netdev - Re: [PATCH RFC] ipv6: use stronger hash for reassembly queue hash table

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Date:	Thu, 14 Mar 2013 08:23:41 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH RFC] ipv6: use stronger hash for reassembly queue hash table

On Thu, Mar 14, 2013 at 08:10:40AM +0100, Jesper Dangaard Brouer wrote:
> On Thu, 2013-03-14 at 02:37 +0100, Hannes Frederic Sowa wrote:
> 
> > [PATCH net] inet: limit length of fragment queue hash table bucket lists
> > 
> > This patch introduces a constant limit of the fragment queue hash
> > table bucket list lengths. Currently the limit 128 is choosen somewhat
> > arbitrary and just ensures that we can fill up the fragment cache with
> > empty packets up to the default ip_frag_high_thresh limits. It should
> > just protect from list iteration eating considerable amounts of cpu.
> > 
> > If we reach the maximum length in one hash bucket a warning is printed.
> > This is implemented on the caller side of inet_frag_find to distinguish
> > between the different users of inet_fragment.c.
> 
> I like the idea of having a safe guard on the fragment queue hash table
> bucket list lengths.  But I'm considering another cleanup/evictor
> strategy, where we drop the LRU list, and do frag eviction on a hash
> bucket level (which will be more cache optimal).  This strategy would
> also involve a list length limit.

I would try to get a simple guard into v3.9. In 3.9 the hashing of the key
of ipv6 fragments changed in such a way that an attacker could generate
fragments which would end up in just one hash chain, thus eating a lot
of cpu time because of list traversal. Later on, when you post your
patches we could simply revert/update this safeguard to your version.

Thanks,

  Hannes

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