From: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
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@xxxxxxxxxx>
Acked-by: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
---
include/uapi/linux/netfilter/nf_conntrack_tcp.h | 3 ++
net/netfilter/nf_conntrack_proto_tcp.c | 35 +++++++++++++++++++++--
2 files changed, 35 insertions(+), 3 deletions(-)
diff --git a/include/uapi/linux/netfilter/nf_conntrack_tcp.h b/include/uapi/linux/netfilter/nf_conntrack_tcp.h
index 9993a42..ef9f80f 100644
--- a/include/uapi/linux/netfilter/nf_conntrack_tcp.h
+++ b/include/uapi/linux/netfilter/nf_conntrack_tcp.h
@@ -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;
diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index 5caa0c4..70383de 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)
--
1.7.10.4
--
To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
