netfilter: nf_ct_tcp: fix accepting invalid RST segments

Robert L Mathews discovered that some clients send evil TCP RST segments,
which are accepted by netfilter conntrack but discarded by the
destination. Thus the conntrack entry is destroyed but the destination
retransmits data until timeout.

The same technique, i.e. sending properly crafted RST segments, can easily
be used to bypass connlimit/connbytes based restrictions (the sample
script written by Robert can be found in the netfilter mailing list
archives).

The patch below adds a new flag and new field to struct ip_ct_tcp_state so
that checking RST segments can be made more strict and thus TCP conntrack
can catch the invalid ones: the RST segment is accepted only if its
sequence number higher than or equal to the highest ack we seen from the
other direction. (The last_ack field cannot be reused because it is used
to catch resent packets.)

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>

1 files changed, 18 insertions, 0 deletions

diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index b5ccf2b4b2e7..97a6e93d742e 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -634,6 +634,14 @@ static bool tcp_in_window(const struct nf_conn *ct,
                         sender->td_end = end;
                         sender->flags |= IP_CT_TCP_FLAG_DATA_UNACKNOWLEDGED;
                 }
+                if (tcph->ack) {
+                        if (!(sender->flags & IP_CT_TCP_FLAG_MAXACK_SET)) {
+                                sender->td_maxack = ack;
+                                sender->flags |= IP_CT_TCP_FLAG_MAXACK_SET;
+                        } else if (after(ack, sender->td_maxack))
+                                sender->td_maxack = ack;
+                }
+
                 /*
                  * Update receiver data.
                  */
@@ -919,6 +927,16 @@ static int tcp_packet(struct nf_conn *ct,
                 return -NF_ACCEPT;
         case TCP_CONNTRACK_CLOSE:
                 if (index == TCP_RST_SET
+                    && (ct->proto.tcp.seen[!dir].flags & IP_CT_TCP_FLAG_MAXACK_SET)
+                    && before(ntohl(th->seq), ct->proto.tcp.seen[!dir].td_maxack)) {
+                        /* Invalid RST  */
+                        write_unlock_bh(&tcp_lock);
+                        if (LOG_INVALID(net, IPPROTO_TCP))
+                                nf_log_packet(pf, 0, skb, NULL, NULL, NULL,
+                                          "nf_ct_tcp: invalid RST ");
+                        return -NF_ACCEPT;
+                }
+                if (index == TCP_RST_SET
                     && ((test_bit(IPS_SEEN_REPLY_BIT, &ct->status)
                          && ct->proto.tcp.last_index == TCP_SYN_SET)
                         || (!test_bit(IPS_ASSURED_BIT, &ct->status)