build_kernel_image: use a salt for verity.

BUG=chromium-os:12138
TEST=Adhoc
TESTED_ON=Kaen
Build an image. Look for "Generating root fs hash tree (salt <foo>)." in the
output. Boot the image, grep for 'salt=' in dmesg. All should be well.

Change-Id: If9dbefbd8a875d06ff45cd54704f166c2511c3b7
Signed-off-by: Elly Jones <ellyjones@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/7174

build_kernel_image.sh

@@ -76,6 +76,14 @@ eval set -- "${FLAGS_ARGV}"
 # Die on error
 set -e
 
+make_salt() {
+  # It is not important that the salt be cryptographically strong; it just needs
+  # to be different for each release. The purpose of the salt is just to ensure
+  # that if someone collides a block in one release, they can't reuse it in
+  # future releases.
+  xxd -l 32 -p -c 32 /dev/urandom
+}
+
 verity_args=
 # Even with a rootfs_image, root= is not changed unless specified.
 if [[ -n "${FLAGS_rootfs_image}" && -n "${FLAGS_rootfs_hash}" ]]; then
@@ -100,14 +108,16 @@ if [[ -n "${FLAGS_rootfs_image}" && -n "${FLAGS_rootfs_hash}" ]]; then
     error "Root file system blocks are not 4k!"
   fi
 
-  info "Generating root fs hash tree."
+  salt=$(make_salt)
+  info "Generating root fs hash tree (salt $salt)."
   # Runs as sudo in case the image is a block device.
   # First argument to verity is reserved/unused and MUST be 0
   table=$(sudo verity mode=create \
                       alg=${FLAGS_verity_hash_alg} \
                       payload=${FLAGS_rootfs_image} \
                       payload_blocks=${root_fs_blocks} \
-                      hashtree=${FLAGS_rootfs_hash})
+                      hashtree=${FLAGS_rootfs_hash} \
+                      salt=$salt)
   if [[ -f "${FLAGS_rootfs_hash}" ]]; then
     sudo chmod a+r "${FLAGS_rootfs_hash}"
   fi