[tls] Poison initial resumption master secret

There should be no circumstance that leads to a session being resumed
without having a valid session resumption secret that was stored by a
successfully established previous connection.

As an additional layer of defence in depth, poison the initial session
resumption master secret so that a predictable all-zero secret can
never be used accidentally in future.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tls.c

@@ -4268,6 +4268,11 @@ static int tls_session ( struct tls_connection *tls, const char *name ) {
 	INIT_LIST_HEAD ( &session->conn );
 	list_add ( &session->list, &tls_sessions );
 
+	/* Poison resumption master secret */
+	tls_ephemeral_label ( tls, "res poison",
+			      session->resumption_master_secret,
+			      sizeof ( session->resumption_master_secret ) );
+
 	/* Record session */
 	tls->session = session;