[SECURITY] CERT/CC VU#464113, SYN plus RST/FIN

[Florian Weimer]

This patch prevents SYN+RST and SYN+FIN segments which arrive in the
LISTEN state from initiating a three-way handshake.

I'm not sure if it is correct, but it's better than nothing (so far, I
haven't seen any patch for this issue).

--- tcp_input.c	2002/10/25 08:45:20	1.1
+++ tcp_input.c	2002/10/25 08:49:21
@@ -3668,6 +3668,8 @@
 	case TCP_LISTEN:
 		if(th->ack)
 			return 1;
+		if(th->rst || th->fin)
+			goto discard;
 
 		if(th->syn) {
 			if(tp->af_specific->conn_request(sk, skb) < 0)