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

[Suren Baghdasaryan]

On Tue, Jan 17, 2023 at 12:36 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> On Tue, Jan 17, 2023 at 8:51 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> > 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,
>
> I still think this change is needed...

Yes, I think you are right. I'll move the check after locking the VMA. Thanks!

>
> > and ensure that the userfaultfd code write-locks the VMA before
> > changing the __VM_UFFD_FLAGS in vma->vm_flags.
>
> Ah, but now I see you already took care of this half of the issue with
> the reset_vm_flags() change in
> https://lore.kernel.org/linux-mm/20230109205336.3665937-16-surenb@xxxxxxxxxx/
> .
>
>
> > >         if (!vma_read_trylock(vma))
> > >                 goto inval;
> > >
> > > --
> > > 2.39.0
> > >