conntrack: RFC5961 challenge ACK confuse conntrack LAST-ACK transition

In compliance with RFC5961, the network stack send challenge ACK in
response to spurious SYN packets, since commit 0c228e833c88 ("tcp:
Restore RFC5961-compliant behavior for SYN packets").

This pose a problem for netfilter conntrack in state LAST_ACK, because
this challenge ACK is (falsely) seen as ACKing last FIN, causing a
false state transition (into TIME_WAIT).

The challenge ACK is hard to distinguish from real last ACK. Thus,
solution introduce a flag that tracks the potential for seeing a
challenge ACK, in case a SYN packet is let through and current state
is LAST_ACK.

When conntrack transition LAST_ACK to TIME_WAIT happens, this flag is
used for determining if we are expecting a challenge ACK.

Scapy based reproducer script avail here:
https://github.com/netoptimizer/network-testing/blob/master/scapy/tcp_hacks_3WHS_LAST_ACK.py

Fixes: 0c228e833c88 ("tcp: Restore RFC5961-compliant behavior for SYN packets")
Signed-off-by: Jesper Dangaard Brouer <[e-mail]>
Acked-by: Jozsef Kadlecsik <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/uapi/linux/netfilter/nf_conntrack_tcp.h

···
 /* The field td_maxack has been set */
 #define IP_CT_TCP_FLAG_MAXACK_SET		0x20
 
+/* Marks possibility for expected RFC5961 challenge ACK */
+#define IP_CT_EXP_CHALLENGE_ACK 		0x40
+
 struct nf_ct_tcp_flags {
 	__u8 flags;
 	__u8 mask;

net/netfilter/nf_conntrack_proto_tcp.c

···
  *	sES -> sES	:-)
  *	sFW -> sCW	Normal close request answered by ACK.
  *	sCW -> sCW
- *	sLA -> sTW	Last ACK detected.
+ *	sLA -> sTW	Last ACK detected (RFC5961 challenged)
  *	sTW -> sTW	Retransmitted last ACK. Remain in the same state.
  *	sCL -> sCL
  */
···
  *	sES -> sES	:-)
  *	sFW -> sCW	Normal close request answered by ACK.
  *	sCW -> sCW
- *	sLA -> sTW	Last ACK detected.
+ *	sLA -> sTW	Last ACK detected (RFC5961 challenged)
  *	sTW -> sTW	Retransmitted last ACK.
  *	sCL -> sCL
  */
···
 					1 : ct->proto.tcp.last_win;
 			ct->proto.tcp.seen[ct->proto.tcp.last_dir].td_scale =
 				ct->proto.tcp.last_wscale;
+			ct->proto.tcp.last_flags &= ~IP_CT_EXP_CHALLENGE_ACK;
 			ct->proto.tcp.seen[ct->proto.tcp.last_dir].flags =
 				ct->proto.tcp.last_flags;
 			memset(&ct->proto.tcp.seen[dir], 0,
···
 		 * may be in sync but we are not. In that case, we annotate
 		 * the TCP options and let the packet go through. If it is a
 		 * valid SYN packet, the server will reply with a SYN/ACK, and
-		 * then we'll get in sync. Otherwise, the server ignores it. */
+		 * then we'll get in sync. Otherwise, the server potentially
+		 * responds with a challenge ACK if implementing RFC5961.
+		 */
 		if (index == TCP_SYN_SET && dir == IP_CT_DIR_ORIGINAL) {
 			struct ip_ct_tcp_state seen = {};
 
···
 				ct->proto.tcp.last_flags |=
 					IP_CT_TCP_FLAG_SACK_PERM;
 			}
+			/* Mark the potential for RFC5961 challenge ACK,
+			 * this pose a special problem for LAST_ACK state
+			 * as ACK is intrepretated as ACKing last FIN.
+			 */
+			if (old_state == TCP_CONNTRACK_LAST_ACK)
+				ct->proto.tcp.last_flags |=
+					IP_CT_EXP_CHALLENGE_ACK;
 		}
 		spin_unlock_bh(&ct->lock);
 		if (LOG_INVALID(net, IPPROTO_TCP))
···
 			nf_log_packet(net, pf, 0, skb, NULL, NULL, NULL,
 				  "nf_ct_tcp: invalid state ");
 		return -NF_ACCEPT;
+	case TCP_CONNTRACK_TIME_WAIT:
+		/* RFC5961 compliance cause stack to send "challenge-ACK"
+		 * e.g. in response to spurious SYNs.  Conntrack MUST
+		 * not believe this ACK is acking last FIN.
+		 */
+		if (old_state == TCP_CONNTRACK_LAST_ACK &&
+		    index == TCP_ACK_SET &&
+		    ct->proto.tcp.last_dir != dir &&
+		    ct->proto.tcp.last_index == TCP_SYN_SET &&
+		    (ct->proto.tcp.last_flags & IP_CT_EXP_CHALLENGE_ACK)) {
+			/* Detected RFC5961 challenge ACK */
+			ct->proto.tcp.last_flags &= ~IP_CT_EXP_CHALLENGE_ACK;
+			spin_unlock_bh(&ct->lock);
+			if (LOG_INVALID(net, IPPROTO_TCP))
+				nf_log_packet(net, pf, 0, skb, NULL, NULL, NULL,
+				      "nf_ct_tcp: challenge-ACK ignored ");
+			return NF_ACCEPT; /* Don't change state */
+		}
+		break;
 	case TCP_CONNTRACK_CLOSE:
 		if (index == TCP_RST_SET
 		    && (ct->proto.tcp.seen[!dir].flags & IP_CT_TCP_FLAG_MAXACK_SET)