Patchwork netfilter: conntrack: drop malformed IPv4 packets

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Submitter Jozsef Kadlecsik
Date April 3, 2012, 7:36 p.m.
Message ID <1333481769-16747-2-git-send-email-kadlec@blackhole.kfki.hu>
Download mbox | patch
Permalink /patch/150533/
State Superseded
Headers show

Comments

Jozsef Kadlecsik - April 3, 2012, 7:36 p.m.
It was reported that the Linux kernel sometimes logs:

klogd: [2629147.402413] kernel BUG at net / netfilter /
nf_conntrack_proto_tcp.c: 447!
klogd: [1072212.887368] kernel BUG at net / netfilter /
nf_conntrack_proto_tcp.c: 392

ipv4_get_l4proto() in nf_conntrack_l3proto_ipv4.c and tcp_error() in
nf_conntrack_proto_tcp.c should catch malformed packets, so the errors
at the indicated lines - TCP options parsing - should not happen.
However, tcp_error() relies on the "dataoff" offset to the TCP header,
calculated by ipv4_get_l4proto().  But ipv4_get_l4proto() does not check
bogus ihl values in IPv4 packets, which then can slip through tcp_error()
and get caught at the TCP options parsing routines.

The patch fixes ipv4_get_l4proto() by dropping packets with bogus
ihl value.

The patch closes netfilter bugzilla id 771.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
---
 net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
Eric Dumazet - April 3, 2012, 7:41 p.m.
On Tue, 2012-04-03 at 21:36 +0200, Jozsef Kadlecsik wrote:
> It was reported that the Linux kernel sometimes logs:
> 
> klogd: [2629147.402413] kernel BUG at net / netfilter /
> nf_conntrack_proto_tcp.c: 447!
> klogd: [1072212.887368] kernel BUG at net / netfilter /
> nf_conntrack_proto_tcp.c: 392
> 
> ipv4_get_l4proto() in nf_conntrack_l3proto_ipv4.c and tcp_error() in
> nf_conntrack_proto_tcp.c should catch malformed packets, so the errors
> at the indicated lines - TCP options parsing - should not happen.
> However, tcp_error() relies on the "dataoff" offset to the TCP header,
> calculated by ipv4_get_l4proto().  But ipv4_get_l4proto() does not check
> bogus ihl values in IPv4 packets, which then can slip through tcp_error()
> and get caught at the TCP options parsing routines.
> 
> The patch fixes ipv4_get_l4proto() by dropping packets with bogus
> ihl value.
> 
> The patch closes netfilter bugzilla id 771.
> 
> Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
> ---
>  net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c |    8 ++++++++
>  1 files changed, 8 insertions(+), 0 deletions(-)
> 
> diff --git a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> index de9da21..c93d052 100644
> --- a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> +++ b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> @@ -84,6 +84,14 @@ static int ipv4_get_l4proto(const struct sk_buff *skb, unsigned int nhoff,
>  	*dataoff = nhoff + (iph->ihl << 2);
>  	*protonum = iph->protocol;
>  
> +	/* Check bogus IP headers */
> +	if (*dataoff > skb->len) {
> +		pr_debug("nf_conntrack_ipv4: drop bogus IPv4 packet: "
> +			 "nhoff %u, ihl %u, skblen %u\n",
> +			 nhoff, iph->ihl << 2, skb->len);
> +		return -NF_ACCEPT;

		return -NF_DROP; ?
		
> +	}
> +
>  	return NF_ACCEPT;
>  }
>  


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Jozsef Kadlecsik - April 3, 2012, 7:55 p.m.
On Tue, 3 Apr 2012, Eric Dumazet wrote:

