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

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Date:	Fri, 18 Jul 2014 23:43:51 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	"Theodore Ts'o" <tytso@....edu>
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

Hi,

On Fri, Jul 18, 2014, at 23:25, Theodore Ts'o wrote:
> 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().

If we always enter account() with r->entropy_count > 0 I think the
checks in place after this patch should suffice. The problem should only
arise in case entropy_count < 0 at the beginning of account (or in
between if we need to do a retry).

I didn't find a way how entropy_count could get smaller than 0 without
hitting a WARN_ON and a reset of r->entropy_count = 0.

Thanks,
Hannes
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