Re: [PATCH v4 0/5] ARM: support THREAD_INFO_IN_TASK

On Tue, 14 Sept 2021 at 11:44, Arnd Bergmann [off-list ref] wrote:

On Mon, Sep 13, 2021 at 12:39 PM Ard Biesheuvel [off-list ref] wrote:

quoted

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?

Yes, not having to rely on the stack pointer for thread_info/current
seems to make this a lot easier. And eith mentioned that he is also
interested in vmap'ed stacks for ARM, so I have a suspicion that we
will end up picking up some IRQ stack changes in the process as well.

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