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

[Laura Abbott]

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.

Thanks,
Laura
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