Re: [PATCH v3] ARM: smp: add support for per-task stack canaries

[Guenter Roeck]

On 3/11/20 10:21 AM, Kees Cook wrote:
> On Mon, Mar 09, 2020 at 09:49:31AM -0700, Guenter Roeck wrote:
>> On Thu, Dec 06, 2018 at 09:32:57AM +0100, Ard Biesheuvel wrote:
>>> On ARM, we currently only change the value of the stack canary when
>>> switching tasks if the kernel was built for UP. On SMP kernels, this
>>> is impossible since the stack canary value is obtained via a global
>>> symbol reference, which means
>>> a) all running tasks on all CPUs must use the same value
>>> b) we can only modify the value when no kernel stack frames are live
>>>    on any CPU, which is effectively never.
>>>
>>> So instead, use a GCC plugin to add a RTL pass that replaces each
>>> reference to the address of the __stack_chk_guard symbol with an
>>> expression that produces the address of the 'stack_canary' field
>>> that is added to struct thread_info. This way, each task will use
>>> its own randomized value.
>>>
>>> Cc: Russell King <linux@xxxxxxxxxxxxxxx>
>>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>>> Cc: Emese Revfy <re.emese@xxxxxxxxx>
>>> Cc: Arnd Bergmann <arnd@xxxxxxxx>
>>> Cc: Laura Abbott <labbott@xxxxxxxxxx>
>>> Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
>>> Acked-by: Nicolas Pitre <nico@xxxxxxxxxx>
>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>>
>> Since this patch is in the tree, cc-option no longer works on
>> the arm architecture if CONFIG_STACKPROTECTOR_PER_TASK is enabled.
>>
>> Any idea how to fix that ? 
> 
> I thought Arnd sent a patch to fix it and it got picked up?
> 

Yes, but the fix is not upstream (it is only in -next), and I missed it.

Guenter