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

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Date:	Fri, 8 Mar 2013 21:53:19 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH RFC] ipv6: use stronger hash for reassembly queue hash table

On Fri, Mar 08, 2013 at 07:23:39AM -0800, Eric Dumazet wrote:
> On Fri, 2013-03-08 at 16:08 +0100, Hannes Frederic Sowa wrote:
> > On Fri, Mar 08, 2013 at 06:53:06AM -0800, Eric Dumazet wrote:
> > > No matter how you hash, a hacker can easily fill your defrag unit with
> > > not complete datagrams, so what's the point ?
> > 
> > I want to harden reassembly logic against all fragments being put in
> > the same hash bucket because of malicious traffic and thus creating
> > long list traversals in the fragment queue hash table.
> 
> Note that the long traversal was a real issue with TCP (thats why I
> introduced ipv6_addr_jhash()), as a single ehash slot could contains
> thousand of sockets.
> 
> But with fragments, we should just limit the depth of any particular
> slot, and drop above a particular threshold.
> 
> reassembly is a best effort mechanism, better make sure it doesnt use
> all our cpu cycles.

On my VM I counted 17500 iterations in one hash bucket and maxing out
one CPU until I got rcu stalls and NMIs. In comparison: If I use the
old hasing code I have max iterations of 370 and don't expirience any
rcu stalls or NMIs (seems to be around 17500/64+-epsilon).

I have not yet drawn my conclusion on this, yet, but I agree some list
length limiting code would be useful independent of the hash function.

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