Re: [PATCH] mm: Revert x86_64 and arm64 ELF_ET_DYN_BASE base

[Kees Cook]

*extreme thread[1] necromancy*

On Mon, Aug 07, 2017 at 01:15:42PM -0700, Kees Cook wrote:
> Moving the x86_64 and arm64 PIE base from 0x555555554000 to 0x000100000000
> broke AddressSanitizer. This is a partial revert of:
> 
>   commit eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")
>   commit 02445990a96e ("arm64: move ELF_ET_DYN_BASE to 4GB / 4MB")
> 
> The AddressSanitizer tool has hard-coded expectations about where
> executable mappings are loaded. The motivation for changing the PIE
> base in the above commits was to avoid the Stack-Clash CVEs that
> allowed executable mappings to get too close to heap and stack. This
> was mainly a problem on 32-bit, but the 64-bit bases were moved too,
> in an effort to proactively protect those systems (proofs of concept
> do exist that show 64-bit collisions, but other recent changes to fix
> stack accounting and setuid behaviors will minimize the impact).

I happened to be looking at this again today, and wondered where things
stood. It seems like ASan's mappings are documented here:
https://github.com/google/sanitizers/wiki/AddressSanitizerAlgorithm#64-bit

This implies that it would be safe to move the ELF_ET_DYN_BASE from
0x555555554000 down to 0x200000000000, since the shadow map ends at
0x10007fff7fff. (Well, anything above there would work, I was just
picking a "round" number above it. We could just as well use
0x100080000000, I think.)

Is this correct? I'd like to open up some more room between mmap and
stack...

Thanks!

-Kees

[1] https://lore.kernel.org/lkml/20170807201542.GA21271@beast/

> 
> The new 32-bit PIE base is fine for ASan (since it matches the ET_EXEC
> base), so only the 64-bit PIE base needs to be reverted to let x86 and
> arm64 ASan binaries run again. Future changes to the 64-bit PIE base on
> these architectures can be made optional once a more dynamic method for
> dealing with AddressSanitizer is found. (e.g. always loading PIE into
> the mmap region for marked binaries.)
> 
> Reported-by: Kostya Serebryany <kcc@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  arch/arm64/include/asm/elf.h | 4 ++--
>  arch/x86/include/asm/elf.h   | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
> index acae781f7359..3288c2b36731 100644
> --- a/arch/arm64/include/asm/elf.h
> +++ b/arch/arm64/include/asm/elf.h
> @@ -114,10 +114,10 @@
>  
>  /*
>   * This is the base location for PIE (ET_DYN with INTERP) loads. On
> - * 64-bit, this is raised to 4GB to leave the entire 32-bit address
> + * 64-bit, this is above 4GB to leave the entire 32-bit address
>   * space open for things that want to use the area for 32-bit pointers.
>   */
> -#define ELF_ET_DYN_BASE		0x100000000UL
> +#define ELF_ET_DYN_BASE		(2 * TASK_SIZE_64 / 3)
>  
>  #ifndef __ASSEMBLY__
>  
> diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> index 1c18d83d3f09..9aeb91935ce0 100644
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -247,11 +247,11 @@ extern int force_personality32;
>  
>  /*
>   * This is the base location for PIE (ET_DYN with INTERP) loads. On
> - * 64-bit, this is raised to 4GB to leave the entire 32-bit address
> + * 64-bit, this is above 4GB to leave the entire 32-bit address
>   * space open for things that want to use the area for 32-bit pointers.
>   */
>  #define ELF_ET_DYN_BASE		(mmap_is_ia32() ? 0x000400000UL : \
> -						  0x100000000UL)
> +						  (TASK_SIZE / 3 * 2))
>  
>  /* This yields a mask that user programs can use to figure out what
>     instruction set this CPU supports.  This could be done in user space,
> -- 
> 2.7.4
> 
> 
> -- 
> Kees Cook
> Pixel Security

-- 
Kees Cook