On Thu, Nov 17, 2016 at 1:51 AM, Michael Ellerman <mpe@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
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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