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 0c228e833c88 ("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: 0c228e833c88 ("tcp: Restore RFC5961-compliant behavior for SYN packets")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 32 insertions, 3 deletions

diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index 5caa0c41bf26..70383de72054 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -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)