linux-kernel - Re: [Security] [PATCH] proc: avoid information leaks to non-privileged processes

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Date:	Tue, 05 May 2009 01:14:05 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Matt Mackall <mpm@...enic.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Jake Edge <jake@....net>, security@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	Eric Paris <eparis@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Roland McGrath <roland@...hat.com>, mingo@...hat.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <greg@...ah.com>, Dave Jones <davej@...hat.com>
Subject: Re: [Security] [PATCH] proc: avoid information leaks to non-privileged processes

Ingo Molnar <mingo@...e.hu> writes:

> * Matt Mackall <mpm@...enic.com> wrote:
>
>> As to what's the appropriate sort of RNG for ASLR to use, finding 
>> a balance between too strong and too weak is tricky. [...]
>
> In exec-shield i mixed 'easily accessible and fast' semi-random 
> state to the get_random_int() result: xor-ed the cycle counter, the 
> pid and a kernel address to it. That strengthened the result in a 
> pretty practical way (without strengthening the theoretical 
> randomless - each of those items are considered guessable) and does 
> so without weakening the entropy of the random pool.

The trouble is, that thinking completely misses the problem, and I
expect that is why we have a problem.  Throwing a bunch of possibly
truly random values into the pot for luck is nice.  But you didn't
throw in a pseudo random number generator.  An unpredictable sequence
that is guaranteed to change from one invocation to the next.

In a very practical sense a pseudo random generator is completely
sufficient.  Throwing in a few truly random numbers guards against
attacks on the random number generator.

What we have now is a hash over an a value that changes every 5 minutes
and some well known values.

Eric
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