Mailing List Archive

Netfilter Core Team Security Advisory

CVE: CAN-2003-0187

Subject:

Netfilter / Connection Tracking Remote DoS

Released:

01 Aug 2003

Effects:

Any remote user may be able to DoS a machine with netfilter connection
tracking when running a specific version of the Linux kernel.

Estimated Severity:
High.

Systems Affected:

Linux 2.4.20 kernels (kernels <= 2.4.19 and >= 2.4.21 NOT affected)
CONFIG_IP_NF_CONNTRACK enabled, or the ip_conntrack module loaded.

Solution:

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

OR: Do not use connection tracking on 2.4.20 based systems.

Details:

The 2.4.20 kernel introduced a change in the behaviour of the generic
linked list support. The connection tracking core relies on the old
behaviour to identify 'UNCONFIRMED' connections.

'UNCONFIRMED' means we've seen traffic only in one direction, but not
in the other. Since connection tracking was unable to identify such
connections correctly anymore, they've been assigned a very high
timeout.

The patch below changes the connection tracking core to no longer rely
on any specific behaviour of the linux linked listed API.

Vendor Statement:

Red Hat: Patches for this issue were first introduced in RHSA-2003:17
Others: unknown

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

Contact:
coreteam@netfilter.org

diff -urN --exclude-from=diff.exclude linux-2.4.20-base/include/linux/netfilter_ipv4/ip_conntrack.h linux-2.4.20-del/include/linux/netfilter_ipv4/ip_conntrack.h
--- linux-2.4.20-base/include/linux/netfilter_ipv4/ip_conntrack.h Fri Nov 29 00:53:15 2002
+++ linux-2.4.20-del/include/linux/netfilter_ipv4/ip_conntrack.h Fri Feb 21 17:01:38 2003
@@ -6,6 +6,7 @@

#include <linux/config.h>
#include <linux/netfilter_ipv4/ip_conntrack_tuple.h>
+#include <linux/bitops.h>
#include <asm/atomic.h>

enum ip_conntrack_info
@@ -41,6 +42,10 @@
/* Conntrack should never be early-expired. */
IPS_ASSURED_BIT = 2,
IPS_ASSURED = (1 << IPS_ASSURED_BIT),
+
+ /* Connection is confirmed: originating packet has left box */
+ IPS_CONFIRMED_BIT = 3,
+ IPS_CONFIRMED = (1 << IPS_CONFIRMED_BIT),
};

#include <linux/netfilter_ipv4/ip_conntrack_tcp.h>
@@ -159,7 +164,7 @@
struct ip_conntrack_tuple_hash tuplehash[IP_CT_DIR_MAX];

/* Have we seen traffic both ways yet? (bitset) */
- volatile unsigned long status;
+ unsigned long status;

/* Timer function; drops refcnt when it goes off. */
struct timer_list timeout;
@@ -254,7 +259,7 @@
/* It's confirmed if it is, or has been in the hash table. */
static inline int is_confirmed(struct ip_conntrack *ct)
{
- return ct->tuplehash[IP_CT_DIR_ORIGINAL].list.next != NULL;
+ return test_bit(IPS_CONFIRMED_BIT, &ct->status);
}

extern unsigned int ip_conntrack_htable_size;
diff -urN --exclude-from=diff.exclude linux-2.4.20-base/net/ipv4/netfilter/ip_conntrack_core.c linux-2.4.20-del/net/ipv4/netfilter/ip_conntrack_core.c
--- linux-2.4.20-base/net/ipv4/netfilter/ip_conntrack_core.c Tue Feb 18 17:08:21 2003
+++ linux-2.4.20-del/net/ipv4/netfilter/ip_conntrack_core.c Fri Feb 21 17:01:39 2003
@@ -292,9 +292,6 @@
{
DEBUGP("clean_from_lists(%p)\n", ct);
MUST_BE_WRITE_LOCKED(&ip_conntrack_lock);
- /* Remove from both hash lists: must not NULL out next ptrs,
- otherwise we'll look unconfirmed. Fortunately, LIST_DELETE
- doesn't do this. --RR */
LIST_DELETE(&ip_conntrack_hash
[hash_conntrack(&ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple)],
&ct->tuplehash[IP_CT_DIR_ORIGINAL]);
@@ -467,6 +464,7 @@
ct->timeout.expires += jiffies;
add_timer(&ct->timeout);
atomic_inc(&ct->ct_general.use);
+ set_bit(IPS_CONFIRMED_BIT, &ct->status);
WRITE_UNLOCK(&ip_conntrack_lock);
return NF_ACCEPT;
}
@@ -585,7 +583,7 @@
connection. Too bad: we're in trouble anyway. */
static inline int unreplied(const struct ip_conntrack_tuple_hash *i)
{
- return !(i->ctrack->status & IPS_ASSURED);
+ return !(test_bit(IPS_ASSURED_BIT, &i->ctrack->status));
}

