[Bug 495] New: Netfilter Connection Tracking Race Condition in Kernel 2.4.x

[bugzilla-daemon at bugzilla.netfilter.org]

https://bugzilla.netfilter.org/bugzilla/show_bug.cgi?id=495

           Summary: Netfilter Connection Tracking Race Condition in Kernel
                    2.4.x
           Product: netfilter/iptables
           Version: linux-2.4.x
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ip_conntrack
        AssignedTo: laforge at netfilter.org
        ReportedBy: Bob.Halley at nominum.com


Background

Our application uses ip_queue in prerouting to divert DNS UDP packets
to a userland daemon which inspects them and then issues a NF_ACCEPT
or NF_DROP verdict back to the kernel.

We found that if several packets with the same conntrack tuple,
i.e. the same src addr, src port, dst addr, and dst port, arrive very
close together, then only the first one accepted by our software
actually makes it back out to the wire; the others are silently
dropped.


Analysis

We instrumented the kernel to find out where the drop was occurring.
The code doing the dropping was ip_refrag() in
net/ipv4/netfilter/ip_conntrack_standalone.c, specifically:

        /* We've seen it coming out the other side: confirm */
        if (ip_confirm(hooknum, pskb, in, out, okfn) != NF_ACCEPT)
                return NF_DROP;

The dropping is caused by a race between the first packet of a given
tuple making it to confirmed state, and the arrival of another packet
with the same tuple.  If a second packet arrives before the first is
confirmed, it is assigned a new connection tracking context instead of
joining that of the first unconfirmed packet.  When the second packet
is finally handled by ip_refrag(), the call to ip_confim() finds that
there is already a confirmed entry in the table, and returns NF_DROP.
>From the comments in __ip_contrack_confirm(), we infer that this is to
deal with duplicated datagrams and some REJECT case, but it's the
wrong thing in this case because the subsequent packets are neither
duplicates nor REJECTs.

We were using RHEL 3 kernel 2.4.21-40 initially.  We looked at later
2.4.x kernels and found some promising looking changes, namely the
addition of an unconfirmed list, in more recent 2.4.x kernels.  We built
a 2.4.32 kernel and tested it, but the problem remained.  We looked into
the nature of the unconfirmed list and discovered that it was solving a
different problem, but could be a useful starting point for a fix.


Fix

We decided to eliminate the race by having subsequent packets with the
same conntrack tuple join the conntrack context of the first packet
instead of creating a new conntrack context for each of them.  Here's
the patch:

--- linux-2.4.32/net/ipv4/netfilter/ip_conntrack_core.c.orig	2005-04-03
18:42:20.000000000 -0700
+++ linux-2.4.32/net/ipv4/netfilter/ip_conntrack_core.c	2006-07-24
13:23:25.000000000 -0700
@@ -777,6 +777,14 @@
 	/* look for tuple match */
 	h = ip_conntrack_find_get(&tuple, NULL);
 	if (!h) {
+		READ_LOCK(&ip_conntrack_lock);
+		h = LIST_FIND(&unconfirmed, conntrack_tuple_cmp,
+			      struct ip_conntrack_tuple_hash *, &tuple, NULL);
+		if (h)
+			atomic_inc(&h->ctrack->ct_general.use);
+		READ_UNLOCK(&ip_conntrack_lock);
+	}
+	if (!h) {
 		h = init_conntrack(&tuple, proto, skb);
 		if (!h)
 			return NULL;

This patch reliably ends the race, and we no longer have mysteriously
disappearing packets.  Not being netfilter experts, we're not certain
that this patch has no other side effects, and would appreciate any
advice or alternative fixes that people who know more than we do have
to offer.

Regards,

Bob Halley <Bob.Halley at nominum.com>
Brian Wellington <Brian.Wellington at nominum.com>

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