tcp: accept RST without ACK flag

commit c3ae62af (tcp: should drop incoming frames without ACK flag
set) added a regression on the handling of RST messages.

RST should be allowed to come even without ACK bit set. We validate
the RST by checking the exact sequence, as requested by RFC 793 and
5961 3.2, in tcp_validate_incoming()
Reported-by: NEric Wong <normalperson@yhbt.net>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NNeal Cardwell <ncardwell@google.com>
Tested-by: NEric Wong <normalperson@yhbt.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -5543,7 +5543,7 @@ int tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
 	if (len < (th->doff << 2) || tcp_checksum_complete_user(sk, skb))
 		goto csum_error;
 
-	if (!th->ack)
+	if (!th->ack && !th->rst)
 		goto discard;
 
 	/*
@@ -5988,7 +5988,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
 			goto discard;
 	}
 
-	if (!th->ack)
+	if (!th->ack && !th->rst)
 		goto discard;
 
 	if (!tcp_validate_incoming(sk, skb, th, 0))