is possible for the name to be changed. Verification can proceed by checking
all available signing keys until one matches.
required
If present this indicates that the key must be verified for the
image / configuration to be considered valid. Only required keys are
normally verified by the FIT image booting algorithm. Valid values are
"image" to force verification of all images, and "conf" to force verification
of the selected configuration (which then relies on hashes in the images to
verify those).
Each signing algorithm has its own additional properties.
For RSA the following are mandatory:
rsa,num-bits
Number of key bits (e.g. 2048)
rsa,modulus
Modulus (N) as a big-endian multi-word integer
rsa,exponent
Public exponent (E) as a 64 bit unsigned integer
rsa,r-squared
(2^num-bits)^2 as a big-endian multi-word integer
rsa,n0-inverse
-1 / modulus[0] mod 2^32
For ECDSA the following are mandatory:
ecdsa,curve
Name of ECDSA curve (e.g. "prime256v1")
ecdsa,x-point
Public key X coordinate as a big-endian multi-word integer
ecdsa,y-point
Public key Y coordinate as a big-endian multi-word integer
These parameters can be added to a binary device tree using parameter -K of the
mkimage command::
tools/mkimage -f fit.its -K control.dtb -k keys -r image.fit
Here is an example of a generated device tree node::
signature {
key-dev {
required = "conf";
algo = "sha256,rsa2048";
rsa,r-squared = <0xb76d1acf 0xa1763ca5 0xeb2f126
0x742edc80 0xd3f42177 0x9741d9d9
0x35bb476e 0xff41c718 0xd3801430
0xf22537cb 0xa7e79960 0xae32a043
0x7da1427a 0x341d6492 0x3c2762f5
0xaac04726 0x5b262d96 0xf984e86d
0xb99443c7 0x17080c33 0x940f6892
0xd57a95d1 0x6ea7b691 0xc5038fa8
0x6bb48a6e 0x73f1b1ea 0x37160841
0xe05715ce 0xa7c45bbd 0x690d82d5
0x99c2454c 0x6ff117b3 0xd830683b
0x3f81c9cf 0x1ca38a91 0x0c3392e4
0xd817c625 0x7b8e9a24 0x175b89ea
0xad79f3dc 0x4d50d7b4 0x9d4e90f8
0xad9e2939 0xc165d6a4 0x0ada7e1b
0xfb1bf495 0xfc3131c2 0xb8c6e604
0xc2761124 0xf63de4a6 0x0e9565f9
0xc8e53761 0x7e7a37a5 0xe99dcdae
0x9aff7e1e 0xbd44b13d 0x6b0e6aa4
0x038907e4 0x8e0d6850 0xef51bc20
0xf73c94af 0x88bea7b1 0xcbbb1b30
0xd024b7f3>;
rsa,modulus = <0xc0711d6cb 0x9e86db7f 0x45986dbe
0x023f1e8c9 0xe1a4c4d0 0x8a0dfdc9
0x023ba0c48 0x06815f6a 0x5caa0654
0x07078c4b7 0x3d154853 0x40729023
0x0b007c8fe 0x5a3647e5 0x23b41e20
0x024720591 0x66915305 0x0e0b29b0
0x0de2ad30d 0x8589430f 0xb1590325
0x0fb9f5d5e 0x9eba752a 0xd88e6de9
0x056b3dcc6 0x9a6b8e61 0x6784f61f
0x000f39c21 0x5eec6b33 0xd78e4f78
0x0921a305f 0xaa2cc27e 0x1ca917af
0x06e1134f4 0xd48cac77 0x4e914d07
0x0f707aa5a 0x0d141f41 0x84677f1d
0x0ad47a049 0x028aedb6 0xd5536fcf
0x03fef1e4f 0x133a03d2 0xfd7a750a
0x0f9159732 0xd207812e 0x6a807375
0x06434230d 0xc8e22dad 0x9f29b3d6
0x07c44ac2b 0xfa2aad88 0xe2429504
0x041febd41 0x85d0d142 0x7b194d65
0x06e5d55ea 0x41116961 0xf3181dde
0x068bf5fbc 0x3dd82047 0x00ee647e
0x0d7a44ab3>;
rsa,exponent = <0x00 0x10001>;
rsa,n0-inverse = <0xb3928b85>;
rsa,num-bits = <0x800>;
key-name-hint = "dev";
};
};
Signed Configurations
---------------------
While signing images is useful, it does not provide complete protection
against several types of attack. For example, it is possible to create a
FIT with the same signed images, but with the configuration changed such
that a different one is selected (mix and match attack). It is also possible
to substitute a signed image from an older FIT version into a newer FIT
(roll-back attack).
