Messages in this thread | | | From | Kees Cook <> | Date | Thu, 17 Nov 2016 12:01:51 -0800 | Subject | Re: [PATCH v2] slab: Add POISON_POINTER_DELTA to ZERO_SIZE_PTR |
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On Thu, Nov 17, 2016 at 1:51 AM, Michael Ellerman <mpe@ellerman.id.au> wrote: > POISON_POINTER_DELTA is defined in poison.h, and is intended to be used > to shift poison values so that they don't alias userspace. > > We should add it to ZERO_SIZE_PTR so that attackers can't use > ZERO_SIZE_PTR as a way to get a non-NULL pointer to userspace. > > Currently ZERO_OR_NULL_PTR() uses a trick of doing a single check that > x <= ZERO_SIZE_PTR, and ignoring the fact that it also matches 1-15. > That no longer really works once we add the poison delta, so split it > into two checks. Assign x to a temporary to avoid evaluating it > twice (suggested by Kees Cook). > > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
I continue to like this idea. If we want to avoid the loss of the 1-15 check, we could just explicitly retain it, see craziness below...
> --- > include/linux/slab.h | 10 +++++++--- > 1 file changed, 7 insertions(+), 3 deletions(-) > > v2: Rework ZERO_OR_NULL_PTR() to do the two checks separately. > > diff --git a/include/linux/slab.h b/include/linux/slab.h > index 084b12bad198..404419d9860f 100644 > --- a/include/linux/slab.h > +++ b/include/linux/slab.h > @@ -12,6 +12,7 @@ > #define _LINUX_SLAB_H > > #include <linux/gfp.h> > +#include <linux/poison.h> > #include <linux/types.h> > #include <linux/workqueue.h> > > @@ -109,10 +110,13 @@ > * ZERO_SIZE_PTR can be passed to kfree though in the same way that NULL can. > * Both make kfree a no-op. > */ > -#define ZERO_SIZE_PTR ((void *)16)
#define __ZERO_SIZE_PTR((void *)16) #define ZERO_SIZE_PTR ((void *)(__ZERO_SIZE_PTR + POISON_POINTER_DELTA))
> > -#define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \ > - (unsigned long)ZERO_SIZE_PTR) > +#define ZERO_OR_NULL_PTR(x) \ > + ({ \ > + void *p = (void *)(x); \ (p < __ZERO_SIZE_PTR || p == ZERO_SIZE_PTR); \ > + })
#undef __ZERO_SIZE_PTR
?
Anyone else have thoughts on this?
-Kees
-- Kees Cook Nexus Security
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