Re: [PATCH] slab: Add POISON_POINTER_DELTA to ZERO_SIZE_PTR

[Balbir Singh]

On 15/11/16 21:57, Michael Ellerman 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 pointer to userspace.
> 
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> ---
>  include/linux/slab.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 084b12bad198..17ddd7aea2dd 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>
>  
> @@ -109,7 +110,7 @@
>   * 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 + POISON_POINTER_DELTA))
>  
>  #define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \
>  				(unsigned long)ZERO_SIZE_PTR)
> 

I wonder if we should make this a variable with boot time entropy
within a certain region

Balbir Singh.