As an example, consider this FIT::
/ {
images {
kernel-1 {
data = <data for kernel1>
signature-1 {
algo = "sha1,rsa2048";
value = <...kernel signature 1...>
};
};
kernel-2 {
data = <data for kernel2>
signature-1 {
algo = "sha1,rsa2048";
value = <...kernel signature 2...>
};
};
fdt-1 {
data = <data for fdt1>;
signature-1 {
algo = "sha1,rsa2048";
value = <...fdt signature 1...>
};
};
fdt-2 {
data = <data for fdt2>;
signature-1 {
algo = "sha1,rsa2048";
value = <...fdt signature 2...>
};
};
};
configurations {
default = "conf-1";
conf-1 {
kernel = "kernel-1";
fdt = "fdt-1";
};
conf-2 {
kernel = "kernel-2";
fdt = "fdt-2";
};
};
};
Since both kernels are signed it is easy for an attacker to add a new
configuration 3 with kernel 1 and fdt 2::
configurations {
default = "conf-1";
conf-1 {
kernel = "kernel-1";
fdt = "fdt-1";
};
conf-2 {
kernel = "kernel-2";
fdt = "fdt-2";
};
conf-3 {
kernel = "kernel-1";
fdt = "fdt-2";
};
};
With signed images, nothing protects against this. Whether it gains an
advantage for the attacker is debatable, but it is not secure.
To solve this problem, we support signed configurations. In this case it
is the configurations that are signed, not the image. Each image has its
own hash, and we include the hash in the configuration signature.
So the above example is adjusted to look like this::
/ {
images {
kernel-1 {
data = <data for kernel1>
hash-1 {
algo = "sha1";
value = <...kernel hash 1...>
};
};
kernel-2 {
data = <data for kernel2>
hash-1 {
algo = "sha1";
value = <...kernel hash 2...>
};
};
fdt-1 {
data = <data for fdt1>;
hash-1 {
algo = "sha1";
value = <...fdt hash 1...>
};
};
fdt-2 {
data = <data for fdt2>;
hash-1 {
algo = "sha1";
value = <...fdt hash 2...>
};
};
};
configurations {
default = "conf-1";
conf-1 {
kernel = "kernel-1";
fdt = "fdt-1";
signature-1 {
algo = "sha1,rsa2048";
value = <...conf 1 signature...>;
};
};
conf-2 {
kernel = "kernel-2";
fdt = "fdt-2";
signature-1 {
algo = "sha1,rsa2048";
value = <...conf 1 signature...>;
};
};
};
};
You can see that we have added hashes for all images (since they are no
longer signed), and a signature to each configuration. In the above example,
mkimage will sign configurations/conf-1, the kernel and fdt that are
pointed to by the configuration (/images/kernel-1, /images/kernel-1/hash-1,
/images/fdt-1, /images/fdt-1/hash-1) and the root structure of the image
(so that it isn't possible to add or remove root nodes). The signature is
written into /configurations/conf-1/signature-1/value. It can easily be
verified later even if the FIT has been signed with other keys in the
meantime.
Details
-------
The signature node contains a property ('hashed-nodes') which lists all the
nodes that the signature was made over. The image is walked in order and each
tag processed as follows:
DTB_BEGIN_NODE
The tag and the following name are included in the signature
if the node or its parent are present in 'hashed-nodes'
DTB_END_NODE
The tag is included in the signature if the node or its parent
are present in 'hashed-nodes'
DTB_PROPERTY
The tag, the length word, the offset in the string table, and
the data are all included if the current node is present in 'hashed-nodes'
and the property name is not 'data'.
DTB_END
The tag is always included in the signature.
DTB_NOP
The tag is included in the signature if the current node is present
in 'hashed-nodes'
In addition, the signature contains a property 'hashed-strings' which contains
the offset and length in the string table of the strings that are to be
included in the signature (this is done last).
IMPORTANT: To verify the signature outside u-boot, it is vital to not only
calculate the hash of the image and verify the signature with that, but also to
calculate the hashes of the kernel, fdt, and ramdisk images and check those
match the hash values in the corresponding 'hash*' subnodes.
