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

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Date:	Thu, 14 Mar 2013 10:18:10 +0100
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
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, 2013-03-14 at 08:28 +0100, Hannes Frederic Sowa wrote:
> On Thu, Mar 14, 2013 at 08:23:41AM +0100, Hannes Frederic Sowa wrote:
> > 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.
> 
> I just wanted to mention that if you plan to target v3.9 with some patches we
> could simply drop this patch.

I will start working on this as soon as Netfilter Workshop is over and I
have recovered from organizing this event.  DaveM told me I needed to
finish my frag patches first, before I could implement all the other
cool ideas we have come up with during the workshop ;-)

But I don't know if my frag changes can make v3.9, maybe v3.10 is more
realistic?  In which case we should use your patch to close this problem
on v3.9 IMHO.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer


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