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

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Message-ID: <04a8c31a-3c43-3dcf-c9fd-82ba225a19f6@roeck-us.net>
Date:   Wed, 11 Mar 2020 11:31:13 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux@...linux.org.uk, Emese Revfy <re.emese@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Laura Abbott <labbott@...hat.com>,
        kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v3] ARM: smp: add support for per-task stack canaries

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@...linux.org.uk>
>>> Cc: Kees Cook <keescook@...omium.org>
>>> Cc: Emese Revfy <re.emese@...il.com>
>>> Cc: Arnd Bergmann <arnd@...db.de>
>>> Cc: Laura Abbott <labbott@...hat.com>
>>> Cc: kernel-hardening@...ts.openwall.com
>>> Acked-by: Nicolas Pitre <nico@...aro.org>
>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>>
>> 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