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

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Date:	Wed, 07 Sep 2011 23:00:42 +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 15:30 -0400, Jarod Wilson wrote:
> Sasha Levin wrote:
> > On Wed, 2011-09-07 at 14:26 -0400, Jarod Wilson wrote:
> >> Sasha Levin wrote:
> >> [..] And anything done in
> >> userspace is going to be full of possible holes [..]
> >
> > Such as? Is there an example of a case which can't be handled in
> > userspace?
> 
> How do you mandate preventing reads from urandom when there isn't 
> sufficient entropy? You likely wind up needing to restrict access to the 
> actual urandom via permissions and selinux policy or similar, and then 
> run a daemon or something that provides a pseudo-urandom that brokers 
> access to the real urandom. Get the permissions or policy wrong, and 
> havoc ensues. An issue with the initscript or udev rule to hide the real 
> urandom, and things can fall down. Its a whole lot more fragile than 
> this approach, and a lot more involved in setting it up.

Replace /dev/urandom with a simple CUSE driver, redirect reads to the
real urandom after applying your threshold.

-- 

Sasha.

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