linux-kernel - Re: [PATCH] random: check for increase of entropy

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Date:	Fri, 18 Jul 2014 17:25:04 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Greg Price <price@....EDU>
Subject: Re: [PATCH] random: check for increase of entropy_count because of
 signed conversion

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@...hat.com>
> Cc: Theodore Ts'o <tytso@....edu>
> Cc: Greg Price <price@....edu>
> Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.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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