From: Marcus Huewe <suse-tux@gmx.de>
Date: Fri, 11 May 2018 10:24:56 +0000 (+0200)
Subject: Do not free a session before calling the remove_session_cb
X-Git-Tag: OpenSSL_1_1_1-pre8~61
X-Git-Url: https://git.librecmc.org/?a=commitdiff_plain;h=c0a58e034d3eff68ca5e0d36d7b4d147425b0599;p=oweals%2Fopenssl.git

Do not free a session before calling the remove_session_cb

If the remove_session_cb accesses the session's data (for instance,
via SSL_SESSION_get_protocol_version), a potential use after free
can occur. For this, consider the following scenario when adding
a new session via SSL_CTX_add_session:

- The session cache is full
  (SSL_CTX_sess_number(ctx) > SSL_CTX_sess_get_cache_size(ctx))
- Only the session cache has a reference to ctx->session_cache_tail
  (that is, ctx->session_cache_tail->references == 1)

Since the cache is full, remove_session_lock is called to remove
ctx->session_cache_tail from the cache. That is, it
SSL_SESSION_free()s the session, which free()s the data. Afterwards,
the free()d session is passed to the remove_session_cb. If the callback
accesses the session's data, we have a use after free.

The free before calling the callback behavior was introduced in
commit e4612d02c53cccd24fa97b08fc01250d1238cca1 ("Remove sessions
from external cache, even if internal cache not used.").

CLA: trivial

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6222)
---

diff --git a/ssl/ssl_sess.c b/ssl/ssl_sess.c
index 525edb3289..0723765366 100644
--- a/ssl/ssl_sess.c
+++ b/ssl/ssl_sess.c
@@ -769,11 +769,11 @@ static int remove_session_lock(SSL_CTX *ctx, SSL_SESSION *c, int lck)
         if (lck)
             CRYPTO_THREAD_unlock(ctx->lock);
 
-        if (ret)
-            SSL_SESSION_free(r);
-
         if (ctx->remove_session_cb != NULL)
             ctx->remove_session_cb(ctx, c);
+
+        if (ret)
+            SSL_SESSION_free(r);
     } else
         ret = 0;
     return ret;