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

[Guenter Roeck]

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 ? 

Thanks,
Guenter