Mailing List Archive

Netfilter Core Team Security Advisory

CVE: CAN-2003-0467

Subject:

Netfilter / NAT Remote DoS

Released:

01 Aug 2003

Effects:

Under limited circumstances, a remote user may be able to crash a
machine doing Network Address Translation (NAT).

Estimated Severity:

Medium.

Systems Affected:

Linux 2.4.20 kernels and recent 2.5 kernels with
CONFIG_IP_NF_NAT_FTP or CONFIG_IP_NF_NAT_IRC enabled, or the
ip_nat_ftp or ip_nat_irc modules loaded, on which ftp and irc users
are not packet filtered out.

Solution:

BEST: Upgrade to Linux kernels 2.4.21 (stable), or apply the patch below.

OR: As a workaround, the modules can be removed, or iptables can
be used to block untrusted users from initiating ftp or irc
connections through the NAT machine.

Details:

This was verified by Rusty Russell on 2.4.20, and verified fixed
with this patch.

Vendor Statement:

Red Hat: All of the 2.4.20-based kernels shipped by Red Hat already
contain the patch and are not vulnerable to this issue.
Others: unknown

Credits:
The problem was found, and the fix implemented by the Netfilter Core Team.

Contact:
coreteam@netfilter.org

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.4.21-pre7/net/ipv4/netfilter/ip_nat_helper.c working-2.4.21-pre7-sackadjust/net/ipv4/netfilter/ip_nat_helper.c
--- linux-2.4.21-pre7/net/ipv4/netfilter/ip_nat_helper.c 2003-04-06 15:26:48.000000000 +1000
+++ working-2.4.21-pre7-sackadjust/net/ipv4/netfilter/ip_nat_helper.c 2003-04-14 23:18:38.000000000 +1000
@@ -366,54 +365,49 @@ sack_adjust(struct tcphdr *tcph,
}


-/* TCP SACK sequence number adjustment, return 0 if sack found and adjusted */
-static inline int
+/* TCP SACK sequence number adjustment. */
+static inline void
ip_nat_sack_adjust(struct sk_buff *skb,
- struct ip_conntrack *ct,
- enum ip_conntrack_info ctinfo)
+ struct ip_conntrack *ct,
+ enum ip_conntrack_info ctinfo)
{
- struct iphdr *iph;
struct tcphdr *tcph;
- unsigned char *ptr;
- int length, dir, sack_adjusted = 0;
+ unsigned char *ptr, *optend;
+ unsigned int dir;

- iph = skb->nh.iph;
- tcph = (void *)iph + iph->ihl*4;
- length = (tcph->doff*4)-sizeof(struct tcphdr);
+ tcph = (void *)skb->nh.iph + skb->nh.iph->ihl*4;
+ optend = (unsigned char *)tcph + tcph->doff*4;
ptr = (unsigned char *)(tcph+1);

dir = CTINFO2DIR(ctinfo);

- while (length > 0) {
- int opcode = *ptr++;
+ while (ptr < optend) {
+ int opcode = ptr[0];
int opsize;

switch (opcode) {
case TCPOPT_EOL:
- return !sack_adjusted;
+ return;
case TCPOPT_NOP:
- length--;
+ ptr++;
continue;
default:
- opsize = *ptr++;
- if (opsize > length) /* no partial opts */
- return !sack_adjusted;
+ opsize = ptr[1];
+ /* no partial opts */
+ if (ptr + opsize > optend || opsize < 2)
+ return;
if (opcode == TCPOPT_SACK) {
/* found SACK */
if((opsize >= (TCPOLEN_SACK_BASE
+TCPOLEN_SACK_PERBLOCK)) &&
!((opsize - TCPOLEN_SACK_BASE)
% TCPOLEN_SACK_PERBLOCK))
- sack_adjust(tcph, ptr-2,
+ sack_adjust(tcph, ptr,
&ct->nat.info.seq[!dir]);
-
- sack_adjusted = 1;
}
- ptr += opsize-2;
- length -= opsize;
+ ptr += opsize;
}
}
- return !sack_adjusted;
}

/* TCP sequence number adjustment */

--
- Harald Welte <laforge@netfilter.org> http://www.netfilter.org/
============================================================================
"Fragmentation is like classful addressing -- an interesting early
architectural error that shows how much experimentation was going
on while IP was being designed." -- Paul Vixie