Re: [PATCH] x86/mm: disable preemption during CR3 read+write

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Aug 5, 2016 at 6:37 AM, Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
> Usually current->mm (and therefore mm->pgd) stays the same during the
> lifetime of a task so it does not matter if a task gets preempted during
> the read and write of the CR3.
>
> But then, there is this scenario on x86-UP:
> TaskA is in do_exit() and exit_mm() sets current->mm = NULL followed by
> mmput() -> exit_mmap() -> tlb_finish_mmu() -> tlb_flush_mmu() ->
> tlb_flush_mmu_tlbonly() -> tlb_flush() -> flush_tlb_mm_range() ->
> __flush_tlb_up() -> __flush_tlb() ->  __native_flush_tlb().
>
> At this point current->mm is NULL but current->active_mm still points to
> the "old" mm.
> Let's preempt taskA _after_ native_read_cr3() by taskB. TaskB has its
> own mm so CR3 has changed.
> Now preempt back to taskA. TaskA has no ->mm set so it borrows taskB's
> mm and so CR3 remains unchanged. Once taskA gets active it continues
> where it was interrupted and that means it writes its old CR3 value
> back. Everything is fine because userland won't need its memory
> anymore.

This should affect kernel threads too, right?

Acked-by: Andy Lutomirski <luto@kernel.org>

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