linux-kernel - Re: [PATCH v3] x86: increase MIN_GAP to include randomized stack

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Date:	Tue, 8 Sep 2009 10:47:46 +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 v3] 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>
> ---
>  arch/x86/mm/mmap.c |   25 +++++++++++++++++++++++--
>  1 files changed, 23 insertions(+), 2 deletions(-)
> 
> I think that this is also stable material and I will repost it to 
> stable@...nel.org once you ack it.
> 
> Changes from v2:
> STACK_RND_MASK has to be defined for CONFIG_X86_32 as well.
> 
> 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..a4566a8 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -30,12 +30,33 @@
>  #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
> +#ifdef CONFIG_X86_64
> +#define STACK_RND_MASK (test_thread_flag(TIF_IA32) ? 0x7ff : 0x3fffff)
> +#else
> +#define STACK_RND_MASK (0x7ff)
> +#endif
> +#endif

I'd still prefer all this macro-magic happening in <asm/elf.h> and 
including this file from arch/x86/mm/mmap.c, otherwise we are duplicating 
the STACK_RND_MASK definition, which is messy.

-- 
Jiri Kosina
SUSE Labs
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