This issue was reported to bugtraq here:
The issue is that if a non root user is able to run umount -r, it is possible to
subvert the restrictions placed on a filesystem such as nosuid.
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on the solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.