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Date: Wed, 16 Nov 2016 11:35:25 +1100
From: Balbir Singh <bsingharora@...il.com>
To: Michael Ellerman <mpe@...erman.id.au>, akpm@...ux-foundation.org
Cc: cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
iamjoonsoo.kim@....com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com, keescook@...omium.org
Subject: Re: [PATCH] slab: Add POISON_POINTER_DELTA to ZERO_SIZE_PTR
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@...erman.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)
>
I wonder if we should make this a variable with boot time entropy
within a certain region
Balbir Singh.
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