Fix unconstrained session cache growth in TLSv1.3

In TLSv1.3 we create a new session object for each ticket that we send.
We do this by duplicating the original session. If SSL_OP_NO_TICKET is in
use then the new session will be added to the session cache. However, if
early data is not in use (and therefore anti-replay protection is being
used), then multiple threads could be resuming from the same session
simultaneously. If this happens and a problem occurs on one of the threads,
then the original session object could be marked as not_resumable. When we
duplicate the session object this not_resumable status gets copied into the
new session object. The new session object is then added to the session
cache even though it is not_resumable.

Subsequently, another bug means that the session_id_length is set to 0 for
sessions that are marked as not_resumable - even though that session is
still in the cache. Once this happens the session can never be removed from
the cache. When that object gets to be the session cache tail object the
cache never shrinks again and grows indefinitely.

CVE-2024-2511

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24042)

diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index fa78906376eb57cc6dca6b9cd7e4ae413129baa5..1474b53a496531042ea896021c25260664649d8a 100644 (file)
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -4476,9 +4476,10 @@ void ssl_update_cache(SSL_CONNECTION *s, int mode)
 
     /*
      * If the session_id_length is 0, we are not supposed to cache it, and it
-     * would be rather hard to do anyway :-)
+     * would be rather hard to do anyway :-). Also if the session has already
+     * been marked as not_resumable we should not cache it for later reuse.
      */
-    if (s->session->session_id_length == 0)
+    if (s->session->session_id_length == 0 || s->session->not_resumable)
         return;
 
     /*

diff --git a/ssl/ssl_sess.c b/ssl/ssl_sess.c
index c6190b92d23b5be2a51c27808c773691948a5ea0..97830d2e6a02dd5a1f97db21a27ea36ae666475b 100644 (file)
--- a/ssl/ssl_sess.c
+++ b/ssl/ssl_sess.c
@@ -127,16 +127,11 @@ SSL_SESSION *SSL_SESSION_new(void)
     return ss;
 }
 
-SSL_SESSION *SSL_SESSION_dup(const SSL_SESSION *src)
-{
-    return ssl_session_dup(src, 1);
-}
-
 /*
  * Create a new SSL_SESSION and duplicate the contents of |src| into it. If
  * ticket == 0 then no ticket information is duplicated, otherwise it is.
  */
-SSL_SESSION *ssl_session_dup(const SSL_SESSION *src, int ticket)
+static SSL_SESSION *ssl_session_dup_intern(const SSL_SESSION *src, int ticket)
 {
     SSL_SESSION *dest;
 
@@ -265,6 +260,27 @@ SSL_SESSION *ssl_session_dup(const SSL_SESSION *src, int ticket)
     return NULL;
 }
 
+SSL_SESSION *SSL_SESSION_dup(const SSL_SESSION *src)
+{
+    return ssl_session_dup_intern(src, 1);
+}
+
+/*
+ * Used internally when duplicating a session which might be already shared.
+ * We will have resumed the original session. Subsequently we might have marked
+ * it as non-resumable (e.g. in another thread) - but this copy should be ok to
+ * resume from.
+ */
+SSL_SESSION *ssl_session_dup(const SSL_SESSION *src, int ticket)
+{
+    SSL_SESSION *sess = ssl_session_dup_intern(src, ticket);
+
+    if (sess != NULL)
+        sess->not_resumable = 0;
+
+    return sess;
+}
+
 const unsigned char *SSL_SESSION_get_id(const SSL_SESSION *s, unsigned int *len)
 {
     if (len)

diff --git a/ssl/statem/statem_srvr.c b/ssl/statem/statem_srvr.c
index 99085a4cc11b0ced11fdf9820f7087ce520b789a..065e0b318a403e25b4198b89b68951b0c3397b7f 100644 (file)
--- a/ssl/statem/statem_srvr.c
+++ b/ssl/statem/statem_srvr.c
@@ -2425,9 +2425,8 @@ CON_FUNC_RETURN tls_construct_server_hello(SSL_CONNECTION *s, WPACKET *pkt)
      * so the following won't overwrite an ID that we're supposed
      * to send back.
      */
-    if (s->session->not_resumable ||
-        (!(SSL_CONNECTION_GET_CTX(s)->session_cache_mode & SSL_SESS_CACHE_SERVER)
-         && !s->hit))
+    if (!(SSL_CONNECTION_GET_CTX(s)->session_cache_mode & SSL_SESS_CACHE_SERVER)
+            && !s->hit)
         s->session->session_id_length = 0;
 
     if (usetls13) {