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

[Donggeun Yoo <donggeunyoo.kernel@gmail.com>]

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