per-task stack canaries for arm64

[Ramana Radhakrishnan]

On 1/17/18 8:45 PM, Kees Cook wrote:
> On Wed, Jan 17, 2018 at 12:32 PM, Ard Biesheuvel
> <ard.biesheuvel at linaro.org> wrote:
>> On 17 January 2018 at 19:10, Kees Cook <keescook at chromium.org> wrote:
>>> On Wed, Jan 17, 2018 at 10:24 AM, Ard Biesheuvel
>>> <ard.biesheuvel at linaro.org> wrote:
>> [...]
>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81708
>>> which was done for x86 only, and provides both:
>>>   -mstack-protector-guard-symbol=...
>>>   -mstack-protector-guard-reg=...
>>>
>>> If this could be extended to arm64, I think we'd be in good shape (and
>>> it could be trivially detected at build time).
>>>
>>
>> I'm not entirely sure what the point is of specifying the name of the
>> symbol on the command line. It is ultimately up to the GCC developers
>> to decide how much point there is to maintaining parity with x86 here.
>>
>> [...]
>>>> Ramana indicated at the time that he would be up for adding, e.g.,
>>>> -fstack-protector-linux-kernel as a command line option, and add the
>>>> contents of tpidr_el1 to every reference of __stack_chk_guard when
>>>> set.

Wow, that was a hall-way conversation eons ago. It took me a while to
page that in.


>>>
>>> I think we want to reuse the command-line names from the x86 options
>>> above, unless there's a good reason not to?
>>
>> I'm perfectly happy to settle for whatever the GCC developers manage
>> to agree on, as long as it gives us the ability to use tpidr_el1 as
>> the offset.
> 
> Ramana, Uroš, what's the best next step? Should we open a GCC bug
> specifically for arm64 here?

The next best step is someone opening a GCC feature request with some
more details - CC'ing me on ramana at gcc.gnu.org should work. What I would
like to see is a feature request on GCC bugzilla along with some
comments / buyin from the AArch64 kernel maintainers whether they would
like to see such a feature and what the behaviour should be and get some
feedback from the AArch64 GCC maintainers upstream before starting the
work.

I think the following (unoptimized) or some derivative of this should work.

        adrp    x19, __stack_chk_guard
        add     x19, x19, :lo12:__stack_chk_guard
	mrs 	x2, tpidr_el1
	add	x2, x2, x19
	ldr 	x2, [x2]


It's also not likely that this will be done in time for GCC 8 as we are
now in stage 4 and we're probably looking at GCC 9 for this assuming
this goes ahead.

Hope this helps.

regards
Ramana

> 
> -Kees
>