linux-kernel - Re: [PATCH][RESEND 3] hwrng: add randomness to system from rng sources

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Date:	Wed, 05 Mar 2014 18:52:27 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Jason Cooper <jason@...edaemon.net>
Cc:	Kees Cook <keescook@...omium.org>, Theodore Ts'o <tytso@....edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Satoru Takeuchi <satoru.takeuchi@...il.com>,
	linux-crypto <linux-crypto@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH][RESEND 3] hwrng: add randomness to system from rng
 sources

On Wed, 2014-03-05 at 16:11 -0500, Jason Cooper wrote:
> > In other words, if there are 4096 bits of "unknownness" in X to start
> > with, and I can get those same 4096 bits of "unknownness" back by
> > unmixing X' and Y, then there must still be 4096 bits of "unknownness"
> > in X'. If X' is 4096 bits long, then we've just proven that
> > reversibility means the attacker can know nothing about the contents of
> > X' by his choice of Y.
> 
> Well, this reinforces my comfortability with loadable modules.  The pool
> is already initialized by the point at which the driver is loaded.
> 
> Unfortunately, any of the drivers in hw_random can be built in.  When
> built in, hwrng_register is going to be called during the kernel
> initialization process.  In that case, the unknownness in X is not 4096
> bits, but far less.  Also, the items that may have seeded X (MAC addr,
> time, etc) are discoverable by a potential attacker.  This is also well
> before random-seed has been fed in.

To which I would respond.. so?

If the pool is in an attacker-knowable state at early boot, adding
attacker-controlled data does not make the situation any worse. In fact,
if the attacker has less-than-perfect control of the inputs, mixing more
things in will make things exponentially harder for the attacker.

Put another way: mixing can't ever removes unknownness from the pool, it
can only add more. So the only reason you should ever choose not to mix
something into the pool is performance.

-- 
Mathematics is the supreme nostalgia of our time.


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