arm64/v4.16-rc1: KASAN: use-after-free Read in finish_task_switch

[Mark Rutland]

Hi,

On Thu, Feb 15, 2018 at 10:08:56PM +0000, Mathieu Desnoyers wrote:
> My current theory: do_exit() gets preempted after having set current->mm
> to NULL, and after having issued mmput(), which brings the mm_count down
> to 0.
>
> Unfortunately, if the scheduler switches from a userspace thread
> to a kernel thread, context_switch() loads prev->active_mm which still
> points to the now-freed mm, mmgrab the mm, and eventually does mmdrop
> in finish_task_switch().

For this to happen, we need to get to the mmput() in exit_mm() with:

  mm->mm_count == 1
  mm->mm_users == 1
  mm == active_mm

... but AFAICT, this cannot happen.

If there's no context_switch between clearing current->mm and the
mmput(), then mm->mm_count >= 2, thanks to the prior mmgrab() and the
active_mm reference (in mm_count) that context_switch+finish_task_switch
manage.

If there is a context_switch between the two, then AFAICT, either:

a) The task re-inherits its old mm as active_mm, and mm_count >= 2. In
   context_switch we mmgrab() the active_mm to inherit it, and in
   finish_task_switch() we drop the oldmm, balancing the mmgrab() with
   an mmput().

   e.g we go task -> kernel_task -> task

b) At some point, another user task is scheduled, and we switch to its
   mm. We don't mmgrab() the active_mm, but we mmdrop() the oldmm, which
   means mm_count >= 1. Since we witched to a new mm, if we switch back
   to the first task, it cannot have its own mm as active_mm.

   e.g. we go task -> other_task -> task

I suspect we have a bogus mmdrop or mmput elsewhere, and do_exit() and
finish_task_switch() aren't to blame.

Thanks,
Mark.