> On Tue, 2012-04-03 at 21:36 +0200, Jozsef Kadlecsik wrote:
> > It was reported that the Linux kernel sometimes logs:
> > 
> > klogd: [2629147.402413] kernel BUG at net / netfilter /
> > nf_conntrack_proto_tcp.c: 447!
> > klogd: [1072212.887368] kernel BUG at net / netfilter /
> > nf_conntrack_proto_tcp.c: 392
> > 
> > ipv4_get_l4proto() in nf_conntrack_l3proto_ipv4.c and tcp_error() in
> > nf_conntrack_proto_tcp.c should catch malformed packets, so the errors
> > at the indicated lines - TCP options parsing - should not happen.
> > However, tcp_error() relies on the "dataoff" offset to the TCP header,
> > calculated by ipv4_get_l4proto().  But ipv4_get_l4proto() does not check
> > bogus ihl values in IPv4 packets, which then can slip through tcp_error()
> > and get caught at the TCP options parsing routines.
> > 
> > The patch fixes ipv4_get_l4proto() by dropping packets with bogus
> > ihl value.
> > 
> > The patch closes netfilter bugzilla id 771.
> > 
> > Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
> > ---
> >  net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c |    8 ++++++++
> >  1 files changed, 8 insertions(+), 0 deletions(-)
> > 
> > diff --git a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > index de9da21..c93d052 100644
> > --- a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > +++ b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > @@ -84,6 +84,14 @@ static int ipv4_get_l4proto(const struct sk_buff *skb, unsigned int nhoff,
> >  	*dataoff = nhoff + (iph->ihl << 2);
> >  	*protonum = iph->protocol;
> >  
> > +	/* Check bogus IP headers */
> > +	if (*dataoff > skb->len) {
> > +		pr_debug("nf_conntrack_ipv4: drop bogus IPv4 packet: "
> > +			 "nhoff %u, ihl %u, skblen %u\n",
> > +			 nhoff, iph->ihl << 2, skb->len);
> > +		return -NF_ACCEPT;
> 
> 		return -NF_DROP; ?

You're right, I'm going to resend it.

Best regards,
Jozsef
-
E-mail  : kadlec@blackhole.kfki.hu, kadlecsik.jozsef@wigner.mta.hu
PGP key : http://www.kfki.hu/~kadlec/pgp_public_key.txt
Address : Wigner Research Centre for Physics, Hungarian Academy of Sciences
          H-1525 Budapest 114, POB. 49, Hungary
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Pablo Neira - April 4, 2012, 11:23 a.m.
On Tue, Apr 03, 2012 at 09:41:19PM +0200, Eric Dumazet wrote:
> On Tue, 2012-04-03 at 21:36 +0200, Jozsef Kadlecsik wrote:
> > It was reported that the Linux kernel sometimes logs:
> > 
> > klogd: [2629147.402413] kernel BUG at net / netfilter /
> > nf_conntrack_proto_tcp.c: 447!
> > klogd: [1072212.887368] kernel BUG at net / netfilter /
> > nf_conntrack_proto_tcp.c: 392
> > 
> > ipv4_get_l4proto() in nf_conntrack_l3proto_ipv4.c and tcp_error() in
> > nf_conntrack_proto_tcp.c should catch malformed packets, so the errors
> > at the indicated lines - TCP options parsing - should not happen.
> > However, tcp_error() relies on the "dataoff" offset to the TCP header,
> > calculated by ipv4_get_l4proto().  But ipv4_get_l4proto() does not check
> > bogus ihl values in IPv4 packets, which then can slip through tcp_error()
> > and get caught at the TCP options parsing routines.
> > 
> > The patch fixes ipv4_get_l4proto() by dropping packets with bogus
> > ihl value.
> > 
> > The patch closes netfilter bugzilla id 771.
> > 
> > Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
> > ---
> >  net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c |    8 ++++++++
> >  1 files changed, 8 insertions(+), 0 deletions(-)
> > 
> > diff --git a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > index de9da21..c93d052 100644
> > --- a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > +++ b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > @@ -84,6 +84,14 @@ static int ipv4_get_l4proto(const struct sk_buff *skb, unsigned int nhoff,
> >  	*dataoff = nhoff + (iph->ihl << 2);
> >  	*protonum = iph->protocol;
> >  
> > +	/* Check bogus IP headers */
> > +	if (*dataoff > skb->len) {
> > +		pr_debug("nf_conntrack_ipv4: drop bogus IPv4 packet: "
> > +			 "nhoff %u, ihl %u, skblen %u\n",
> > +			 nhoff, iph->ihl << 2, skb->len);
> > +		return -NF_ACCEPT;
> 
> 		return -NF_DROP; ?

