Commit b3cad287 authored by Jesper Dangaard Brouer's avatar Jesper Dangaard Brouer Committed by Pablo Neira Ayuso

conntrack: RFC5961 challenge ACK confuse conntrack LAST-ACK transition

In compliance with RFC5961, the network stack send challenge ACK in
response to spurious SYN packets, since commit 0c228e83 ("tcp:
Restore RFC5961-compliant behavior for SYN packets").

This pose a problem for netfilter conntrack in state LAST_ACK, because
this challenge ACK is (falsely) seen as ACKing last FIN, causing a
false state transition (into TIME_WAIT).

The challenge ACK is hard to distinguish from real last ACK.  Thus,
solution introduce a flag that tracks the potential for seeing a
challenge ACK, in case a SYN packet is let through and current state
is LAST_ACK.

When conntrack transition LAST_ACK to TIME_WAIT happens, this flag is
used for determining if we are expecting a challenge ACK.

Scapy based reproducer script avail here:
 https://github.com/netoptimizer/network-testing/blob/master/scapy/tcp_hacks_3WHS_LAST_ACK.py

Fixes: 0c228e83 ("tcp: Restore RFC5961-compliant behavior for SYN packets")
Signed-off-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
Acked-by: default avatarJozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: default avatarPablo Neira Ayuso <pablo@netfilter.org>
parent 595ca588
......@@ -42,6 +42,9 @@ enum tcp_conntrack {
/* The field td_maxack has been set */
#define IP_CT_TCP_FLAG_MAXACK_SET 0x20
/* Marks possibility for expected RFC5961 challenge ACK */
#define IP_CT_EXP_CHALLENGE_ACK 0x40
struct nf_ct_tcp_flags {
__u8 flags;
__u8 mask;
......
......@@ -202,7 +202,7 @@ static const u8 tcp_conntracks[2][6][TCP_CONNTRACK_MAX] = {
* sES -> sES :-)
* sFW -> sCW Normal close request answered by ACK.
* sCW -> sCW
* sLA -> sTW Last ACK detected.
* sLA -> sTW Last ACK detected (RFC5961 challenged)
* sTW -> sTW Retransmitted last ACK. Remain in the same state.
* sCL -> sCL
*/
......@@ -261,7 +261,7 @@ static const u8 tcp_conntracks[2][6][TCP_CONNTRACK_MAX] = {
* sES -> sES :-)
* sFW -> sCW Normal close request answered by ACK.
* sCW -> sCW
* sLA -> sTW Last ACK detected.
* sLA -> sTW Last ACK detected (RFC5961 challenged)
* sTW -> sTW Retransmitted last ACK.
* sCL -> sCL
*/
......@@ -906,6 +906,7 @@ static int tcp_packet(struct nf_conn *ct,
1 : ct->proto.tcp.last_win;
ct->proto.tcp.seen[ct->proto.tcp.last_dir].td_scale =
ct->proto.tcp.last_wscale;
ct->proto.tcp.last_flags &= ~IP_CT_EXP_CHALLENGE_ACK;
ct->proto.tcp.seen[ct->proto.tcp.last_dir].flags =
ct->proto.tcp.last_flags;
memset(&ct->proto.tcp.seen[dir], 0,
......@@ -923,7 +924,9 @@ static int tcp_packet(struct nf_conn *ct,
* may be in sync but we are not. In that case, we annotate
* the TCP options and let the packet go through. If it is a
* valid SYN packet, the server will reply with a SYN/ACK, and
* then we'll get in sync. Otherwise, the server ignores it. */
* then we'll get in sync. Otherwise, the server potentially
* responds with a challenge ACK if implementing RFC5961.
*/
if (index == TCP_SYN_SET && dir == IP_CT_DIR_ORIGINAL) {
struct ip_ct_tcp_state seen = {};
......@@ -939,6 +942,13 @@ static int tcp_packet(struct nf_conn *ct,
ct->proto.tcp.last_flags |=
IP_CT_TCP_FLAG_SACK_PERM;
}
/* Mark the potential for RFC5961 challenge ACK,
* this pose a special problem for LAST_ACK state
* as ACK is intrepretated as ACKing last FIN.
*/
if (old_state == TCP_CONNTRACK_LAST_ACK)
ct->proto.tcp.last_flags |=
IP_CT_EXP_CHALLENGE_ACK;
}
spin_unlock_bh(&ct->lock);
if (LOG_INVALID(net, IPPROTO_TCP))
......@@ -970,6 +980,25 @@ static int tcp_packet(struct nf_conn *ct,
nf_log_packet(net, pf, 0, skb, NULL, NULL, NULL,
"nf_ct_tcp: invalid state ");
return -NF_ACCEPT;
case TCP_CONNTRACK_TIME_WAIT:
/* RFC5961 compliance cause stack to send "challenge-ACK"
* e.g. in response to spurious SYNs. Conntrack MUST
* not believe this ACK is acking last FIN.
*/
if (old_state == TCP_CONNTRACK_LAST_ACK &&
index == TCP_ACK_SET &&
ct->proto.tcp.last_dir != dir &&
ct->proto.tcp.last_index == TCP_SYN_SET &&
(ct->proto.tcp.last_flags & IP_CT_EXP_CHALLENGE_ACK)) {
/* Detected RFC5961 challenge ACK */
ct->proto.tcp.last_flags &= ~IP_CT_EXP_CHALLENGE_ACK;
spin_unlock_bh(&ct->lock);
if (LOG_INVALID(net, IPPROTO_TCP))
nf_log_packet(net, pf, 0, skb, NULL, NULL, NULL,
"nf_ct_tcp: challenge-ACK ignored ");
return NF_ACCEPT; /* Don't change state */
}
break;
case TCP_CONNTRACK_CLOSE:
if (index == TCP_RST_SET
&& (ct->proto.tcp.seen[!dir].flags & IP_CT_TCP_FLAG_MAXACK_SET)
......
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