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

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 23 June 2017 at 14:02, Kees Cook <keescook@chromium.org> wrote:
> On Fri, Jun 23, 2017 at 6:52 AM, Kees Cook <keescook@chromium.org> wrote:
>> On Thu, Jun 22, 2017 at 11:57 PM, Ard Biesheuvel
>> <ard.biesheuvel@linaro.org> wrote:
>>> Hi Kees,
>>>
>>> On 22 June 2017 at 18:06, Kees Cook <keescook@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@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.
>

Oh, is that what I heard?