netdev - Re: ipv4: Simplify ARP hash function.

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Date:	Fri, 8 Jul 2011 19:40:55 +0200
From:	Martin Mares <mj@....cz>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: ipv4: Simplify ARP hash function.

Hello!

> Using Jenkins is over the top.
> 
> If the premise is that the hash_rnd is a random unpredictable key,
> then:
> 
> 	key ^ dev->ifindex ^ hash_rnd
> 
> results in an unpredictable hash result, even if an attacker
> controls 'key' and 'dev->ifindex' completely.
> 
> Therefore, if this hash result is unpredictable, then the
> final fold phase of:
> 
> 	(val >> 8) ^ (val >> 16) ^ (val >> 24)
> 
> is unpredictable as well.

If I understand the new hash function correctly, it should be very easy
for an outside attacker to force arbitrary collisions.

The hash function is linear, so it can be reduced to:

	a = key ^ dev->ifindex
	return (a >> 8) ^ (a >> 16) ^ (a >> 24)				// (1)
	     ^ (hash_rnd >> 8) ^ (hash_rnd >> 16) ^ (hash_rnd >> 24)	// (2)

Where (1) is under control of the attacker and while (2) is not, the
only effect of (2) is a random permutation on the hash buckets.

I.e., the attacker can generate arbitrarily long collision chains,
although he cannot pick the bucket where the collisions happen :)

Am I right?

				Have a nice fortnight
-- 
Martin `MJ' Mares                          <mj@....cz>   http://mj.ucw.cz/
Faculty of Math and Physics, Charles University, Prague, Czech Rep., Earth
If going to a church makes you a Christian, does going to a garage make you a car?
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