The connection tracking system should not drop packets. Well, we
sometimes do in special cases.

I think -NF_ACCEPT is fine, the user can add one rule to drop INVALID
packets (like this one) from iptables.
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Jozsef Kadlecsik - April 4, 2012, 12:07 p.m.
On Wed, 4 Apr 2012, Pablo Neira Ayuso wrote:

> On Tue, Apr 03, 2012 at 09:41:19PM +0200, Eric Dumazet wrote:
> > On Tue, 2012-04-03 at 21:36 +0200, Jozsef Kadlecsik wrote:
> > > It was reported that the Linux kernel sometimes logs:
> > > 
> > > klogd: [2629147.402413] kernel BUG at net / netfilter /
> > > nf_conntrack_proto_tcp.c: 447!
> > > klogd: [1072212.887368] kernel BUG at net / netfilter /
> > > nf_conntrack_proto_tcp.c: 392
> > > 
> > > ipv4_get_l4proto() in nf_conntrack_l3proto_ipv4.c and tcp_error() in
> > > nf_conntrack_proto_tcp.c should catch malformed packets, so the errors
> > > at the indicated lines - TCP options parsing - should not happen.
> > > However, tcp_error() relies on the "dataoff" offset to the TCP header,
> > > calculated by ipv4_get_l4proto().  But ipv4_get_l4proto() does not check
> > > bogus ihl values in IPv4 packets, which then can slip through tcp_error()
> > > and get caught at the TCP options parsing routines.
> > > 
> > > The patch fixes ipv4_get_l4proto() by dropping packets with bogus
> > > ihl value.
> > > 
> > > The patch closes netfilter bugzilla id 771.
> > > 
> > > Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
> > > ---
> > >  net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c |    8 ++++++++
> > >  1 files changed, 8 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > > index de9da21..c93d052 100644
> > > --- a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > > +++ b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > > @@ -84,6 +84,14 @@ static int ipv4_get_l4proto(const struct sk_buff *skb, unsigned int nhoff,
> > >  	*dataoff = nhoff + (iph->ihl << 2);
> > >  	*protonum = iph->protocol;
> > >  
> > > +	/* Check bogus IP headers */
> > > +	if (*dataoff > skb->len) {
> > > +		pr_debug("nf_conntrack_ipv4: drop bogus IPv4 packet: "
> > > +			 "nhoff %u, ihl %u, skblen %u\n",
> > > +			 nhoff, iph->ihl << 2, skb->len);
> > > +		return -NF_ACCEPT;
> > 
> > 		return -NF_DROP; ?
> 
> The connection tracking system should not drop packets. Well, we
> sometimes do in special cases.
> 
> I think -NF_ACCEPT is fine, the user can add one rule to drop INVALID
> packets (like this one) from iptables.

That's a nontrivial question and currently we are inconsistent: in 
ipv4_get_l4proto() we drop all broken packets (IP header is incomplete or 
fragments) and the condition above just adds a third category. However, in 
ipv6_get_l4proto() we simply mark them as INVALID and in tcp_error() we do 
the same.

So, should we convert ipv4_get_l4proto() to mark all broken packets as 
INVALID?

