[kernel-hardening] [PATCH 2/4] arm64: Reduce ELF_ET_DYN_BASE

[Kees Cook]

On Fri, Jun 23, 2017 at 6:52 AM, Kees Cook <keescook at chromium.org> wrote:
> On Thu, Jun 22, 2017 at 11:57 PM, Ard Biesheuvel
> <ard.biesheuvel at linaro.org> wrote:
>> Hi Kees,
>>
>> On 22 June 2017 at 18:06, Kees Cook <keescook at chromium.org> wrote:
>>> Now that explicitly executed loaders are loaded in the mmap region,
>>> position PIE binaries lower in the address space to avoid possible
>>> collisions with mmap or stack regions. For 64-bit, align to 4GB to
>>> allow runtimes to use the entire 32-bit address space for 32-bit
>>> pointers.
>>>
>>> Signed-off-by: Kees Cook <keescook at chromium.org>
>>> ---
>>>  arch/arm64/include/asm/elf.h | 13 ++++++-------
>>>  1 file changed, 6 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
>>> index 5d1700425efe..f742af8f7c42 100644
>>> --- a/arch/arm64/include/asm/elf.h
>>> +++ b/arch/arm64/include/asm/elf.h
>>> @@ -113,12 +113,13 @@
>>>  #define ELF_EXEC_PAGESIZE      PAGE_SIZE
>>>
>>>  /*
>>> - * This is the location that an ET_DYN program is loaded if exec'ed.  Typical
>>> - * use of this is to invoke "./ld.so someprog" to test out a new version of
>>> - * the loader.  We need to make sure that it is out of the way of the program
>>> - * that it will "exec", and that there is sufficient room for the brk.
>>> + * 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
>>> + * space open for things that want to use the area for 32-bit pointers.
>>>   */
>>> -#define ELF_ET_DYN_BASE        (2 * TASK_SIZE_64 / 3)
>>> +#define ELF_ET_DYN_BASE                (test_thread_flag(TIF_32BIT) ?  \
>>> +                                       0x000400000UL :         \
>>> +                                       0x100000000UL)
>>>
>>
>> Why are you merging this with the COMPAT definition?
>
> It seemed like the right thing to do since a single definition could
> handle both cases. Is there something I'm overlooking in the arm64
> case?

And like 5 minutes later there was a loud head-slapping noise in my
office. Durr, yeah, arm64 doesn't have to deal with a "native 32-bit"
mode, so the merge isn't needed. Yes yes, I will split it back up and
drop the thread flag test.

Thanks!

-Kees

>>>  #ifndef __ASSEMBLY__
>>>
>>> @@ -173,8 +174,6 @@ extern int arch_setup_additional_pages(struct linux_binprm *bprm,
>>>
>>>  #ifdef CONFIG_COMPAT
>>>
>>> -#define COMPAT_ELF_ET_DYN_BASE         (2 * TASK_SIZE_32 / 3)
>>> -
>>>  /* AArch32 registers. */
>>>  #define COMPAT_ELF_NGREG               18
>>>  typedef unsigned int                   compat_elf_greg_t;
>>> --
>>> 2.7.4
>>>
>
>
>
> --
> Kees Cook
> Pixel Security



-- 
Kees Cook
Pixel Security