backup: verify if archive chunks are not reordered

Now, when file name is also integrity protected (prefixed to the
passphrase), we can make sure that input files are given in the same
order. And are parts of the same VM.

QubesOS/qubes-issues#971

qubes/backup.py

@@ -1303,7 +1303,19 @@ class ExtractWorker3(ExtractWorker2):
                 os.remove(filename)
                 continue
             else:
+                (basename, ext) = os.path.splitext(self.tar2_current_file)
+                previous_chunk_number = int(ext[1:])
+                expected_filename = basename + '.%03d' % (
+                    previous_chunk_number+1)
+                if expected_filename != filename:
+                    self.cleanup_tar2(wait=True, terminate=True)
+                    self.log.error(
+                        'Unexpected file in archive: {}, expected {}'.format(
+                            filename, expected_filename))
+                    os.remove(filename)
+                    continue
                 self.log.debug("Releasing next chunck")
+
             self.tar2_current_file = filename
 
             run_error = handle_streams(