[PATCH 23/46] mm/userfaultfd: write-lock the VMA before waking waiters when unregistering

[Suren Baghdasaryan]

On Tue, Oct 27, 2026 at 1:43 AM Lorenzo Stoakes (ARM) <ljs at kernel.org> wrote:
>
> On Tue, Oct 27, 2026 at 12:17:43AM +0100, David Hildenbrand (Arm) wrote:
> > userfaultfd_unregister() wakes pending missing faults before clearing the
> > VMA's userfaultfd registration. With per-VMA fault locking, holding
> > mmap_lock for write does not prevent another fault from queuing after
> > the wakeup but before userfaultfd_clear_vma() write-locks the VMA. That
> > waiter can then remain asleep indefinitely.
>
> Oof.
>
> I guess userfaultfd_clear_vma() only implicitly takes the VMA write lock via
> vma_modify_flags_uffd().
>
> >
> > Call vma_start_write() before waking waiters. Existing fault handlers
> > queue themselves before dropping the VMA read lock, and new readers are
> > excluded until mmap_write_unlock().
>
> Clearly an oversight from the VMA locks work. I guess was always a Herculean
> task to catch every single point at which there is an implicit assumption that
> mmap write lock ~ readers (esp. faults) excluded.
>
> >
> > Fixes: 29a22b9e08d7 ("mm: handle userfaults under VMA lock")
> > Cc: stable at vger.kernel.org
> > Reported-by: Josef Bacik <josef at toxicpanda.com>
> > Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
>
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>

Reviewed-by: Suren Baghdasaryan <surenb at google.com>

Thanks!

>
> > ---
> >  mm/userfaultfd.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 669f71d485d02..7e30b5ff17be1 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -3991,6 +3991,9 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> >                       start = vma->vm_start;
> >               vma_end = min(end, vma->vm_end);
> >
> > +             /* Prevent new userfaults from being queued after the wakeup. */
> > +             vma_start_write(vma);
> > +
> >               if (userfaultfd_missing(vma)) {
> >                       /*
> >                        * Wake any concurrent pending userfault while
> >
> > --
> > 2.43.0
> >
>
> --
> Cheers, Lorenzo