netfilter: Validate the sequence number of dataless ACK packets as well

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 <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index 2458cc933b8508273642c7e5a78134c268c811e9..57ad4666b24e452740ab71174429a70a43dfadf9 100644 (file)
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -625,15 +625,9 @@ static bool tcp_in_window(const struct nf_conn *ct,
                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;