[patch 4/7] userfaultfd: fix remap event with MREMAP_DONTUNMAP

[Andrew Morton <akpm@linux-foundation.org>]

From: Brian Geffon <bgeffon@google.com>
Subject: 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.

Link: http://lkml.kernel.org/r/20200506172158.218366-1-bgeffon@google.com
Fixes: e346b3813067d4b ("mm/mremap: add MREMAP_DONTUNMAP to mremap()")
Signed-off-by: Brian Geffon <bgeffon@google.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: "Michael S . Tsirkin" <mst@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Sonny Rao <sonnyrao@google.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/mremap.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/mremap.c~userfaultfd-fix-remap-event-with-mremap_dontunmap
+++ a/mm/mremap.c
@@ -794,7 +794,7 @@ out:
 	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;
 }
_