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4f02e3da67
Connection takeover was implemented for H2 in 2.2 by commit cd4159f03
("MEDIUM: mux_h2: Implement the takeover() method."). It does have one
corner case related to memory allocation failure: in case the task or
tasklet allocation fails, the connection gets released synchronously.
Unfortunately the situation is bad there, because the lower layers are
already switched to the new thread while the tasklet is either NULL or
still the old one, and calling h2_release() will also result in
h2_process() and h2_process_demux() that may process any possibly
pending frames. Even the session remains the old one on the old thread,
so that some sess_log() that are called when facing certain demux errors
will be associated with the previous thread, possibly accessing a number
of elements belonging to another thread. There are even code paths where
the thread will try to grab the lock of its own idle conns list, believing
the connection is there while it has no useful effect. However, if the
owner thread was doing the same at the same moment, and ended up trying
to pick from the current thread (which could happen if picking a connection
for a different name), the two could even deadlock.
The risk is extremely low, but Fred managed to reproduce use-after-free
errors in conn_backend_get() after a takeover() failed by playing with
-dMfail, indicating that h2_release() had been successfully called. In
practise it's sufficient to have h2 on the server side with reuse-always
and to inject lots of request on it with -dMfail.
This patch takes a simple but radically different approach. Instead of
starting to migrate the connection before risking to face allocation
failures, it first pre-allocates a new task and tasklet, then assigns
them to the connection if the migration succeeds, otherwise it just
frees them. This way it's no longer needed to manipulate the connection
until it's fully migrated, and as a bonus this means the connection will
continue to exist and the use-after-free condition is solved at the same
time.
This should be backported to 2.2. Thanks to Fred for the initial analysis
of the problem!
The HAProxy documentation has been split into a number of different files for ease of use. Please refer to the following files depending on what you're looking for : - INSTALL for instructions on how to build and install HAProxy - BRANCHES to understand the project's life cycle and what version to use - LICENSE for the project's license - CONTRIBUTING for the process to follow to submit contributions The more detailed documentation is located into the doc/ directory : - doc/intro.txt for a quick introduction on HAProxy - doc/configuration.txt for the configuration's reference manual - doc/lua.txt for the Lua's reference manual - doc/SPOE.txt for how to use the SPOE engine - doc/network-namespaces.txt for how to use network namespaces under Linux - doc/management.txt for the management guide - doc/regression-testing.txt for how to use the regression testing suite - doc/peers.txt for the peers protocol reference - doc/coding-style.txt for how to adopt HAProxy's coding style - doc/internals for developer-specific documentation (not all up to date)