The copy_to_user function in the PAL emulation functionality for Xen 3.1.2 and earlier, when running on ia64 systems, allows HVM guest users to access arbitrary physical memory by triggering certain mapping operations.
http://secunia.com/advisories/28146
http://secunia.com/advisories/28643
http://www.redhat.com/support/errata/RHSA-2008-0089.html
http://www.securityfocus.com/bid/26954
http://xenbits.xensource.com/ext/ia64/xen-unstable.hg?rev/e6069a715fd7
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A9840