Re: [PATCH] slab: Add POISON_POINTER_DELTA to ZERO_SIZE_PTR

[Kees Cook <keescook@chromium.org>]

On Tue, Nov 15, 2016 at 2:57 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 pointer to userspace.

Ah, when dealing with a 0-sized malloc or similar? Do you have
pointers to exploits that rely on this?

Regardless, normally PAN/SMAP-like things should be sufficient to
protect against this. Additionally, on everything but x86_64 and
arm64, POISON_POINTER_DELTA == 0, if I'm reading correctly:

#ifdef CONFIG_ILLEGAL_POINTER_VALUE
# define POISON_POINTER_DELTA _AC(CONFIG_ILLEGAL_POINTER_VALUE, UL)
#else
# define POISON_POINTER_DELTA 0
#endif

...

config ILLEGAL_POINTER_VALUE
       hex
       default 0 if X86_32
       default 0xdead000000000000 if X86_64

...

config ILLEGAL_POINTER_VALUE
        hex
        default 0xdead000000000000

Is the plan to add ILLEGAL_POINTER_VALUE for powerpc too? And either
way, this patch, IIUC, will break the ZERO_OR_NULL_PTR() check, since
suddenly all of userspace will match it. (Though maybe that's okay?)

-Kees

>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>  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)
> --
> 2.7.4
>



-- 
Kees Cook
Nexus Security

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