Fix RAND_enable_fork_unsafe_buffering when called after fork

If a process calls fork(), then the child process never forks again, the
child may wish to call RAND_enable_fork_unsafe_buffering(). However,
doing so exposes a bug: we assume that, if the flag is set, we don't
need to worry about fork-safety. But it is possible that the PRNG state
was cloned from another process which does not work.

Concretely, consider a zygote process, e.g. Chromium's. A zygote process
would retain fork-safety, but pass along its PRNG state to each of its
children. If the children never fork, they might disable fork-safety,
hitting this bug. (Chromium does not call this API. This is just a
hypothetical scenario.)

Fix this by reseeding whenever the fork-safety bit changes. This fix
does not strictly depend on the atomics work, but it causes us to
unconditionally sample rand_fork_unsafe_buffering_enabled(). This no
longer causes contention because it's just another atomic load.

This only affects systems without MADV_WIPEONFORK and without fast
RDRAND. If RDRAND is fast, we're always fork-safe and MADV_WIPEONFORK
allows us to efficiently detect forks.

Cq-Include-Trybots: luci.boringssl.try:linux_clang_rel_tsan
Change-Id: I6d0c471c62c951254faf85420a7dc3f4a9d65ee0
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/59850
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

0 files changed, 0 insertions, 0 deletions