Verification
------------
FITs are verified when loaded. After the configuration is selected a list
of required images is produced. If there are 'required' public keys, then
each image must be verified against those keys. This means that every image
that might be used by the target needs to be signed with 'required' keys.
This happens automatically as part of a bootm command when FITs are used.
For Signed Configurations, the default verification behavior can be changed by
the following optional property in /signature node in U-Boot's control FDT.
required-mode
Valid values are "any" to allow verified boot to succeed if
the selected configuration is signed by any of the 'required' keys, and "all"
to allow verified boot to succeed if the selected configuration is signed by
all of the 'required' keys.
This property can be added to a binary device tree using fdtput as shown in
below examples::
fdtput -t s control.dtb /signature required-mode any
fdtput -t s control.dtb /signature required-mode all
Enabling FIT Verification
-------------------------
In addition to the options to enable FIT itself, the following CONFIGs must
be enabled:
CONFIG_FIT_SIGNATURE
enable signing and verification in FITs
CONFIG_RSA
enable RSA algorithm for signing
CONFIG_ECDSA
enable ECDSA algorithm for signing
WARNING: When relying on signed FIT images with required signature check
the legacy image format is default disabled by not defining
CONFIG_LEGACY_IMAGE_FORMAT
Testing
-------
An easy way to test signing and verification is to use the test script
provided in test/vboot/vboot_test.sh. This uses sandbox (a special version
of U-Boot which runs under Linux) to show the operation of a 'bootm'
command loading and verifying images.
A sample run is show below::
$ make O=sandbox sandbox_config
$ make O=sandbox
$ O=sandbox ./test/vboot/vboot_test.sh
Simple Verified Boot Test
-------------------------
Please see :doc:`verified-boot` for more information::
/home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000
Build keys
do sha1 test
Build FIT with signed images
Test Verified Boot Run: unsigned signatures:: OK
Sign images
Test Verified Boot Run: signed images: OK
Build FIT with signed configuration
Test Verified Boot Run: unsigned config: OK
Sign images
Test Verified Boot Run: signed config: OK
check signed config on the host
Signature check OK
OK
Test Verified Boot Run: signed config: OK
Test Verified Boot Run: signed config with bad hash: OK
do sha256 test
Build FIT with signed images
Test Verified Boot Run: unsigned signatures:: OK
Sign images
Test Verified Boot Run: signed images: OK
Build FIT with signed configuration
Test Verified Boot Run: unsigned config: OK
Sign images
Test Verified Boot Run: signed config: OK
check signed config on the host
Signature check OK
OK
Test Verified Boot Run: signed config: OK
Test Verified Boot Run: signed config with bad hash: OK
Test passed
Software signing: keydir vs keyfile
-----------------------------------
In the simplest case, signing is done by giving mkimage the 'keyfile'. This is
the path to a file containing the signing key.
The alternative is to pass the 'keydir' argument. In this case the filename of
the key is derived from the 'keydir' and the "key-name-hint" property in the
FIT. In this case the "key-name-hint" property is mandatory, and the key must
exist in "<keydir>/<key-name-hint>.<ext>" Here the extension "ext" is
specific to the signing algorithm.
Hardware Signing with PKCS#11 or with HSM
-----------------------------------------
Securely managing private signing keys can challenging, especially when the
keys are stored on the file system of a computer that is connected to the
Internet. If an attacker is able to steal the key, they can sign malicious FIT
images which will appear genuine to your devices.
An alternative solution is to keep your signing key securely stored on hardware
device like a smartcard, USB token or Hardware Security Module (HSM) and have
them perform the signing. PKCS#11 is standard for interfacing with these crypto
device.
Requirements:
- Smartcard/USB token/HSM which can work with some openssl engine
- openssl
For pkcs11 engine usage:
- libp11 (provides pkcs11 engine)
- p11-kit (recommended to simplify setup)
- opensc (for smartcards and smartcard like USB devices)
- gnutls (recommended for key generation, p11tool)
For generic HSMs respective openssl engine must be installed and locateable by
openssl. This may require setting up LD_LIBRARY_PATH if engine is not installed
to openssl's default search paths.
PKCS11 engine support forms "key id" based on "keydir" and with
"key-name-hint". "key-name-hint" is used as "object" name (if not defined in
keydir). "keydir" (if defined) is used to define (prefix for) which PKCS11 source
is being used for lookup up for the key.
