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

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>

1 files changed, 2 insertions, 8 deletions

diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index aba98f94297..e046b3756aa 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -630,15 +630,9 @@ 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;