Re: ipv4: Simplify ARP hash function.

[David Miller <davem@davemloft.net>]

From: Roland Dreier <roland@purestorage.com>
Date: Fri, 8 Jul 2011 12:26:17 -0700

> On Fri, Jul 8, 2011 at 11:06 AM, David Miller <davem@davemloft.net> wrote:
>> Ok, there was also an unintended bug in my original patch,
>> I lost the bottom 8 bits in the fold, the hash function
>> should instead be:
>>
>> +static inline u32 arp_hashfn(u32 key, const struct net_device *dev, u32 hash_rnd)
>> +{
>> +       u32 val = key ^ dev->ifindex ^ hash_rnd;
>> +
>> +       return val ^ (val >> 8) ^ (val >> 16) ^ (val >> 24);
>> +}
> 
> Doesn't seem to matter much -- this is now equivalent to
> 
>       a = key ^ dev->ifindex
>        return (a ^ (a >> 8) ^ (a >> 16) ^ (a >> 24))           // (1)
>             ^ (rnd ^ (rnd >> 8) ^ (rnd >> 16) ^ (rnd >> 24))   // (2)
> 
> where again the attacker controls (1), and (2) is a constant.

Right, but how can you attack it?  Show me how you can grow
a hash chain of arbitrary length by modulating the key in
a deterministic way.

Nobody has done this yet.