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

On Mon, Sep 13, 2021 at 12:39:56PM +0200, Ard Biesheuvel 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.

Overall, this looks much simpler than Keith's v3 to me, so probably
less likely to run into issues in the future. The only suggestion I
hvae is the comment I've made on patch 4.

Thanks.

-- 
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