Code to thread-safety in ChangeCipherState

The server-side ChangeCipherState processing stores the new cipher
in the SSL_SESSION object, so that the new state can be used if
this session gets resumed.  However, writing to the session is only
thread-safe for initial handshakes, as at other times the session
object may be in a shared cache and in use by another thread at the
same time.  Reflect this invariant in the code by only writing to
s->session->cipher when it is currently NULL (we do not cache sessions
with no cipher).  The code prior to this change would never actually
change the (non-NULL) cipher value in a session object, since our
server enforces that (pre-TLS-1.3) resumptions use the exact same
cipher as the initial connection, and non-abbreviated renegotiations
have produced a new session object before we get to this point.
Regardless, include logic to detect such a condition and abort the
handshake if it occurs, to avoid any risk of inadvertently using
the wrong cipher on a connection.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10943)

1 files changed, 9 insertions, 1 deletions

diff --git a/ssl/statem/statem_srvr.c b/ssl/statem/statem_srvr.c
index 00905eb760..1cc106876c 100644
--- a/ssl/statem/statem_srvr.c
+++ b/ssl/statem/statem_srvr.c
@@ -744,7 +744,15 @@ WORK_STATE ossl_statem_server_pre_work(SSL *s, WORK_STATE wst)
     case TLS_ST_SW_CHANGE:
         if (SSL_IS_TLS13(s))
             break;
-        s->session->cipher = s->s3.tmp.new_cipher;
+        /* Writes to s->session are only safe for initial handshakes */
+        if (s->session->cipher == NULL) {
+            s->session->cipher = s->s3.tmp.new_cipher;
+        } else if (s->session->cipher != s->s3.tmp.new_cipher) {
+            SSLfatal(s, SSL_AD_INTERNAL_ERROR,
+                     SSL_F_OSSL_STATEM_SERVER_PRE_WORK,
+                     ERR_R_INTERNAL_ERROR);
+            return WORK_ERROR;
+        }
         if (!s->method->ssl3_enc->setup_key_block(s)) {
             /* SSLfatal() already called */
             return WORK_ERROR;