linux-kernel - Re: [PATCH v2] x86: increase MIN_GAP to include randomized stack

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Date:	Mon, 7 Sep 2009 17:18:44 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Michal Hocko <mhocko@...e.cz>, Ingo Molnar <mingo@...hat.com>
Cc:	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 v2] x86: increase MIN_GAP to include randomized stack

On Fri, 4 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>
> ---
>  arch/x86/mm/mmap.c |   21 +++++++++++++++++++--
>  1 files changed, 19 insertions(+), 2 deletions(-)
> 
> I wasn't sure about STACK_RND_MASK because it is defined also in 
> arch/x86/include/asm/elf.h and I couldn't find a common header file for 
> both of them. If you have any idea I would like to help with that.
> Or should we just let it for later cleanup?

What will break if we include <asm/elf.h> from mmap.c?

> I think that this is also stable material and I will repost it to 
> stable@...nel.org once you ack it.
> 
> Changes from v1:
> Fixed unsigned int overflow in MIN_GAP calculation.
> 
> 
> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index 1658296..ac41955 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -30,12 +30,29 @@
>  #include <linux/limits.h>
>  #include <linux/sched.h>
>  
> +/* 1GB for 64bit, 8MB for 32bit definition taken from arch/x86/include/asm/elf.h */
> +#ifndef STACK_RND_MASK
> +#define STACK_RND_MASK (test_thread_flag(TIF_IA32) ? 0x7ff : 0x3fffff)
> +#endif
> +
> +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)

Apart from doubling the STACK_RND_MASK definition

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

Thanks,

-- 
Jiri Kosina
SUSE Labs
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