l2tp: make datapath sequence number support RFC-compliant

The L2TP datapath is not currently RFC-compliant when sequence numbers
are used in L2TP data packets. According to the L2TP RFC, any received
sequence number NR greater than or equal to the next expected NR is
acceptable, where the "greater than or equal to" test is determined by
the NR wrap point. This differs for L2TPv2 and L2TPv3, so add state in
the session context to hold the max NR value and the NR window size in
order to do the acceptable sequence number value check. These might be
configurable later, but for now we derive it from the tunnel L2TP
version, which determines the sequence number field size.
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -414,10 +414,7 @@ static void l2tp_recv_dequeue_skb(struct l2tp_session *session, struct sk_buff *
 	if (L2TP_SKB_CB(skb)->has_seq) {
 		/* Bump our Nr */
 		session->nr++;
-		if (tunnel->version == L2TP_HDR_VER_2)
-			session->nr &= 0xffff;
-		else
-			session->nr &= 0xffffff;
+		session->nr &= session->nr_max;
 
 		l2tp_dbg(session, L2TP_MSG_SEQ, "%s: updated nr to %hu\n",
 			 session->name, session->nr);
@@ -542,11 +539,34 @@ static inline int l2tp_verify_udp_checksum(struct sock *sk,
 	return __skb_checksum_complete(skb);
 }
 
+static int l2tp_seq_check_rx_window(struct l2tp_session *session, u32 nr)
+{
+	u32 nws;
+
+	if (nr >= session->nr)
+		nws = nr - session->nr;
+	else
+		nws = (session->nr_max + 1) - (session->nr - nr);
+
+	return nws < session->nr_window_size;
+}
+
 /* If packet has sequence numbers, queue it if acceptable. Returns 0 if
  * acceptable, else non-zero.
  */
 static int l2tp_recv_data_seq(struct l2tp_session *session, struct sk_buff *skb)
 {
+	if (!l2tp_seq_check_rx_window(session, L2TP_SKB_CB(skb)->ns)) {
+		/* Packet sequence number is outside allowed window.
+		 * Discard it.
+		 */
+		l2tp_dbg(session, L2TP_MSG_SEQ,
+			 "%s: pkt %u len %d discarded, outside window, nr=%u\n",
+			 session->name, L2TP_SKB_CB(skb)->ns,
+			 L2TP_SKB_CB(skb)->length, session->nr);
+		goto discard;
+	}
+
 	if (session->reorder_timeout != 0) {
 		/* Packet reordering enabled. Add skb to session's
 		 * reorder queue, in order of ns.
@@ -556,7 +576,8 @@ static int l2tp_recv_data_seq(struct l2tp_session *session, struct sk_buff *skb)
 		/* Packet reordering disabled. Discard out-of-sequence
 		 * packets
 		 */
-		if (L2TP_SKB_CB(skb)->ns != session->nr) {
+		if ((L2TP_SKB_CB(skb)->ns != session->nr) &&
+		    (!session->reorder_skip)) {
 			atomic_long_inc(&session->stats.rx_seq_discards);
 			l2tp_dbg(session, L2TP_MSG_SEQ,
 				 "%s: oos pkt %u len %d discarded, waiting for %u, reorder_q_len=%d\n",
@@ -1826,6 +1847,11 @@ struct l2tp_session *l2tp_session_create(int priv_size, struct l2tp_tunnel *tunn
 		session->session_id = session_id;
 		session->peer_session_id = peer_session_id;
 		session->nr = 0;
+		if (tunnel->version == L2TP_HDR_VER_2)
+			session->nr_max = 0xffff;
+		else
+			session->nr_max = 0xffffff;
+		session->nr_window_size = session->nr_max / 2;
 
 		sprintf(&session->name[0], "sess %u/%u",
 			tunnel->tunnel_id, session->session_id);

net/l2tp/l2tp_core.h

@@ -102,6 +102,8 @@ struct l2tp_session {
 	u32			nr;		/* session NR state (receive) */
 	u32			ns;		/* session NR state (send) */
 	struct sk_buff_head	reorder_q;	/* receive reorder queue */
+	u32			nr_max;		/* max NR. Depends on tunnel */
+	u32			nr_window_size;	/* NR window size */
 	struct hlist_node	hlist;		/* Hash list node */
 	atomic_t		ref_count;