Re: [PATCH] netfilter: conntrack: drop malformed IPv4 packets

[Pablo Neira Ayuso <pablo@netfilter.org>]

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.