l2tp: handle hash key collisions in l2tp_v3_session_get

To handle colliding l2tpv3 session IDs, l2tp_v3_session_get searches a
hashed list keyed by ID and sk. Although unlikely, if hash keys
collide, it is possible that hash_for_each_possible loops over a
session which doesn't have the ID that we are searching for. So check
for session ID match when looping over possible hash key matches.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -291,7 +291,8 @@ struct l2tp_session *l2tp_v3_session_get(const struct net *net, struct sock *sk,
 			 */
 			struct l2tp_tunnel *tunnel = READ_ONCE(session->tunnel);
 
-			if (tunnel && tunnel->sock == sk &&
+			if (session->session_id == session_id &&
+			    tunnel && tunnel->sock == sk &&
 			    refcount_inc_not_zero(&session->ref_count)) {
 				rcu_read_unlock_bh();
 				return session;