netfilter: Mark SYN/ACK packets as invalid from original direction

Clients should not send such packets. By accepting them, we open
up a hole by wich ephemeral ports can be discovered in an off-path
attack.

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>

diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index a5ac11ebef331895f39af5e86ca4406e56562ceb..aba98f942979a01c8266fbd5eb967bb1ee01e705 100644 (file)
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -158,21 +158,18 @@ static const u8 tcp_conntracks[2][6][TCP_CONNTRACK_MAX] = {
  *     sCL -> sSS
  */
 /*          sNO, sSS, sSR, sES, sFW, sCW, sLA, sTW, sCL, sS2   */
-/*synack*/ { sIV, sIV, sIG, sIG, sIG, sIG, sIG, sIG, sIG, sSR },
+/*synack*/ { sIV, sIV, sSR, sIV, sIV, sIV, sIV, sIV, sIV, sSR },
 /*
  *     sNO -> sIV      Too late and no reason to do anything
  *     sSS -> sIV      Client can't send SYN and then SYN/ACK
  *     sS2 -> sSR      SYN/ACK sent to SYN2 in simultaneous open
- *     sSR -> sIG
- *     sES -> sIG      Error: SYNs in window outside the SYN_SENT state
- *                     are errors. Receiver will reply with RST
- *                     and close the connection.
- *                     Or we are not in sync and hold a dead connection.
- *     sFW -> sIG
- *     sCW -> sIG
- *     sLA -> sIG
- *     sTW -> sIG
- *     sCL -> sIG
+ *     sSR -> sSR      Late retransmitted SYN/ACK in simultaneous open
+ *     sES -> sIV      Invalid SYN/ACK packets sent by the client
+ *     sFW -> sIV
+ *     sCW -> sIV
+ *     sLA -> sIV
+ *     sTW -> sIV
+ *     sCL -> sIV
  */
 /*          sNO, sSS, sSR, sES, sFW, sCW, sLA, sTW, sCL, sS2   */
 /*fin*/    { sIV, sIV, sFW, sFW, sLA, sLA, sLA, sTW, sCL, sIV },