static int early_drop(struct list_head *chain)
@@ -720,7 +718,7 @@
conntrack, expected);
/* Welcome, Mr. Bond. We've been expecting you... */
IP_NF_ASSERT(master_ct(conntrack));
- conntrack->status = IPS_EXPECTED;
+ __set_bit(IPS_EXPECTED_BIT, &conntrack->status);
conntrack->master = expected;
expected->sibling = conntrack;
LIST_DELETE(&ip_conntrack_expect_list, expected);
@@ -768,11 +766,11 @@
*set_reply = 1;
} else {
/* Once we've had two way comms, always ESTABLISHED. */
- if (h->ctrack->status & IPS_SEEN_REPLY) {
+ if (test_bit(IPS_SEEN_REPLY_BIT, &h->ctrack->status)) {
DEBUGP("ip_conntrack_in: normal packet for %p\n",
h->ctrack);
*ctinfo = IP_CT_ESTABLISHED;
- } else if (h->ctrack->status & IPS_EXPECTED) {
+ } else if (test_bit(IPS_EXPECTED_BIT, &h->ctrack->status)) {
DEBUGP("ip_conntrack_in: related packet for %p\n",
h->ctrack);
*ctinfo = IP_CT_RELATED;
diff -urN --exclude-from=diff.exclude linux-2.4.20-base/net/ipv4/netfilter/ip_conntrack_proto_tcp.c linux-2.4.20-del/net/ipv4/netfilter/ip_conntrack_proto_tcp.c
--- linux-2.4.20-base/net/ipv4/netfilter/ip_conntrack_proto_tcp.c Tue Feb 18 17:07:26 2003
+++ linux-2.4.20-del/net/ipv4/netfilter/ip_conntrack_proto_tcp.c Fri Feb 21 17:03:35 2003
@@ -192,7 +192,7 @@
have an established connection: this is a fairly common
problem case, so we can delete the conntrack
immediately. --RR */
- if (!(conntrack->status & IPS_SEEN_REPLY) && tcph->rst) {
+ if (!test_bit(IPS_SEEN_REPLY_BIT, &conntrack->status) && tcph->rst) {
WRITE_UNLOCK(&tcp_lock);
if (del_timer(&conntrack->timeout))
conntrack->timeout.function((unsigned long)conntrack);
diff -urN --exclude-from=diff.exclude linux-2.4.20-base/net/ipv4/netfilter/ip_conntrack_proto_udp.c linux-2.4.20-del/net/ipv4/netfilter/ip_conntrack_proto_udp.c
--- linux-2.4.20-base/net/ipv4/netfilter/ip_conntrack_proto_udp.c Fri Nov 29 00:53:15 2002
+++ linux-2.4.20-del/net/ipv4/netfilter/ip_conntrack_proto_udp.c Fri Feb 21 17:01:39 2003
@@ -51,7 +51,7 @@
{
/* If we've seen traffic both ways, this is some kind of UDP
stream. Extend timeout. */
- if (conntrack->status & IPS_SEEN_REPLY) {
+ if (test_bit(IPS_SEEN_REPLY_BIT, &conntrack->status)) {
ip_ct_refresh(conntrack, UDP_STREAM_TIMEOUT);
/* Also, more likely to be important, and not a probe */
set_bit(IPS_ASSURED_BIT, &conntrack->status);
diff -urN --exclude-from=diff.exclude linux-2.4.20-base/net/ipv4/netfilter/ip_conntrack_standalone.c linux-2.4.20-del/net/ipv4/netfilter/ip_conntrack_standalone.c
--- linux-2.4.20-base/net/ipv4/netfilter/ip_conntrack_standalone.c Fri Nov 29 00:53:15 2002
+++ linux-2.4.20-del/net/ipv4/netfilter/ip_conntrack_standalone.c Fri Feb 21 21:10:37 2003
@@ -77,7 +77,7 @@
}

static unsigned int
-print_conntrack(char *buffer, const struct ip_conntrack *conntrack)
+print_conntrack(char *buffer, struct ip_conntrack *conntrack)
{
unsigned int len;
struct ip_conntrack_protocol *proto
@@ -95,12 +95,12 @@
len += print_tuple(buffer + len,
&conntrack->tuplehash[IP_CT_DIR_ORIGINAL].tuple,
proto);
- if (!(conntrack->status & IPS_SEEN_REPLY))
+ if (!(test_bit(IPS_SEEN_REPLY_BIT, &conntrack->status)))
len += sprintf(buffer + len, "[UNREPLIED] ");
len += print_tuple(buffer + len,
&conntrack->tuplehash[IP_CT_DIR_REPLY].tuple,
proto);
- if (conntrack->status & IPS_ASSURED)
+ if (test_bit(IPS_ASSURED_BIT, &conntrack->status))
len += sprintf(buffer + len, "[ASSURED] ");
len += sprintf(buffer + len, "use=%u ",
atomic_read(&conntrack->ct_general.use));

--
- 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