netdev - Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF

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Date:   Fri, 16 Dec 2016 15:43:58 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     George Spelvin <linux@...encehorizons.net>
Cc:     Jason@...c4.com, ak@...ux.intel.com, davem@...emloft.net,
        David.Laight@...lab.com, djb@...yp.to, ebiggers3@...il.com,
        hannes@...essinduktion.org, jeanphilippe.aumasson@...il.com,
        kernel-hardening@...ts.openwall.com, linux-crypto@...r.kernel.org,
        linux-kernel@...r.kernel.org, luto@...capital.net,
        netdev@...r.kernel.org, tom@...bertland.com,
        torvalds@...ux-foundation.org, vegard.nossum@...il.com
Subject: Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF

On Fri, Dec 16, 2016 at 03:17:39PM -0500, George Spelvin wrote:
> > That's a nice analysis. Might one conclude from that that hsiphash is
> > not useful for our purposes? Or does it still remain useful for
> > network facing code?
> 
> I think for attacks where the threat is a DoS, it's usable.  The point
> is you only have to raise the cost to equal that of a packet flood.
> (Just like in electronic warfare, the best you can possibly do is force
> the enemy to use broadband jamming.)
> 
> Hash collision attacks just aren't that powerful.  The original PoC
> was against an application that implemented a hard limit on hash chain
> length as a DoS defense, which the attack then exploited to turn it into
> a hard DoS.

What should we do with get_random_int() and get_random_long()?  In
some cases it's being used in performance sensitive areas, and where
anti-DoS protection might be enough.  In others, maybe not so much.

If we rekeyed the secret used by get_random_int() and
get_random_long() frequently (say, every minute or every 5 minutes),
would that be sufficient for current and future users of these
interfaces?

						- Ted

P.S.  I'll note that my performance figures when testing changes to
get_random_int() were done on a 32-bit x86; Jason, I'm guessing your
figures were using a 64-bit x86 system?.  I haven't tried 32-bit ARM
or smaller CPU's (e.g., mips, et. al.) that might be more likely to be
used on IoT devices, but I'm worried about those too, of course.