Re: [PATCH] userfaultfd: prevent registration of special VMAs

[Mike Rapoport]

On Thu, Jun 18, 2026 at 11:25:31AM +0200, David Hildenbrand (Arm) wrote:
> On 6/18/26 11:21, Mike Rapoport wrote:
> > On Thu, Jun 18, 2026 at 10:47:19AM +0200, David Hildenbrand (Arm) wrote:
> >> On 6/18/26 10:43, Mike Rapoport wrote:
> >>>
> >>> Ah, hugetlb sets VM_DONTEXPAND, so it must me excluded to allow uffd with
> >>> hugetlb.
> >>
> >> It would probably be cleaner to just allow hugetlb, and then check for
> >> VM_SPECIAL if not hugetlb.
> > 
> > Cleaner in what sense?
> > Will be uglier for sure, just take a look at vma_can_userfault().
> 
> I was thinking of this:
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 180bad42fc79..8a6803618a91 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -2029,7 +2029,10 @@ bool vma_can_userfault(struct vm_area_struct *vma,
> vm_flags_t vm_flags,
>  {
>         const struct vm_uffd_ops *ops = vma_uffd_ops(vma);
> 
> -       if (vma->vm_flags & VM_DROPPABLE)
> +       if (vma->vm_flags & (VM_DROPPABLE | VM_SHADOW_STACK))
> +               return false;
> +
> +       if (!is_vm_hugetlb_page(vma) && (vma->vm_flags & VM_SPECIAL))
>                 return false;

In a way that's an extra check for hugetlb, but it will work.
Will respin.

>         vm_flags &= __VM_UFFD_FLAGS;
> 
> -- 
> Cheers,
> David

-- 
Sincerely yours,
Mike.