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

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Message-ID: <CAGXu5j+ddoy_v4y-cwgZmiKCe5DTARgkQYNntfNaY5msyADvtg@mail.gmail.com>
Date:	Wed, 5 Mar 2014 17:34:48 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Jason Cooper <jason@...edaemon.net>,
	"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, Mar 5, 2014 at 4:52 PM, Matt Mackall <mpm@...enic.com> wrote:
> 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.

Excellent. So it sounds like you're okay with my original patch as-is?

-Kees

-- 
Kees Cook
Chrome OS Security
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