PKCS11 engine key ids
"pkcs11:<keydir>;object=<key-name-hint>;type=<public|private>"
or, if keydir contains "object="
"pkcs11:<keydir>;type=<public|private>"
or
"pkcs11:object=<key-name-hint>;type=<public|private>",
Generic HSM engine support forms "key id" based on "keydir" and with
"key-name-hint". If "keydir" is specified for mkimage it is used as a prefix in
"key id" and is appended with "key-name-hint".
Generic engine key ids:
"<keydir><key-name-hint>"
or
"< key-name-hint>"
In order to set the pin in the HSM, an environment variable "MKIMAGE_SIGN_PIN"
can be specified.
The following examples use the Nitrokey Pro using pkcs11 engine. Instructions
for other devices may vary.
Notes on pkcs11 engine setup:
Make sure p11-kit, opensc are installed and that p11-kit is setup to use opensc.
/usr/share/p11-kit/modules/opensc.module should be present on your system.
Generating Keys On the Nitrokey::
$ gpg --card-edit
Reader ...........: Nitrokey Nitrokey Pro (xxxxxxxx0000000000000000) 00 00
Application ID ...: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Version ..........: 2.1
Manufacturer .....: ZeitControl
Serial number ....: xxxxxxxx
Name of cardholder: [not set]
Language prefs ...: de
Sex ..............: unspecified
URL of public key : [not set]
Login data .......: [not set]
Signature PIN ....: forced
Key attributes ...: rsa2048 rsa2048 rsa2048
Max. PIN lengths .: 32 32 32
PIN retry counter : 3 0 3
Signature counter : 0
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]
General key info..: [none]
gpg/card> generate
Make off-card backup of encryption key? (Y/n) n
Please note that the factory settings of the PINs are
PIN = '123456' Admin PIN = '12345678'
You should change them using the command --change-pin
What keysize do you want for the Signature key? (2048) 4096
The card will now be re-configured to generate a key of 4096 bits
Note: There is no guarantee that the card supports the requested size.
If the key generation does not succeed, please check the
documentation of your card to see what sizes are allowed.
What keysize do you want for the Encryption key? (2048) 4096
The card will now be re-configured to generate a key of 4096 bits
What keysize do you want for the Authentication key? (2048) 4096
The card will now be re-configured to generate a key of 4096 bits
Please specify how long the key should be valid.
0 = key does not expire
<n> = key expires in n days
<n>w = key expires in n weeks
<n>m = key expires in n months
<n>y = key expires in n years
Key is valid for? (0)
Key does not expire at all
Is this correct? (y/N) y
GnuPG needs to construct a user ID to identify your key.
Real name: John Doe
Email address: john.doe@email.com
Comment:
You selected this USER-ID:
"John Doe <john.doe@email.com>"
Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o
Using p11tool to get the token URL:
Depending on system configuration, gpg-agent may need to be killed first::
$ p11tool --provider /usr/lib/opensc-pkcs11.so --list-tokens
Token 0:
URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29
Label: OpenPGP card (User PIN (sig))
Type: Hardware token
Manufacturer: ZeitControl
Model: PKCS#15 emulated
Serial: 000xxxxxxxxx
Module: (null)
Token 1:
URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%29
Label: OpenPGP card (User PIN)
Type: Hardware token
Manufacturer: ZeitControl
Model: PKCS#15 emulated
Serial: 000xxxxxxxxx
Module: (null)
Use the portion of the signature token URL after "pkcs11:" as the keydir argument (-k) to mkimage below.
Use the URL of the token to list the private keys::
$ p11tool --login --provider /usr/lib/opensc-pkcs11.so --list-privkeys \
"pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29"
Token 'OpenPGP card (User PIN (sig))' with URL 'pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29' requires user PIN
Enter PIN:
Object 0:
URL: pkcs11:model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29;id=%01;object=Signature%20key;type=private
Type: Private key
Label: Signature key
Flags: CKA_PRIVATE; CKA_NEVER_EXTRACTABLE; CKA_SENSITIVE;
ID: 01
Use the label, in this case "Signature key" as the key-name-hint in your FIT.
Create the fitImage::
$ ./tools/mkimage -f fit-image.its fitImage
Sign the fitImage with the hardware key::
$ ./tools/mkimage -F -k \
"model=PKCS%2315%20emulated;manufacturer=ZeitControl;serial=000xxxxxxxxx;token=OpenPGP%20card%20%28User%20PIN%20%28sig%29%29" \
-K u-boot.dtb -N pkcs11 -r fitImage
Future Work
-----------
- Roll-back protection using a TPM is done using the tpm command. This can
be scripted, but we might consider a default way of doing this, built into
bootm.
