UFFDIO_REGISTER can be issued on a range that is already registered in
the same userfaultfd context, replacing the VMA's userfaultfd tracking
mode.  For example, a range can be registered with
UFFDIO_REGISTER_MODE_WP and later re-registered with
UFFDIO_REGISTER_MODE_MISSING.

When the second registration removes VM_UFFD_WP, the VMA flags are
updated but existing uffd-wp state in page-table entries is left behind.
That stale state can survive in swap PTEs.  On swapin, do_swap_page()
restores _PAGE_UFFD_WP from the swap PTE and can then install a writable
PTE, triggering page_table_check:

  pte_uffd_wp(pte) && pte_write(pte)

Handle removal of WP mode through UFFDIO_REGISTER the same way as
UFFDIO_UNREGISTER: resolve the per-PTE uffd-wp state before dropping
VM_UFFD_WP from the VMA.

Also make the same-context fast path require an exact UFFD mode match.
The old subset check treats MISSING|WP -> MISSING as a no-op, even though
WP mode is being removed.

Fixes: f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration")
Reported-by: syzbot+18d274a59b87cf80e86d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=18d274a59b87cf80e86d
Signed-off-by: Jianhui Zhou <jianhuizzzzz@gmail.com>
---
 mm/userfaultfd.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 180bad42fc79..dc0b3eba768b 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -2153,13 +2153,21 @@ int userfaultfd_register_range(struct userfaultfd_ctx *ctx,
 		 * userfaultfd and with the right tracking mode too.
 		 */
 		if (vma->vm_userfaultfd_ctx.ctx == ctx &&
-		    vma_test_all_mask(vma, vma_flags))
+		    (vma->vm_flags & __VM_UFFD_FLAGS) == vm_flags)
 			goto skip;
 
 		if (vma->vm_start > start)
 			start = vma->vm_start;
 		vma_end = min(end, vma->vm_end);
 
+		/*
+		 * Re-registering into the same userfaultfd can remove WP mode.
+		 * Clear any per-PTE uffd-wp state before dropping VM_UFFD_WP,
+		 * matching the UFFDIO_UNREGISTER cleanup semantics.
+		 */
+		if (userfaultfd_wp(vma) && !(vm_flags & VM_UFFD_WP))
+			uffd_wp_range(vma, start, vma_end - start, false);
+
 		new_vma_flags = vma->flags;
 		vma_flags_clear_mask(&new_vma_flags, __VMA_UFFD_FLAGS);
 		vma_flags_set_mask(&new_vma_flags, vma_flags);

base-commit: e43ffb69e0438cddd72aaa30898b4dc446f664f8
-- 
2.43.0