From: Florian Westphal <fw@xxxxxxxxx>
When a connection is re-used, following can happen:
[ connection starts to close, fin sent in either direction ]
> syn # initator quickly reuses connection
< ack # peer sends a challenge ack
> rst # rst, sequence number == ack_seq of previous challenge ack
> syn # this syn is expected to pass
Problem is that the rst will fail window validation, so it gets
tagged as invalid.
If ruleset drops such packets, we get repeated syn-retransmits until
initator gives up or peer starts responding with syn/ack.
Before the commit indicated in the "Fixes" tag below this used to work:
The challenge-ack made conntrack re-init state based on the challenge
ack itself, so the following rst would pass window validation.
Add challenge-ack support: If we get ack for syn, record the ack_seq,
and then check if the rst sequence number matches the last ack number
seen in reverse direction.
Fixes: c7aab4f17021 ("netfilter: nf_conntrack_tcp: re-init for syn packets only")
Reported-by: Michal Tesar <mtesar@xxxxxxxxxx>
Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
---
net/netfilter/nf_conntrack_proto_tcp.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index 656631083177..3ac1af6f59fc 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -1068,6 +1068,13 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
ct->proto.tcp.last_flags |=
IP_CT_EXP_CHALLENGE_ACK;
}
+
+ /* possible challenge ack reply to syn */
+ if (old_state == TCP_CONNTRACK_SYN_SENT &&
+ index == TCP_ACK_SET &&
+ dir == IP_CT_DIR_REPLY)
+ ct->proto.tcp.last_ack = ntohl(th->ack_seq);
+
spin_unlock_bh(&ct->lock);
nf_ct_l4proto_log_invalid(skb, ct, state,
"packet (index %d) in dir %d ignored, state %s",
@@ -1193,6 +1200,14 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
* segments we ignored. */
goto in_window;
}
+
+ /* Reset in response to a challenge-ack we let through earlier */
+ if (old_state == TCP_CONNTRACK_SYN_SENT &&
+ ct->proto.tcp.last_index == TCP_ACK_SET &&
+ ct->proto.tcp.last_dir == IP_CT_DIR_REPLY &&
+ ntohl(th->seq) == ct->proto.tcp.last_ack)
+ goto in_window;
+
break;
default:
/* Keep compilers happy. */
--
2.30.2
