On 3/11/20 1:45 PM, Russell King - ARM Linux admin wrote: > On Wed, Mar 11, 2020 at 11:47:20AM -0700, Kees Cook wrote: >> On Wed, Mar 11, 2020 at 11:31:13AM -0700, Guenter Roeck wrote: >>> 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. >> >> Ah, yes, I found it again now too; it went through rmk's tree. >> >> For thread posterity: >> >> ARM: 8961/2: Fix Kbuild issue caused by per-task stack protector GCC plugin >> https://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=8961/2 > > It's in my fixes branch, waiting for me to do my (now usual) push > of fixes to Linus. > > I'm not sure whether the above discussion is suggesting that there's > a problem with this patch, or whether it's trying to encourage me > to send it up to Linus. I _think_ there's the suggestion that it > causes a regression, hmm? > ARM: 8961/2: Fix Kbuild issue caused by per-task stack protector GCC plugin Commit 189af4657186da08 ("ARM: smp: add support for per-task stack canaries") caused a regression. "ARM: 8961/2: Fix Kbuild issue caused by per-task stack protector GCC plugin" fixes it. Guenter
