Re: [PATCH 32/41] mm: prevent userfaults to be handled under per-vma lock

[Jann Horn]

On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> Due to the possibility of handle_userfault dropping mmap_lock, avoid fault
> handling under VMA lock and retry holding mmap_lock. This can be handled
> more gracefully in the future.
>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Suggested-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  mm/memory.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 20806bc8b4eb..12508f4d845a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5273,6 +5273,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
>         if (!vma->anon_vma)
>                 goto inval;
>
> +       /*
> +        * Due to the possibility of userfault handler dropping mmap_lock, avoid
> +        * it for now and fall back to page fault handling under mmap_lock.
> +        */
> +       if (userfaultfd_armed(vma))
> +               goto inval;

This looks racy wrt concurrent userfaultfd_register(). I think you'll
want to do the userfaultfd_armed(vma) check _after_ locking the VMA,
and ensure that the userfaultfd code write-locks the VMA before
changing the __VM_UFFD_FLAGS in vma->vm_flags.

>         if (!vma_read_trylock(vma))
>                 goto inval;
>
> --
> 2.39.0
>