Re: [PATCH 6.18.y] userfaultfd: prevent registration of special VMAs

On Thu, Aug 06, 2026 at 11:34:59PM -0400, Sasha Levin wrote:
> From: "Mike Rapoport (Microsoft)" <[email protected]>
> 
> [ Upstream commit 3c58f641e813c3c71039f8fd4d4e2a3aab713288 ]
> 
> 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_SPECIAL so that if a driver would implement
> vm_uffd_ops, it wouldn't be possible to register special VMAs with
> userfaultfd.
> 
> Since VM_SPECIAL includes VM_DONTEXPAND which is set but hugetlb, exclude
> hugetlb VMAs from the check for VM_SPECIAL.
> 
> Link: https://lore.kernel.org/[email protected]
> Fixes: 54007f818206 ("mm: Introduce VM_SHADOW_STACK for shadow stack memory")
> Signed-off-by: Mike Rapoport (Microsoft) <[email protected]>
> Reported-by: vova tokarev <[email protected]>
> Acked-by: David Hildenbrand (Arm) <[email protected]>
> Reviewed-by: Lorenzo Stoakes <[email protected]>
> Cc: Al Viro <[email protected]>
> Cc: Christian Brauner <[email protected]>
> Cc: Jan Kara <[email protected]>
> Cc: Linus Torvalds <[email protected]>
> Cc: Mike Rapoport <[email protected]>
> Cc: Oleg Nesterov <[email protected]>
> Cc: Peter Xu <[email protected]>
> Cc: <[email protected]>
> Signed-off-by: Andrew Morton <[email protected]>
> Signed-off-by: Sasha Levin <[email protected]>
> ---
>  include/linux/userfaultfd_k.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index c0e716aec26aa..796a380664bd8 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -214,7 +214,10 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
>  {
>  	vm_flags &= __VM_UFFD_FLAGS;
>  
> -	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))

Yeah, that's the right place :)

Thanks!

>  		return false;
>  
>  	if ((vm_flags & VM_UFFD_MINOR) &&
> -- 
> 2.53.0
> 

-- 
Sincerely yours,
Mike.