[PATCH] mm: Revert x86_64 and arm64 ELF_ET_DYN_BASE base

[keescook@chromium.org (Kees Cook)]

On Tue, Aug 8, 2017 at 2:34 AM, Will Deacon <will.deacon@arm.com> wrote:
> 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).
>>
>> 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@google.com>
>> Cc: stable at vger.kernel.org
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>>  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__
>
> Acked-by: Will Deacon <will.deacon@arm.com>
>
> I assume this is going via akpm like the original commit?

That's my expectation, yup. Thanks for the Ack!

-Kees

-- 
Kees Cook
Pixel Security