[PATCH 1/1] userfaultfd: allow registration of WP_ASYNC for any VMA

[Mike Rapoport (Microsoft)]

Registration of a VMA with WP_ASYNC userfaulfd context in write-protect
mode does not require any VMA-specific resolution of user faults and
these faults are completely handled by the generic page fault handler.

This functionality existed since the introduction of WP_ASYNC mode and
it allows tracking writes to SysV shared memory mappings (shmget(2) and
shmat(2)).

Move the check for WP mode before checking for presence of ->uffd_ops in a
VMA to restore the original behaviour.

Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
---
 mm/userfaultfd.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index b55d4a8d88cc..436795bf218e 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -2044,22 +2044,22 @@ bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
 {
 	const struct vm_uffd_ops *ops = vma_uffd_ops(vma);
 
-	/* only VMAs that implement vm_uffd_ops are supported */
-	if (!ops)
-		return false;
-
 	vm_flags &= __VM_UFFD_FLAGS;
 
-	if (vma->vm_flags & VM_DROPPABLE)
-		return false;
-
 	/*
-	 * If wp async enabled, and WP is the only mode enabled, allow any
+	 * If WP is the only mode enabled and context is wp async, allow any
 	 * memory type.
 	 */
 	if (wp_async && (vm_flags == VM_UFFD_WP))
 		return true;
 
+	/* For any other mode reject VMAs that don't implement vm_uffd_ops */
+	if (!ops)
+		return false;
+
+	if (vma->vm_flags & VM_DROPPABLE)
+		return false;
+
 	/*
 	 * If user requested uffd-wp but not enabled pte markers for
 	 * uffd-wp, then only anonymous memory is supported
-- 
2.51.0