userfaultfd: fix deadlock warning when locking src and dst VMAs

Use down_read_nested() to avoid the warning.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 867a43a ("userfaultfd: use per-vma locks in userfaultfd operations")
Reported-by: [email protected]
Signed-off-by: Lokesh Gidra <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Brian Geffon <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: Jann Horn <[email protected]> [Bug #2]
Cc: Kalesh Singh <[email protected]>
Cc: Lokesh Gidra <[email protected]>
Cc: Mike Rapoport (IBM) <[email protected]>
Cc: Nicolas Geoffray <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: SENSEIIIII

mm/userfaultfd.c

@@ -1444,7 +1444,8 @@ static int uffd_move_lock(struct mm_struct *mm,
 		 */
 		down_read(&(*dst_vmap)->vm_lock->lock);
 		if (*dst_vmap != *src_vmap)
-			down_read(&(*src_vmap)->vm_lock->lock);
+			down_read_nested(&(*src_vmap)->vm_lock->lock,
+					 SINGLE_DEPTH_NESTING);
 	}
 	mmap_read_unlock(mm);
 	return err;