Re: [PATCH] arm64: kernel: Fix incorrect brk randomization

[Jon Medhurst (Tixy)]

On Tue, 2016-05-10 at 10:55 -0700, Kees Cook wrote:
> This fixes two issues with the arm64 brk randomziation. First, the
> STACK_RND_MASK was being used incorrectly. The original code was:
> 
> 	unsigned long range_end = base + (STACK_RND_MASK << PAGE_SHIFT) + 1;
> 
> STACK_RND_MASK is 0x7ff (32-bit) or 0x3ffff (64-bit), with 4K pages where
> PAGE_SHIFT is 12:
> 
> 	#define STACK_RND_MASK	(test_thread_flag(TIF_32BIT) ? \
> 						0x7ff >> (PAGE_SHIFT - 12) : \
> 						0x3ffff >> (PAGE_SHIFT - 12))
> 
> This means the resulting offset from base would be 0x7ff0001 or 0x3ffff0001,
> which is wrong since it creates an unaligned end address. It was likely
> intended to be:
> 
> 	unsigned long range_end = base + ((STACK_RND_MASK + 1) << PAGE_SHIFT)
> 
> Which would result in offsets of 0x800000 (32-bit) and 0x40000000 (64-bit).
> 
> However, even this corrected 32-bit compat offset (0x00800000) is much
> smaller than native ARM's brk randomization value (0x02000000):
> 
> 	unsigned long arch_randomize_brk(struct mm_struct *mm)
> 	{
> 	        unsigned long range_end = mm->brk + 0x02000000;
> 	        return randomize_range(mm->brk, range_end, 0) ? : mm->brk;
> 	}
> 
> So, instead of basing arm64's brk randomization on mistaken STACK_RND_MASK
> calculations, just use specific corrected values for compat (0x2000000)
> and native arm64 (0x40000000).
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

There seems to be a helper 'is_compat_task()' that does
'test_thread_flag(TIF_32BIT)' so could perhaps be used instead, but
that's too nit-picky. This change makes things more consistent with
arch/arm and other arches, and stops use of STACK_RND_MASK for things
that aren't stack related so seems good all round to me.

Reviewed-by: Jon Medhurst <tixy@xxxxxxxxxx>

> ---
>  arch/arm64/kernel/process.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 80624829db61..0d0969bcd76d 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -382,13 +382,14 @@ unsigned long arch_align_stack(unsigned long sp)
>  	return sp & ~0xf;
>  }
>  
> -static unsigned long randomize_base(unsigned long base)
> -{
> -	unsigned long range_end = base + (STACK_RND_MASK << PAGE_SHIFT) + 1;
> -	return randomize_range(base, range_end, 0) ? : base;
> -}
> -
>  unsigned long arch_randomize_brk(struct mm_struct *mm)
>  {
> -	return randomize_base(mm->brk);
> +	unsigned long range_end = mm->brk;
> +
> +	if (test_thread_flag(TIF_32BIT))
> +		range_end += 0x02000000;
> +	else
> +		range_end += 0x40000000;
> +
> +	return randomize_range(mm->brk, range_end, 0) ? : mm->brk;
>  }
> -- 
> 2.6.3
> 
>