userfaultfd: fix remap event with MREMAP_DONTUNMAP

A user is not required to set a new address when using MREMAP_DONTUNMAP
as it can be used without MREMAP_FIXED.  When doing so the remap event
will use new_addr which may not have been set and we didn't propagate it
back other then in the return value of remap_to.

Because ret is always the new address it's probably more correct to use
it rather than new_addr on the remap_event_complete call, and it
resolves this bug.

Fixes: e346b38 ("mm/mremap: add MREMAP_DONTUNMAP to mremap()")
Reported-by: Randy Dunlap <[email protected]>
Signed-off-by: Brian Geffon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Lokesh Gidra <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: "Michael S . Tsirkin" <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Sonny Rao <[email protected]>
Cc: Joel Fernandes <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

mm/mremap.c

@@ -794,7 +794,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	if (locked && new_len > old_len)
 		mm_populate(new_addr + old_len, new_len - old_len);
 	userfaultfd_unmap_complete(mm, &uf_unmap_early);
-	mremap_userfaultfd_complete(&uf, addr, new_addr, old_len);
+	mremap_userfaultfd_complete(&uf, addr, ret, old_len);
 	userfaultfd_unmap_complete(mm, &uf_unmap);
 	return ret;
 }