l2tp: Support different protocol versions with same IP/port quadruple

628bc3e ("l2tp: Support several sockets with same IP/port quadruple")
added support for several L2TPv2 tunnels using the same IP/port quadruple,
but if an L2TPv3 socket exists it could eat all the trafic. We thus have to
first use the version from the packet to get the proper tunnel, and only
then check that the version matches.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: James Chapman <jchapman@katalix.com>
Link: https://lore.kernel.org/r/20240509205812.4063198-1-samuel.thibault@ens-lyon.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/l2tp/l2tp_core.c

@@ -820,13 +820,8 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
 	/* Get L2TP header flags */
 	hdrflags = ntohs(*(__be16 *)ptr);
 
-	/* Check protocol version */
+	/* Get protocol version */
 	version = hdrflags & L2TP_HDR_VER_MASK;
-	if (version != tunnel->version) {
-		pr_debug_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",
-				     tunnel->name, version, tunnel->version);
-		goto invalid;
-	}
 
 	/* Get length of L2TP packet */
 	length = skb->len;
@@ -838,7 +833,7 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
 	/* Skip flags */
 	ptr += 2;
 
-	if (tunnel->version == L2TP_HDR_VER_2) {
+	if (version == L2TP_HDR_VER_2) {
 		/* If length is present, skip it */
 		if (hdrflags & L2TP_HDRFLAG_L)
 			ptr += 2;
@@ -855,7 +850,7 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
 			struct l2tp_tunnel *alt_tunnel;
 
 			alt_tunnel = l2tp_tunnel_get(tunnel->l2tp_net, tunnel_id);
-			if (!alt_tunnel || alt_tunnel->version != L2TP_HDR_VER_2)
+			if (!alt_tunnel)
 				goto pass;
 			tunnel = alt_tunnel;
 		}
@@ -869,6 +864,13 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
 		ptr += 4;
 	}
 
+	/* Check protocol version */
+	if (version != tunnel->version) {
+		pr_debug_ratelimited("%s: recv protocol version mismatch: got %d expected %d\n",
+				     tunnel->name, version, tunnel->version);
+		goto invalid;
+	}
+
 	/* Find the session context */
 	session = l2tp_tunnel_get_session(tunnel, session_id);
 	if (!session || !session->recv_skb) {