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

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Date:	Sat, 19 Jul 2014 08:20:11 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Theodore Ts'o <tytso@....edu>
Cc:	linux-kernel@...r.kernel.org, davej@...hat.com, price@....edu
Subject: Re: [PATCH] random: check for increase of entropy_count because of
 signed conversion

On Sa, 2014-07-19 at 01:42 -0400, Theodore Ts'o wrote:
> On Sat, Jul 19, 2014 at 01:35:48AM +0200, Hannes Frederic Sowa wrote:
> > > +	nfrac = ibytes << (ENTROPY_SHIFT + 3);
> > > +	if (entropy_count < 0) {
> > 
> > Minor nit: maybe also add an unlikely() here?
> 
> Yep, done.
> 
> > > +	if ((unsigned) entropy_count > nfrac)
> > 
> > (unsigned) -> (size_t)
> >
> > size_t could also be (unsigned long) so the plain (unsigned) is
> > misleading.
> 
> Good point, done.
> 
> > (Maybe I wouldn't have done the cast at all, as we compile the kernel
> > with -Wno-sign-compare and we have the < 0 check right above, but I
> > don't have a strong opinion on that.)
> 
> I also wanted to shut up other static code checkers like Coverity.  :-)
> 
> > > +	nbytes = min_t(size_t, nbytes, INT_MAX >> ENTROPY_SHIFT);
> > 
> > Hmm, not sure, nfracs unit is 1/8 bits, so don't we have to limit nbytes
> > to INT_MAX >> (ENTROPY_SHIFT + 3) here?
> 
> Good catch, done.
> 
> > And if we want to be even more correct here, we could switch from
> > INT_MAX to SIZE_MAX, as we do all nfrac calculations in the size_t
> > domain.
> 
> The main reason why I used INT_MAX was as a further safety check to
> protect the:
> 
> 	entropy_count -= nfrac;
> 
> calculation, since nfrac is size_t and entropy_count is int.
> 
> In fact I think this online change ("nbytes = min_t(size_t, nbytes,
> INT_MAX >> (ENTROPY_SHIFT + 3));") would have been enough to fix the
> problem all by itself, but the other changes results in code which is
> cleaner and easier to understand, and I'm a firm believer in multiple
> layers of protection.  :-)

I see and can agree here. :)

I think the patch is good to go.

Thanks you,
Hannes


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