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@xxxxxxxxxxxxxxxxx>
> > > ---
> > > 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@xxxxxxxxxxxxxxxxx, kadlecsik.jozsef@xxxxxxxxxxxxx
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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