netdev - Re: ipv6 fragmentation-related panic in netfilter

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Date:	Tue, 19 Nov 2013 12:11:24 +0100
From:	Wolfgang Walter <linux@...m.de>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Steffen Klassert <steffen.klassert@...unet.com>,
	Tomas Hlavacek <tmshlvck@...il.com>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: ipv6 fragmentation-related panic in netfilter

Am Freitag, 1. November 2013, 09:25:37 schrieb Patrick McHardy:
> On Fri, Nov 01, 2013 at 09:45:29AM +0100, Steffen Klassert wrote:
> > On Wed, Oct 30, 2013 at 12:07:11AM +0000, Patrick McHardy wrote:
> > > The problem is that the reassembled packet is referenced by the
> > > individual
> > > fragments, so we trigger the BUG_ON in pskb_expand_head(). In this
> > > particular case the case we BUG() on is actually OK, but I'm looking at
> > > a way we can fix this without special casing. Hope to have a patch for
> > > testing in the next hours.
> > 
> > Just for the record. I'm observing similar, quite reproducable crashes
> > when
> > receiving fragmented icmp echo request packets on an IPsec gateway with
> > nf_conntrack_ipv6.
> > 
> > Since git commit 58a317f10 ("netfilter: ipv6: add IPv6 NAT support")
> > netfilter might insert a reassembled ipv6 packet with a shared skb and
> > local_df = 1 to the ok function. In case of xfrm, __xfrm6_output()
> > fragments the packet again and when adjusting the headroom later, we
> > crash because of a shared skb.
> > 
> > I can fix it by checking for a shared skb in ip6_fragment() and do
> > slow path fragmentation then. But we never needed such a check in
> > ip6_fragment(), so it's maybe better to fix it in netfilter.
> 
> So what seems to be happening is that this case in __ipv6_conntrack_in()
> triggers:
> 
>         /* Conntrack helpers need the entire reassembled packet in the
>          * POST_ROUTING hook. In case of unconfirmed connections NAT
>          * might reassign a helper, so the entire packet is also
>          * required.
>          */
>         ct = nf_ct_get(reasm, &ctinfo);
>         if (ct != NULL && !nf_ct_is_untracked(ct)) {
>                 help = nfct_help(ct);
>                 if ((help && help->helper) || !nf_ct_is_confirmed(ct)) {
>                         nf_conntrack_get_reasm(reasm);
>                         NF_HOOK_THRESH(NFPROTO_IPV6, hooknum, reasm,
>                                        (struct net_device *)in,
>                                        (struct net_device *)out,
>                                        okfn, NF_IP6_PRI_CONNTRACK + 1);
> 
> Since this code is called while walking through the fragment chain, we have
> extra references to the reassembled skb. So I think what we need to do is
> to release the fragment chain before calling NF_HOOK_THRESH() and indicate
> this to nf_ct_frag6_output() so it will stop processing the chain
> immediately.
> 
> I'll give it a try, will let you know when I have a patch for testing.
> --
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Are there patches available? I can crash my 3.12 kernel easily doing

	fping -p 20 -l -b 4000 bla

3.11.x does not expose this problem.

Regards,
-- 
Wolfgang Walter
Studentenwerk München
Anstalt des öffentlichen Rechts
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