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

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Message-ID: <CAFB3abwrjp61LLOk3RWwXYEq1g4QZUC8OTKuO2cAr+GbV3n4rw@mail.gmail.com>
Date:	Fri, 9 Jan 2015 12:48:24 +0530
From:	Rahul Sharma <rsharma@...sta.com>
To:	Pablo Neira Ayuso <pablo@...filter.org>
Cc:	Hannes Frederic Sowa <hannes@...essinduktion.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,

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.

Thanks,
Rahul
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