Re: [PATCH 1/1] userfaultfd: avoid huge_zero_page in UFFDIO_MOVE

[Suren Baghdasaryan]

On Fri, Jan 12, 2024 at 12:57 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 12.01.24 02:39, Suren Baghdasaryan wrote:
> > While testing UFFDIO_MOVE ioctl, syzbot triggered VM_BUG_ON_PAGE caused
> > by a call to PageAnonExclusive() with a huge_zero_page as a parameter.
> > UFFDIO_MOVE does not yet handle zeropages and returns EBUSY when one is
> > encountered. Add an early huge_zero_page check in the PMD move path
> > to avoid this situation.
> >
> > Reported-by: syzbot+705209281e36404998f6@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > ---
> > Applies cleanly over linux-next, mm-stable and mm-unstable branches
> >
> >   mm/userfaultfd.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> >
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 216ab4c8621f..20e3b0d9cf7e 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -1393,6 +1393,12 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
> >                               err = -ENOENT;
> >                               break;
> >                       }
> > +                     /* Avoid moving zeropages for now */
> > +                     if (is_huge_zero_pmd(*src_pmd)) {
> > +                             spin_unlock(ptl);
> > +                             err = -EBUSY;
> > +                             break;
> > +                     }
>
> As an alternative (and more future proof?) just reject anything that is
> not an anon folio as well?

Yes, but I'm going to remove this check shortly, once zeropage
movement patch is ready.

>
> Anyhow, this should handle the issue at hand.
>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks!

>
> --
> Cheers,
>
> David / dhildenb
>