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

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Date:	Thu, 08 Sep 2011 12:13:47 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	sgrubb@...hat.com
Cc:	hch@...radead.org, stephan.mueller@...ec.com, tytso@....edu,
	jarod@...hat.com, levinsasha928@...il.com,
	linux-crypto@...r.kernel.org, mpm@...enic.com, nhorman@...hat.com,
	herbert.xu@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] random: add blocking facility to urandom

From: Steve Grubb <sgrubb@...hat.com>
Date: Thu, 8 Sep 2011 07:48:27 -0400

> On Thursday, September 08, 2011 04:44:20 AM Christoph Hellwig wrote:
>> On Wed, Sep 07, 2011 at 11:27:12PM +0200, Stephan Mueller wrote:
>> > And exactly that is the concern from organizations like BSI. Their
>> > cryptographer's concern is that due to the volume of data that you can
>> > extract from /dev/urandom, you may find cycles or patterns that increase
>> > the probability to guess the next random value compared to brute force
>> > attack. Note, it is all about probabilities.
>> 
>> So don't use /dev/urandom if you don't like the behaviour.  Breaking all
>> existing application because of a certification is simply not an option.
> 
> This patch does not _break_ all existing applications. If a system were under attack, 
> they might pause momentarily, but they do not break. Please, try the patch and use a 
> nice large number like 2000000 and see for yourself. Right now, everyone arguing about 
> this breaking things have not tried it to see if in fact things do break and how they 
> break if they do.

If the application holds a critical resource other threads want when it
blocks on /dev/urandom, then your change breaks things.  I can come up
with more examples if you like.

Please get off this idea that you can just change the blocking behavior
for a file descriptor and nothing of consequence will happen.

When this happens in the networking due to a bug or similar, we know
it does break things.
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