linux-kernel - Re: [PATCH 07/10] random: add new get_random_bytes

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Message-ID: <20120725151000.GA30996@thunk.org>
Date:	Wed, 25 Jul 2012 11:10:00 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	torvalds@...ux-foundation.org, w@....eu, ewust@...ch.edu,
	zakir@...ch.edu, greg@...ah.com, mpm@...enic.com,
	nadiah@...ucsd.edu, jhalderm@...ch.edu, tglx@...utronix.de,
	davem@...emloft.net, stable@...nel.org,
	DJ Johnson <dj.johnson@...el.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 07/10] random: add new get_random_bytes_arch() function

On Tue, Jul 24, 2012 at 08:37:23PM -0700, H. Peter Anvin wrote:
> 
> As a compromise I offer the following patch; in terms of performance
> it is "the worst of both worlds" but it should provide the combined
> security of either; even if RDRAND is completely compromised by the
> NSA, Microsoft and the Illuminati all at once it will do no worse
> than the existing code, and (since RDRAND is so much faster than the
> existing code) it has only a modest performance cost.  More
> realistically, it will let many more users take advantage of a high
> entropy quick-reseeding random number generator, thus ending up with
> a major gain in security.

RDRAND is already getting mixed in already in xfer_secondary_pool() so
we are already taking advantage of Bull Mountain (or any other
CPU/architecture-specific hw RNG) if it is present.

Aside from whether it's better to do this step in
xfer_secondary_pool() or extract_entropy(), your patch looks very
wrong to me.  Nothing is actually *using* hash.l[], which is where the
results of the RDRAND generator is placed.

I assume you were planning on xor'ing hash.w and hash.l, but that's
not present in your patch.

I also don't understand why you are using a blind union here; it has
no real advantage that I can see, and so it's all downside.  It bloats
the patch (making it harder to see that your patch results in a net
*decrease* in security, since it removes the use of RDRAND in
xfer_security_pool, and replaces it with a no-op).

		    		    	   - Ted
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