linux-kernel - Re: [PATCH] Change ZERO_SIZE_PTR to point at unmapped space

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Date:	Sun, 31 May 2009 00:32:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Larry H." <research@...reption.com>
Cc:	linux-mm@...ck.org, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Change ZERO_SIZE_PTR to point at unmapped space

On Sat, 2009-05-30 at 12:28 -0700, Larry H. wrote:
> [PATCH] Change ZERO_SIZE_PTR to point at unmapped space
> 
> This patch changes the ZERO_SIZE_PTR address to point at top memory
> unmapped space, instead of the original location which could be
> mapped from userland to abuse a NULL (or offset-from-null) pointer
> dereference scenario.

Same goes for the regular NULL pointer, we have bits to disallow
userspace mapping the NULL page, so I'm not exactly seeing what this
patch buys us.

> The ZERO_OR_NULL_PTR macro is changed accordingly. This patch does
> not modify its behavior nor has any performance nor functionality
> impact.

It does generate longer asm.

> The original change was written first by the PaX team for their
> patch.
> 
> Signed-off-by: Larry Highsmith <larry@...reption.com>
> 
> Index: linux-2.6/include/linux/slab.h
> ===================================================================
> --- linux-2.6.orig/include/linux/slab.h
> +++ linux-2.6/include/linux/slab.h
> @@ -73,10 +73,9 @@
>   * 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 *)-1024L)
>  
> -#define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \
> -				(unsigned long)ZERO_SIZE_PTR)
> +#define ZERO_OR_NULL_PTR(x) (!(x) || (x) == ZERO_SIZE_PTR)
>  
>  /*
>   * struct kmem_cache related prototypes
> --
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