Linux kernel before 2.6.9, when running on the AMD64 and Intel EM64T architectures, allows local users to write to privileged IO ports via the OUTS instruction.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A10320
http://www.trustix.org/errata/2006/0006
http://www.securityfocus.com/bid/12598
http://www.redhat.com/support/errata/RHSA-2005-293.html