Re: [PATCH v1 1/2] userfaultfd: clear the inherited uffd bit in move_swap_pte()

[Kiryl Shutsemau <kirill@shutemov.name>]

On Sat, Sep 19, 2026 at 09:46:29AM +0900, Donggeun Yoo wrote:
> UFFDIO_MOVE on a swapped-out page installs the source PTE at the
> destination unchanged, so a uffd bit set on the source lands in a
> destination VMA that was never registered for write protection.  It is
> then permanent: the bit is dropped only by change_protection() under
> MM_CP_UFFD_{WP,RWP}_RESOLVE, which uffd_wp_range(), mrwprotect_range()
> and userfaultfd_clear_vma() issue only for a VMA registered in that
> mode.  pagemap reports the page as uffd-tracked, and MADV_COLLAPSE
> refuses the range while the bit is set, because collapse_scan_pmd() is
> strict about uffd on swap entries.
> 
> move_present_ptes() and move_zeropage_pte() build the destination PTE
> from dst_vma->vm_page_prot and arm RWP only when dst_vma asks for it, so
> the destination's own registration decides the result.  move_swap_pte()
> copies the source PTE instead and only ever sets the bit, never clears
> it, so one UFFDIO_MOVE behaves differently depending on whether the page
> happened to be resident.
> 
> Clear the uffd bit on the moved swap entry unless the destination is
> RWP-registered, as copy_nonpresent_pte() does where it installs a PTE
> into a destination that may not be armed.  A WP-registered destination
> stops inheriting the bit as well, which is already what it gets when the
> moved page is resident.
> 
> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
> ---
>  mm/userfaultfd.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 74f04c323c50..6495666c596b 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1452,6 +1452,8 @@ static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma,
>  	/* Re-arm RWP on the moved swap entry if dst_vma is RWP-registered. */
>  	if (userfaultfd_rwp(dst_vma))
>  		orig_src_pte = pte_swp_mkuffd(orig_src_pte);
> +	else
> +		orig_src_pte = pte_swp_clear_uffd(orig_src_pte);

Putting it in the 'else' is wrong. Clear uffd on the source
unconditionally and re-arm based on the target VMA:

	orig_src_pte = pte_swp_clear_uffd(orig_src_pte);
	/* Re-arm RWP on the moved swap entry if dst_vma is RWP-registered. */
	if (userfaultfd_rwp(dst_vma))
		orig_src_pte = pte_swp_mkuffd(orig_src_pte);

That is what move_pages_huge_pmd() does with clear_uffd_wp_pmd() before
its own re-arm, and it is the logically correct way to do this: the
source's state is gone, the destination decides.

>  	set_pte_at(mm, dst_addr, dst_pte, orig_src_pte);
>  	double_pt_unlock(dst_ptl, src_ptl);
>  
> -- 
> 2.53.0
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov