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authorJesper Dangaard Brouer <brouer@redhat.com>2015-05-07 14:54:16 +0200
committerPablo Neira Ayuso <pablo@netfilter.org>2015-05-15 20:50:56 +0200
commitb3cad287d13b5f6695c6b4aab72969cd64bf0171 (patch)
tree35cb16bad9e76c3cf30e01e71b4ea5031de0a564 /net
parent595ca5880b37d4aa3c292d75531577175d36b225 (diff)
downloadkernel-b3cad287d13b5f6695c6b4aab72969cd64bf0171.tar.gz
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>
Diffstat (limited to 'net')
-rw-r--r--net/netfilter/nf_conntrack_proto_tcp.c35
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)