[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-Id: <20110708.122742.1006323245708104141.davem@davemloft.net>
Date: Fri, 08 Jul 2011 12:27:42 -0700 (PDT)
From: David Miller <davem@...emloft.net>
To: roland@...estorage.com
Cc: johnwheffner@...il.com, mj@....cz, netdev@...r.kernel.org
Subject: Re: ipv4: Simplify ARP hash function.
From: Roland Dreier <roland@...estorage.com>
Date: Fri, 8 Jul 2011 12:26:17 -0700
> On Fri, Jul 8, 2011 at 11:06 AM, David Miller <davem@...emloft.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.
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Powered by blists - more mailing lists