It doesn't appear that logic (added in upstream's
b7cfcfb7f8e17c17f457b3384010eb027f3aad72) is protecting against anything. On
the contrary, it prohibits implementing CRYPTO_add with real atomic operations!
There's no guarantee that those operations will interact with the locked
implementation.
https://www.mail-archive.com/openssl-users@openssl.org/msg63176.html
As long as ssl->session points to the same session, we know the session won't
be freed. There is no lock protecting, say, SSL_set_session, but a single SSL*
does not appear to be safe to use across threads. If this were to be supported,
both should be guarded by CRYPTO_LOCK_SSL (which is barely used).
CRYPTO_LOCK_SSL_SESSION isn't sufficient anyway; it could sample while
SSL_set_session is busy swapping out the now freed old session with the new.
Change-Id: I54623d0690c55c2c86720406ceff545e2e5f2f8f
Reviewed-on: https://boringssl-review.googlesource.com/3345
Reviewed-by: Adam Langley <agl@google.com>
23a8ca1f10