linux-kernel - Re: [PATCH v4] x86: increase MIN_GAP to include randomized stack

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Date:	Tue, 8 Sep 2009 11:09:36 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, "H . Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4] x86: increase MIN_GAP to include randomized stack

On Tue, 8 Sep 2009, Michal Hocko wrote:

> Currently we are not including randomized stack size when calculating
> mmap_base address in arch_pick_mmap_layout for topdown case. This might
> cause that mmap_base starts in the stack reserved area because stack is
> randomized by 1GB for 64b (8MB for 32b) and the minimum gap is 128MB.
> 
> If the stack really grows down to mmap_base then we can get silent mmap
> region overwrite by the stack values.
> 
> Let's include maximum stack randomization size into MIN_GAP which is
> used as the low bound for the gap in mmap.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.cz>

Acked-by: Jiri Kosina <jkosina@...e.cz>

Ingo, could you please consider applying this?

> ---
>  arch/x86/include/asm/elf.h |    2 ++
>  arch/x86/mm/mmap.c         |   17 +++++++++++++++--
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> index 83c1bc8..456a304 100644
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -299,6 +299,8 @@ do {									\
>  
>  #ifdef CONFIG_X86_32
>  
> +#define STACK_RND_MASK (0x7ff)
> +
>  #define VDSO_HIGH_BASE		(__fix_to_virt(FIX_VDSO))
>  
>  #define ARCH_DLINFO		ARCH_DLINFO_IA32(vdso_enabled)
> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index 1658296..c8191de 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -29,13 +29,26 @@
>  #include <linux/random.h>
>  #include <linux/limits.h>
>  #include <linux/sched.h>
> +#include <asm/elf.h>
> +
> +static unsigned int stack_maxrandom_size(void)
> +{
> +	unsigned int max = 0;
> +	if ((current->flags & PF_RANDOMIZE) &&
> +		!(current->personality & ADDR_NO_RANDOMIZE)) {
> +		max = ((-1U) & STACK_RND_MASK) << PAGE_SHIFT;
> +	}
> +
> +	return max;
> +}
> +
>  
>  /*
>   * Top of mmap area (just below the process stack).
>   *
> - * Leave an at least ~128 MB hole.
> + * Leave an at least ~128 MB hole with possible stack randomization.
>   */
> -#define MIN_GAP (128*1024*1024)
> +#define MIN_GAP (128*1024*1024UL + stack_maxrandom_size())
>  #define MAX_GAP (TASK_SIZE/6*5)
>  
>  /*
> -- 
> 1.6.3.3
> 

-- 
Jiri Kosina
SUSE Labs
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