[patch] tcp connection tracking 2.4.19

Martin Renold
2002-10-08T20:47:39Z

hi,

There is a bug in the stable 2.4.19 kernel in the ip_conntrack code that
allows the final ACK of a SYN - SYN/ACK - ACK tcp handshake to establish
an ASSURED connection even if it has a wrong sequence number. The current
code only checks the ACK number.

This allows a DoS attack that will make it impossible to establish *real*
connections for some days, once the maximum is reached. Somebody sent me
an exploit:

http://old.homeip.net/martin/cdos.tgz

So I wrote a simple patch against 2.4.19, but I must admit that I do not
really understand the code around it, especially why it does not mark
such a packet as invalid (I'm new to most things here).

diff -urN -X dontdiff kernel-source-2.4.19.origin/include/linux/netfilter_ipv4/ip_conntrack_tcp.h kernel-source-2.4.19.patch/include/linux/netfilter_ipv4/ip_conntrack_tcp.h
--- kernel-source-2.4.19.origin/include/linux/netfilter_ipv4/ip_conntrack_tcp.h Fri Aug 4 22:07:24 2000
+++ kernel-source-2.4.19.patch/include/linux/netfilter_ipv4/ip_conntrack_tcp.h Sat Oct 5 19:07:44 2002
@@ -24,8 +24,9 @@
{
enum tcp_conntrack state;

- /* Poor man's window tracking: sequence number of valid ACK
- handshake completion packet */
+ /* Poor man's window tracking: expected sequence and acknowledge
+ number of valid ACK handshake completion packet */
+ u_int32_t handshake_seq;
u_int32_t handshake_ack;
};

diff -urN -X dontdiff kernel-source-2.4.19.origin/net/ipv4/netfilter/ip_conntrack_proto_tcp.c kernel-source-2.4.19.patch/net/ipv4/netfilter/ip_conntrack_proto_tcp.c
--- kernel-source-2.4.19.origin/net/ipv4/netfilter/ip_conntrack_proto_tcp.c Fri Oct 4 08:13:38 2002
+++ kernel-source-2.4.19.patch/net/ipv4/netfilter/ip_conntrack_proto_tcp.c Sat Oct 5 20:45:49 2002
@@ -180,6 +180,8 @@
if (oldtcpstate == TCP_CONNTRACK_SYN_SENT
&& CTINFO2DIR(ctinfo) == IP_CT_DIR_REPLY
&& tcph->syn && tcph->ack)
+ conntrack->proto.tcp.handshake_seq
+ = tcph->ack_seq;
conntrack->proto.tcp.handshake_ack
= htonl(ntohl(tcph->seq) + 1);
WRITE_UNLOCK(&tcp_lock);
@@ -196,6 +198,7 @@
if (oldtcpstate == TCP_CONNTRACK_SYN_RECV
&& CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL
&& tcph->ack && !tcph->syn
+ && tcph->seq == conntrack->proto.tcp.handshake_seq
&& tcph->ack_seq == conntrack->proto.tcp.handshake_ack)
set_bit(IPS_ASSURED_BIT, &conntrack->status);

--
Thunder's just a noise, boys, lightnin' does the work
-- (Chad Brock/John Hadley/Kelly Garrett)