[PATCH v4 0/5] ARM: support THREAD_INFO_IN_TASK

[Arnd Bergmann]

On Mon, Sep 13, 2021 at 12:39 PM Ard Biesheuvel <ardb at kernel.org> wrote:
>
> Placing thread_info in the kernel stack leaves it vulnerable to stack
> overflow attacks. This short series addresses that by using the existing
> THREAD_INFO_IN_TASK infrastructure.
>
> This v4 is a follow-up to Keith's v3 [0], which did not address all
> concerns I raised in response to v2. After collaborating with Keith
> off-list, we decided that I should go ahead and post our joint v4.
>
> Changes since v3:
>
> - Leave the CPU field in thread_info, and keep it in sync at context
>   switch time. This is by far the easiest and cleanest way to work
>   around the fact that it is infeasible to implement
>   raw_smp_processor_id() in terms of task_struct::cpu (for reasons of
>   header soup).
>
> - Drop the VFP changes, they are no longer necessary given the previous
>   point.
>
> - Drop the change to pass the CPU number to secondary_start_kernel().
>   Given that we also need to pass the idle task pointer, which carries
>   the CPU number, passing the CPU number directly is redundant.
>
> - Use the TPIDRURO register to carry 'current' while running in the
>   kernel, and keep using TPIDRPRW for the per-CPU offset as before. This
>   way, there is no need to make any changes to the way the per-CPU offsets
>   are programmed. It also avoids the concurrency issues that would
>   result from carrying the 'current' pointer in a per-CPU variable.
>
> - Update the per-task stack protector plugin to pull the stack canary
>   value directly from the task struct.

I haven't looked in detail, but it all looks great to me so far.

On a related note, this reminds me of the CONFIG_IRQSTACK series
that was posted last year[1] and probably attempted before that.
I'd have to get back to understanding all the details of that discussion,
but I have some hope that adding irqstacks would become much
easier after your series. Any thoughts on that?

       Arnd

[1] https://lore.kernel.org/linux-arm-kernel/1602141333-17822-1-git-send-email-maninder1.s@samsung.com/