mm/userfaultfd: reset ptes when close() for wr-protected ones

Userfaultfd unregister includes a step to remove wr-protect bits from all
the relevant pgtable entries, but that only covered an explicit
UFFDIO_UNREGISTER ioctl, not a close() on the userfaultfd itself.  Cover
that too.  This fixes a WARN trace.

The only user visible side effect is the user can observe leftover
wr-protect bits even if the user close()ed on an userfaultfd when
releasing the last reference of it.  However hopefully that should be
harmless, and nothing bad should happen even if so.

This change is now more important after the recent page-table-check
patch we merged in mm-unstable (446dd9a ("mm/page_table_check:
support userfault wr-protect entries")), as we'll do sanity check on
uffd-wp bits without vma context.  So it's better if we can 100%
guarantee no uffd-wp bit leftovers, to make sure each report will be
valid.

Link: https://lore.kernel.org/all/[email protected]/
Fixes: f369b07 ("mm/uffd: reset write protection when unregister with wp-mode")
Analyzed-by: David Hildenbrand <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Reported-by: [email protected]
Signed-off-by: Peter Xu <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

fs/userfaultfd.c

@@ -895,6 +895,10 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 			prev = vma;
 			continue;
 		}
+		/* Reset ptes for the whole vma range if wr-protected */
+		if (userfaultfd_wp(vma))
+			uffd_wp_range(vma, vma->vm_start,
+				      vma->vm_end - vma->vm_start, false);
 		new_flags = vma->vm_flags & ~__VM_UFFD_FLAGS;
 		vma = vma_modify_flags_uffd(&vmi, prev, vma, vma->vm_start,
 					    vma->vm_end, new_flags,