[PATCH] userfaultfd: fix remap event with MREMAP_DONTUNMAP.

[Brian Geffon]

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.

Signed-off-by: Brian Geffon <bgef...@google.com>
---
 mm/mremap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index c881abeba0bf..6aa6ea605068 100644
--- a/mm/mremap.c
+++ b/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;
 }
-- 
2.26.2.526.g744177e7f7-goog