Subject: Re: Where exactly will arch_fast_hash be used

From: Hannes Frederic Sowa <[email-masked]>
Date: Thu, 04 Dec 2014 13:34:28 +0100

Hi Herbert,

On Do, 2014-12-04 at 16:11 +0800, Herbert Xu wrote:
> While working on rhashtable it came to me that this whole concept
> of arch_fast_hash is flawed.  CRCs are linear functions so it's
> fairly easy for an attacker to identify collisions or at least
> eliminate a large amount of search space (e.g., controlling the
> last bit of the hash result is almost trivial, even when you add
> a random seed).
> 
> So what exactly are we going to use arch_fast_hash for? Presumably
> it's places where security is never goint to be an issue, right?
> 
> Even if security wasn't an issue, straight CRC32 has really poor
> lower-order bit distribution, which makes it a terrible choice for
> a hash table that simply uses the lower-order bits.

I wondered the same while trying to use arch_fast_hash in a lot more
places (I did a new implementation in assembler I'll send later on, it
is mostly optimized to deal with ovs flow keys).

While the uniformity of crc32 does actually look good and IMHO this even
holds for the lower bits of the hash, I totally agree on the linearity
matters.

The easiest way to make arch_fast_hash non-linear would be to build up
on the crc32 instruction like e.g. the cityhash function family does and
it seems not too hard to do that by combining two crc32c outputs of the
original and cyclic shifted input data. I have doubts if this is faster
than jhash in the end. There are proposals from Intel to do so, but they
are patent encumbered. :/

For most consumers in the networking stack, security and DoS resistence
is an issue. OVS, for which this was designed at first does do rehashing
from time to time, but still there is a possible DoS attack vector with
this hashing algorithm.

Bye,
Hannes