netdev - Re: [PATCH net] ipv6: Prevent ipv6_find_hdr() from returning ENOENT for valid non-first fragments

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Date:	Mon, 12 Jan 2015 16:38:16 +0530
From:	Rahul Sharma <rsharma@...sta.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Pablo Neira Ayuso <pablo@...filter.org>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH net] ipv6: Prevent ipv6_find_hdr() from returning ENOENT
 for valid non-first fragments

Hi Pablo, Hannes

On Fri, Jan 9, 2015 at 9:20 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Fr, 2015-01-09 at 12:45 +0100, Pablo Neira Ayuso wrote:
>> Hi Hannes,
>>
>> On Fri, Jan 09, 2015 at 12:34:15PM +0100, Hannes Frederic Sowa wrote:
>> > On Fri, Jan 9, 2015, at 08:18, Rahul Sharma wrote:
>> > > Hi Pablo,
>> > >
>> > > On Fri, Jan 9, 2015 at 5:35 AM, Pablo Neira Ayuso <pablo@...filter.org>
>> > > wrote:
>> > > > On Thu, Jan 08, 2015 at 11:39:16PM +0100, Hannes Frederic Sowa wrote:
>> > > >> Hi Pablo,
>> > > >>
>> > > >> On Thu, Jan 8, 2015, at 21:53, Pablo Neira Ayuso wrote:
>> > > >> > I'm afraid we cannot just get rid of that !ipv6_ext_hdr() check. The
>> > > >> > ipv6_find_hdr() function is designed to return the transport protocol.
>> > > >> > After the proposed change, it will return extension header numbers.
>> > > >> > This will break existing ip6tables rulesets since the `-p' option
>> > > >> > relies on this function to match the transport protocol.
>> > > >> >
>> > > >> > Note that the AH header is skipped (see code a bit below this
>> > > >> > problematic fragmentation handling) so the follow up header after the
>> > > >> > AH header is returned as the transport header.
>> > > >> >
>> > > >> > We can probably return the AH protocol number for non-1st fragments.
>> > > >> > However, that would be something new to ip6tables since nobody has
>> > > >> > ever seen packet matching `-p ah' rules. Thus, we restore control to
>> > > >> > the user to allow this, but we would accept all kind of fragmented AH
>> > > >> > traffic through the firewall since we cannot know what transport
>> > > >> > protocol contains from non-1st fragments (unless I'm missing anything,
>> > > >> > I need to have a closer look at this again tomorrow with fresher
>> > > >> > mind).
>> > > >>
>> > > >> The code in question is guarded by (_frag_off != 0), so we are
>> > > >> definitely processing a non-1st fragment currently. The -p match would
>> > > >> happen at the time when the packet is reassembled and thus ipv6_find_hdr
>> > > >> will find the real transport (final) header at this point (I hope I
>> > > >> followed the code correctly here).
>> > > >
>> > > > Then, Rahul should get things working by modprobing nf_defrag_ipv6.
>> > >
>> > > I already had nf_defrag_ipv6 installed when the issue occured. But I
>> > > see ip6table_raw_hook returning NF_DROP for the second fragment.
>> >
>> > That's what I expected. I think the change only affects hooks before
>> > reassembly.
>>
>> reassembly happens at NF_IP6_PRI_CONNTRACK_DEFRAG (-400), so that
>> happens before NF_IP6_PRI_RAW (-300) in IPv6 which is where the raw
>> table is placed.
>
> I tried to reproduce it, but couldn't get non-1st fragments getting
> dropped during traversal of the raw table. They get dropped earlier at
> during reassembly or pass.
>
> I agree with Pablo, I also would like to see more data.
>
> Thanks,
> Hannes
>
>

I enabled pr_debug() and there was no error in nf_ct_frag6_gather().
It seems to have defragmented the packet correctly. As expected,
ipv6_defrag() returns NF_STOLEN for the first packet after queuing it.
For the next fragment, ipv6_defrag() calls nf_ct_frag6_output() after
after reassembling it. I checked skb->len inside nf_ct_frag6_output()
and it implies that they were indeed reassembled. Now from what I
follow from the code, this function seems to iterate over both the
fragments with corresponding skb's and incrementing
thresh(NF_IP6_PRI_CONNTRACK_DEFRAG) by 1 so the subsequent hooks
(ip6table_raw_hook, ipv6_conntrack_in, ip6table_mangle_hook) should
get called in order for both the fragments. This works fine for the
first fragment as I had mentioned, but the ip6table_raw_hook returns
NF_DROP for the second fragment. Please let me know if you need more
inputs.

Thanks,
Rahul
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