Re: [PATCH v2 2/3] userfaultfd: refuse to __mfill_atomic_pte() for unsupported VMAs

[Mike Rapoport]

On Wed, May 27, 2026 at 08:09:01PM +0100, David CARLIER wrote:
> Hi,
> 
> just chiming it
> 
> On Wed, 27 May 2026 at 19:48, Mike Rapoport <rppt@xxxxxxxxxx> wrote:
> >
> > From: "Mike Rapoport (Microsoft)" <rppt@xxxxxxxxxx>
> >
> > __mfill_atomic_pte() unconditionally dereferences ops because there is an
> > assumption that VMAs that can undergo mfill_* operations are vetted on
> > registration and must have valid vm_uffd_ops.
> >
> > Add a guard against potential bugs and make sure __mfill_atomic_pte() bails
> > out if ops is NULL.
> >
> > Suggested-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> > Fixes: ad9ac3081332 ("userfaultfd: introduce vm_uffd_ops->alloc_folio()")
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
> > ---
> >  mm/userfaultfd.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index e5d2fb3ce2c1..2872c71bbf36 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -552,6 +552,11 @@ static int __mfill_atomic_pte(struct mfill_state *state,
> >         struct folio *folio;
> >         int ret;
> >
> > +       if (!ops) {
> > +               VM_WARN_ONCE(1, "UFFDIO_COPY for unsupported VMA");
> 
> nit: Is the warning message a bit too specific ? i.e. Also handle
> MFILL_ATOMIC_ZEROPAGE case.

It handles ZEROPAGE case by actually copying a zeroed page there so in a
sense it's still a copy.

Differentiating COPY and ZEROPAGE here will add complexity that is not
justified for a message that should be never printed.
 
> > +               return -EOPNOTSUPP;
> > +       }
> > +
> >         folio = ops->alloc_folio(state->vma, state->dst_addr);
> >         if (!folio)
> >                 return -ENOMEM;
> > --
> > 2.53.0
> >
> 
> Cheers.
> 

-- 
Sincerely yours,
Mike.