3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jozsef Kadlecsik <redacted>

commit 4a70bbfaef0361d27272629d1a250a937edcafe4 upstream.

We spare nothing by not validating the sequence number of dataless
ACK packets and enabling it makes harder off-path attacks.

See: "Reflection scan: an Off-Path Attack on TCP" by Jan Wrobel,
http://arxiv.org/abs/1201.2074

Signed-off-by: Jozsef Kadlecsik <redacted>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 net/netfilter/nf_conntrack_proto_tcp.c |   10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c

@@ -627,15 +627,9 @@ static bool tcp_in_window(const struct n
 		ack = sack = receiver->td_end;
 	}
 
-	if (seq == end
-	    && (!tcph->rst
-		|| (seq == 0 && state->state == TCP_CONNTRACK_SYN_SENT)))
+	if (tcph->rst && seq == 0 && state->state == TCP_CONNTRACK_SYN_SENT)
 		/*
-		 * Packets contains no data: we assume it is valid
-		 * and check the ack value only.
-		 * However RST segments are always validated by their
-		 * SEQ number, except when seq == 0 (reset sent answering
-		 * SYN.
+		 * RST sent answering SYN.
 		 */
 		seq = end = sender->td_end;