linux-kernel - Re: [Security] [patch] random: make get_random

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Date:	Sat, 16 May 2009 05:02:42 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Willy Tarreau <w@....eu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	security@...nel.org, Linux@...a.kernel.org, stable@...nel.org,
	Cox <alan@...rguk.ukuu.org.uk>, Arjan@...a.kernel.org,
	List <linux-kernel@...r.kernel.org>, Alan@...a.kernel.org,
	Eric Paris <eparis@...hat.com>, Jake Edge <jake@....net>,
	linux-security-module@...r.kernel.org, mingo@...hat.com,
	Matt Mackall <mpm@...enic.com>, Dave Jones <davej@...hat.com>,
	James Morris <jmorris@...ei.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>,
	de Ven <arjan@...radead.org>
Subject: Re: [Security] [patch] random: make get_random_int() more random

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

> * Willy Tarreau <w@....eu> wrote:
>
>
> Bad idea IMHO ...
>
> It is a bad idea because such sort of tunables do not really help 
> the user as those who tweak are a distinct minority.
>
> Also, having a two-way hack _hinders_ your good idea from being 
> adopted for example. Why bother with a faster hash and with using 
> the resulting bits sparingly if we can get an 'easy' tunable in and 
> can have two sub-par solutions instead of one (harder to implement) 
> good solution?
>
> So tunables are really counter-productive - and this is a pet peeve 
> of mine.
>
> Every time we have such a tunable for something fundamental we've 
> not improved the kernel, we've documented a _failure_ in kernel 
> design and implementation.
>
> Sure, we do use tunables for physical constants, limits and other 
> natural parameters - and _sometimes_ we just grudingly admit defeat 
> and admit that something is really impossible to implement. IMHO 
> here we are not at that point yet, at all.

In the lwn comment section there was a suggestion to use a high
quality stream cipher (AES?) instead of sha1 or the half md4 thing.
Apparently those should be both stronger and faster.

I don't know enough about it except to say that sounds right in
principle.

Apparently some of the BSDs do something similar with arc4random.
arc4 is old and in some case broken so it is unlikely to make a good
choice at this point, but the overall design of a stream cipher
that is rekeyed ever 5 minutes seems sound.

Eric
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