Re: [kernel-hardening] [RFC][PATCH 6/7] mm: Add Kconfig option for slab sanitization

[Laura Abbott]

On 1/5/16 4:29 PM, Kees Cook wrote:
> On Tue, Dec 22, 2015 at 11:13 AM, Laura Abbott <laura@xxxxxxxxxxxx> wrote:
> > On 12/22/15 10:19 AM, Dave Hansen wrote:
> > > On 12/22/2015 10:08 AM, Christoph Lameter wrote:
> > > > On Tue, 22 Dec 2015, Dave Hansen wrote:
> > > > > > Why would you use zeros? The point is just to clear the information
> > > > > > right?
> > > > > > The regular poisoning does that.
> > > > >
> > > > >
> > > > > It then allows you to avoid the zeroing at allocation time.
> > > >
> > > >
> > > > Well much of the code is expecting a zeroed object from the allocator and
> > > > its zeroed at that time. Zeroing makes the object cache hot which is an
> > > > important performance aspect.
> > >
> > >
> > > Yes, modifying this behavior has a performance impact.  It absolutely
> > > needs to be evaluated, and I wouldn't want to speculate too much on how
> > > good or bad any of the choices are.
> > >
> > > Just to reiterate, I think we have 3 real choices here:
> > >
> > > 1. Zero at alloc, only when __GFP_ZERO
> > >      (behavior today)
> > > 2. Poison at free, also Zero at alloc (when __GFP_ZERO)
> > >      (this patch's proposed behavior, also what current poisoning does,
> > >       doubles writes)
> > > 3. Zero at free, *don't* Zero at alloc (when __GFP_ZERO)
> > >      (what I'm suggesting, possibly less perf impact vs. #2)
> >
> > poisoning with non-zero memory makes it easier to determine that the error
> > came from accessing the sanitized memory vs. some other case. I don't think
> > the feature would be as strong if the memory was only zeroed vs. some other
> > data value.
>
> I would tend to agree. If there are significant perf improvements for
> "3" above, that should be easy to add on later as another choice.

I was looking at the sanitization for the buddy allocator that exists in
grsecurity and that does option #3 (zero at free, skip __GFP_ZERO).
I'm going to look into adding that as an option for the slab allocator
and see what the performance numbers show.

Thanks,
Laura
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