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

[David CARLIER]

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.


> +               return -EOPNOTSUPP;
> +       }
> +
>         folio = ops->alloc_folio(state->vma, state->dst_addr);
>         if (!folio)
>                 return -ENOMEM;
> --
> 2.53.0
>

Cheers.