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

[Mike Rapoport]

On Thu, Jun 18, 2026 at 10:19:17AM +0200, David Hildenbrand (Arm) wrote:
> On 6/17/26 21:40, Mike Rapoport wrote:
> > From: "Mike Rapoport (Microsoft)" <rppt@xxxxxxxxxx>
> > 
> > Vova Tokarev says:
> > 
> >   userfaultfd allows registration on shadow stack VMAs.  With userfaultfd
> >   access, you can register on the shadow stack, discard a page ... and
> >   inject a page with chosen return addresses via UFFDIO_COPY.
> > 
> > Update vma_can_userfault() to reject VM_SHADOW_STACK.
> > 
> > While on it, also reject VM_IO, VM_MIXEDMAP and VM_PFNMAP so that if a
> > driver would implement vm_uffd_ops, it wouldn't be possible to register
> > special VMAs with userfaultfd.
> > 
> > Reported-by: vova tokarev <vladimirelitokarev@xxxxxxxxx>
> > Fixes: 54007f818206 ("mm: Introduce VM_SHADOW_STACK for shadow stack memory")
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
> > ---
> >  mm/userfaultfd.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 246af12bf801..b8d2d87ce8d7 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -2111,7 +2111,8 @@ static 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_IO | VM_MIXEDMAP | VM_PFNMAP |
> > +			     VM_SHADOW_STACK))
> 
> I'm sure you considered VM_SPECIAL, which additionally includes VM_DONTEXPAND.
> 
> Would that be better, or what was the reason to allow VM_DONTEXPAND?

By itself VM_DONTEXPAND won't matter, as uffd can't resize a VMA.
But thinking more about it, it's better to make vma_can_userfault() more
restrictive and just use VM_SPECIAL.
 
> -- 
> Cheers,
> 
> David

-- 
Sincerely yours,
Mike.