Best regards,
Jozsef
-
E-mail  : kadlec@blackhole.kfki.hu, kadlecsik.jozsef@wigner.mta.hu
PGP key : http://www.kfki.hu/~kadlec/pgp_public_key.txt
Address : Wigner Research Centre for Physics, Hungarian Academy of Sciences
          H-1525 Budapest 114, POB. 49, Hungary
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Pablo Neira - April 4, 2012, 3:14 p.m.
On Wed, Apr 04, 2012 at 02:07:45PM +0200, Jozsef Kadlecsik wrote:
> On Wed, 4 Apr 2012, Pablo Neira Ayuso wrote:
> 
> > On Tue, Apr 03, 2012 at 09:41:19PM +0200, Eric Dumazet wrote:
> > > On Tue, 2012-04-03 at 21:36 +0200, Jozsef Kadlecsik wrote:
> > > > It was reported that the Linux kernel sometimes logs:
> > > > 
> > > > klogd: [2629147.402413] kernel BUG at net / netfilter /
> > > > nf_conntrack_proto_tcp.c: 447!
> > > > klogd: [1072212.887368] kernel BUG at net / netfilter /
> > > > nf_conntrack_proto_tcp.c: 392
> > > > 
> > > > ipv4_get_l4proto() in nf_conntrack_l3proto_ipv4.c and tcp_error() in
> > > > nf_conntrack_proto_tcp.c should catch malformed packets, so the errors
> > > > at the indicated lines - TCP options parsing - should not happen.
> > > > However, tcp_error() relies on the "dataoff" offset to the TCP header,
> > > > calculated by ipv4_get_l4proto().  But ipv4_get_l4proto() does not check
> > > > bogus ihl values in IPv4 packets, which then can slip through tcp_error()
> > > > and get caught at the TCP options parsing routines.
> > > > 
> > > > The patch fixes ipv4_get_l4proto() by dropping packets with bogus
> > > > ihl value.
> > > > 
> > > > The patch closes netfilter bugzilla id 771.
> > > > 
> > > > Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
> > > > ---
> > > >  net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c |    8 ++++++++
> > > >  1 files changed, 8 insertions(+), 0 deletions(-)
> > > > 
> > > > diff --git a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > > > index de9da21..c93d052 100644
> > > > --- a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > > > +++ b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
> > > > @@ -84,6 +84,14 @@ static int ipv4_get_l4proto(const struct sk_buff *skb, unsigned int nhoff,
> > > >  	*dataoff = nhoff + (iph->ihl << 2);
> > > >  	*protonum = iph->protocol;
> > > >  
> > > > +	/* Check bogus IP headers */
> > > > +	if (*dataoff > skb->len) {
> > > > +		pr_debug("nf_conntrack_ipv4: drop bogus IPv4 packet: "
> > > > +			 "nhoff %u, ihl %u, skblen %u\n",
> > > > +			 nhoff, iph->ihl << 2, skb->len);
> > > > +		return -NF_ACCEPT;
> > > 
> > > 		return -NF_DROP; ?
> > 
> > The connection tracking system should not drop packets. Well, we
> > sometimes do in special cases.
> > 
> > I think -NF_ACCEPT is fine, the user can add one rule to drop INVALID
> > packets (like this one) from iptables.
> 
> That's a nontrivial question and currently we are inconsistent: in 
> ipv4_get_l4proto() we drop all broken packets (IP header is incomplete or 
> fragments) and the condition above just adds a third category. However, in 
> ipv6_get_l4proto() we simply mark them as INVALID and in tcp_error() we do 
> the same.

Thanks for spotting that.

> So, should we convert ipv4_get_l4proto() to mark all broken packets as 
> INVALID?

Yes, I think we should stick to -NF_ACCEPT so we let the user drop the
packet via iptables.

BTW, if you send a follow-up patch for this, it would be also good to
log invalid packets in ipv4_get_l4proto. There's no nf_log_packet
invocation for these.
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Patch

diff --git a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
index de9da21..c93d052 100644
--- a/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
+++ b/net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c
@@ -84,6 +84,14 @@  static int ipv4_get_l4proto(const struct sk_buff *skb, unsigned int nhoff,
 	*dataoff = nhoff + (iph->ihl << 2);
 	*protonum = iph->protocol;
 
+	/* Check bogus IP headers */
+	if (*dataoff > skb->len) {
+		pr_debug("nf_conntrack_ipv4: drop bogus IPv4 packet: "
+			 "nhoff %u, ihl %u, skblen %u\n",
+			 nhoff, iph->ihl << 2, skb->len);
+		return -NF_ACCEPT;
+	}
+
 	return NF_ACCEPT;
 }