We broke this to varying degrees ages ago.
This is the logic to implement the variations of rules in TLS to discard
sessions after a failed connection, where a failed connection could be
one of:
- A connection that was not cleanly shut down.
- A connection that received a fatal alert.
The first one is nonsense since close_notify does not actually work in
the real world. The second is a vaguely more plausible but...
- A stateless ticket-based server can't drop sessions anyway.
- In TLS 1.3, a client may receive many tickets over the lifetime of a
single connection. With an external session cache like ours which may,
in theory, but multithreaded, this will be a huge hassle to track.
- A client may well attempt to establish a connection and reuse the
session before we receive the fatal alert, so any application state we
hope to manage won't really work.
- An attacker can always close the connection before the fatal alert, so
whatever security policy clearing the session gave is easily
bypassable.
Implementation-wise, this has basically never worked. The
ssl_clear_bad_session logic called into SSL_CTX_remove_session which
relied on the internal session cache. (Sessions not in the internal
session cache don't get removed.) The internal session cache was only
useful for a server, where tickets prevent this mechanism from doing
anything. For a client, we since removed the internal session cache, so
nothing got removed. The API for a client also did not work as it gave
the SSL_SESSION, not the SSL, so a consumer would not know the key to
invalidate anyway.
The recent session state splitting change further broke this.
Moreover, calling into SSL_CTX_remove_session logic like that is
extremely dubious because it mutates the not_resumable flag on the
SSL_SESSION which isn't thread-safe.
Spec-wise, TLS 1.3 has downgraded the MUST to a SHOULD.
Given all that mess, just remove this code. It is no longer necessary to
call SSL_shutdown just to make session caching work.
Change-Id: Ib601937bfc5f6b40436941e1c86566906bb3165d
Reviewed-on: https://boringssl-review.googlesource.com/9091
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
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