linux-kernel - Re: [PATCH] random: only use gathered bytes from arch_get_random

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Date:	Sat, 7 Jul 2012 14:23:16 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Kees Cook <keescook@...omium.org>
Cc:	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	ewust@...ch.edu, zakir@...ch.edu, nadiah@...ucsd.edu,
	jhalderm@...ch.edu, stable@...r.kernel.org
Subject: Re: [PATCH] random: only use gathered bytes from arch_get_random_long

On Sat, Jul 07, 2012 at 10:11:22AM -0700, Kees Cook wrote:
> While very unlikely, it is possible for arch_get_random_long() to fail
> in the middle of the loop in xfer_secondary_pool(), which would mean
> that the loop could stop with only part of u.hwrand populated, leading
> to mix_pool_bytes() injecting uninitialized or already injected bytes
> instead of fresh bytes. This changes the mix_pool_bytes() call to only
> inject the successfully gathered bytes.

I don't believe there is a major problem with injecting uninitialized
or even known bytes into the pool; worst case we're wastiing a tiny
amount of CPU in this unlikely case (versus the CPU costs of doing the
multiplication each time).  Not that I think really matters one way or
the other... 

Is there a reason why you're particularly concerned about what might
happen in the case where arch_get_random_long() fails mid-loop (which
can happen if RDRAND returns an error for whatever reason, granted)?

Regards,

						- Ted
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