linux-kernel - Re: [PATCH] random: add blocking facility to urandom

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Date:	Wed, 07 Sep 2011 21:12:59 +0300
From:	Sasha Levin <levinsasha928@...il.com>
To:	Jarod Wilson <jarod@...hat.com>
Cc:	linux-crypto@...r.kernel.org, Matt Mackall <mpm@...enic.com>,
	Neil Horman <nhorman@...hat.com>,
	Herbert Xu <herbert.xu@...hat.com>,
	Steve Grubb <sgrubb@...hat.com>,
	Stephan Mueller <stephan.mueller@...ec.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] random: add blocking facility to urandom

On Wed, 2011-09-07 at 13:38 -0400, Jarod Wilson wrote:
> Certain security-related certifications and their respective review
> bodies have said that they find use of /dev/urandom for certain
> functions, such as setting up ssh connections, is acceptable, but if and
> only if /dev/urandom can block after a certain threshold of bytes have
> been read from it with the entropy pool exhausted. Initially, we were
> investigating increasing entropy pool contributions, so that we could
> simply use /dev/random, but since that hasn't (yet) panned out, and
> upwards of five minutes to establsh an ssh connection using an
> entropy-starved /dev/random is unacceptable, we started looking at the
> blocking urandom approach.

Can't you accomplish this in userspace by trying to read as much as you
can out of /dev/random without blocking, then reading out
of /dev/urandom the minimum between allowed threshold and remaining
bytes, and then blocking on /dev/random?

For example, lets say you need 100 bytes of randomness, and your
threshold is 30 bytes. You try reading out of /dev/random and get 50
bytes, at that point you'll read another 30 (=threshold) bytes
out /dev/urandom and then you'll need to block on /dev/random until you
get the remaining 20 bytes.

-- 

Sasha.

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