Re: [RFC][PATCH 0/7] Sanitization of slabs based on grsecurity/PaX

[Laura Abbott]

On 1/5/16 4:09 PM, Kees Cook wrote:
> On Tue, Dec 22, 2015 at 12:04 PM, Laura Abbott <laura@xxxxxxxxxxxx> wrote:
> > On 12/22/15 8:08 AM, Christoph Lameter wrote:
> > > On Mon, 21 Dec 2015, Laura Abbott wrote:
> > >
> > > > The biggest change from PAX_MEMORY_SANTIIZE is that this feature
> > > > sanitizes
> > > > the SL[AOU]B allocators only. My plan is to work on the buddy allocator
> > > > santization after this series gets picked up. A side effect of this is
> > > > that allocations which go directly to the buddy allocator (i.e. large
> > > > allocations) aren't sanitized. I'd like feedback about whether it's worth
> > > > it to add sanitization on that path directly or just use the page
> > > > allocator sanitization when that comes in.
>
> This looks great! I love the added lkdtm tests, too. Very cool.
>
> > > I am not sure what the point of this patchset is. We have a similar effect
> > > to sanitization already in the allocators through two mechanisms:
> > >
> > > 1. Slab poisoning
> > > 2. Allocation with GFP_ZERO
> > >
> > > I do not think we need a third one. You could accomplish your goals much
> > > easier without this code churn by either
> > >
> > > 1. Improve the existing poisoning mechanism. Ensure that there are no
> > >      gaps. Security sensitive kernel slab caches can then be created with
> > >      the  POISONING flag set. Maybe add a Kconfig flag that enables
> > >      POISONING for each cache? What was the issue when you tried using
> > >      posining for sanitization?
> >
> > The existing poisoning does work for sanitization but it's still a debug
> > feature. It seemed more appropriate to keep debug features and non-debug
> > features separate hence the separate option and configuration.
>
> What stuff is intertwined in the existing poisoning that makes it
> incompatible/orthogonal?

It's not the poisoning per se that's incompatible, it's how the poisoning is
set up. At least for slub, the current poisoning is part of SLUB_DEBUG which
enables other consistency checks on the allocator. Trying to pull out just
the poisoning for use when SLUB_DEBUG isn't on would result in roughly what
would be here anyway. I looked at trying to reuse some of the existing poisoning
and came to the conclusion it was less intrusive to the allocator to keep it
separate.

Thanks,
Laura
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