This issue was reported to bugtraq here: http://marc.theaimsgroup.com/?l=bugtraq&m=112656096125857&w=2 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. http://rhn.redhat.com/errata/RHSA-2005-782.html