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Date: Fri, 18 Jul 2014 17:25:04 -0400
From: "Theodore Ts'o" <tytso@mit.edu>
To: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Dave Jones <davej@redhat.com>, Linux Kernel <linux-kernel@vger.kernel.org>,
        Greg Price <price@mit.edu>
Subject: Re: [PATCH] random: check for increase of entropy_count because of
 signed conversion
Message-ID: <20140718212504.GC18775@thunk.org>
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	Hannes Frederic Sowa <hannes@stressinduktion.org>,
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	Greg Price <price@MIT.EDU>
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On Wed, Jul 16, 2014 at 09:18:15PM +0200, Hannes Frederic Sowa wrote:
> The expression entropy_count -= ibytes << (ENTROPY_SHIFT + 3) could
> actually increase entropy_count if during assignment of the unsigned
> expression on the RHS (mind the -=) we reduce the value modulo
> 2^width(int) and assign it to entropy_count. Trinity found this.
> 
> Reported-by: Dave Jones <davej@redhat.com>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Cc: Greg Price <price@mit.edu>
> Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
> ---
> As indicated by credit_entropy_bits entropy_count cannot get negative,
> so I don't see any reason to include a check for entropy_count < 0
> here. Do you agree?

No, the check is important; after we subtract ibytes << (ENTROPY_SHIFT
+ 3) we could drive entropy_count negative, and we don't want to
trigger the WARN_ON().

I'll modify the patch to keep the check.

						- Ted

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