Possible Future Work
--------------------
- More sandbox tests for failure modes
- Passwords for keys/certificates
- Perhaps implement OAEP
- Enhance bootm to permit scripted signature verification (so that a script
can verify an image but not actually boot it)
.. sectionauthor:: Simon Glass <sjg@chromium.org>, 1-1-13

77
doc/uImage.FIT/verified-boot.txt → doc/usage/fit/verified-boot.rst
View File

@ -1,8 +1,11 @@
.. SPDX-License-Identifier: GPL-2.0+
U-Boot Verified Boot
====================
Introduction
------------
Verified boot here means the verification of all software loaded into a
machine during the boot process to ensure that it is authorised and correct
for that machine.
@ -21,6 +24,7 @@ memory, so that firmware can easily be upgraded in a secure manner.
Signing
-------
Verified boot uses cryptographic algorithms to 'sign' software images.
Images are signed using a private key known only to the signer, but can
be verified using a public key. As its name suggests the public key can be
@ -28,31 +32,31 @@ made available without risk to the verification process. The private and
public keys are mathematically related. For more information on how this
works look up "public key cryptography" and "RSA" (a particular algorithm).
The signing and verification process looks something like this:
The signing and verification process looks something like this::
Signing Verification
======= ============
Signing Verification
======= ============
+--------------+ *
| RSA key pair | * +---------------+
| .key .crt | * | Public key in |
+--------------+ +------> public key ----->| trusted place |
| | * +---------------+
| | * |
v | * v
+---------+ | * +--------------+
| |----------+ * | |
| signer | * | U-Boot |
| |----------+ * | signature |--> yes/no
+---------+ | * | verification |
^ | * | |
| | * +--------------+
| | * ^
+----------+ | * |
| Software | +----> signed image -------------+
| image | *
+----------+ *
+--------------+ *
| RSA key pair | * +---------------+
| .key .crt | * | Public key in |
+--------------+ +------> public key ----->| trusted place |
| | * +---------------+
| | * |
v | * v
+---------+ | * +--------------+
| |---------+ * | |
| signer | * | U-Boot |
| |---------+ * | signature |--> yes/no
+---------+ | * | verification |
^ | * | |
| | * +--------------+
| | * ^
+----------+ | * |
| Software | +----> signed image -------------+
| image | *
+----------+ *
The signature algorithm relies only on the public key to do its work. Using
@ -70,23 +74,25 @@ the verification is worthless.
Chaining Images
---------------
The above method works for a signer providing images to a run-time U-Boot.
It is also possible to extend this scheme to a second level, like this:
1. Master private key is used by the signer to sign a first-stage image.
2. Master public key is placed in read-only memory.
2. Secondary private key is created and used to sign second-stage images.
3. Secondary public key is placed in first stage images
4. We use the master public key to verify the first-stage image. We then
use the secondary public key in the first-stage image to verify the second-
state image.
5. This chaining process can go on indefinitely. It is recommended to use a
different key at each stage, so that a compromise in one place will not
affect the whole change.
#. Master private key is used by the signer to sign a first-stage image.
#. Master public key is placed in read-only memory.
#. Secondary private key is created and used to sign second-stage images.
#. Secondary public key is placed in first stage images
#. We use the master public key to verify the first-stage image. We then
use the secondary public key in the first-stage image to verify the second-
state image.
#. This chaining process can go on indefinitely. It is recommended to use a
different key at each stage, so that a compromise in one place will not
affect the whole change.
Flattened Image Tree (FIT)
--------------------------
The FIT format is already widely used in U-Boot. It is a flattened device
tree (FDT) in a particular format, with images contained within. FITs
include hashes to verify images, so it is relatively straightforward to
@ -96,9 +102,6 @@ The public key can be stored in U-Boot's CONFIG_OF_CONTROL device tree in
a standard place. Then when a FIT is loaded it can be verified using that
public key. Multiple keys and multiple signatures are supported.
See signature.txt for more information.
See :doc:`signature` for more information.
Simon Glass
sjg@chromium.org
1-1-13
.. sectionauthor:: Simon Glass <sjg@chromium.org> 1-1-13