The xen_failsafe_callback function in Xen for the Linux kernel 2.6.23 and other versions, when running a 32-bit PVOPS guest, allows local users to cause a denial of service (guest crash) by triggering an iret fault, leading to use of an incorrect stack pointer and stack corruption.
http://rhn.redhat.com/errata/RHSA-2013-0496.html
http://www.openwall.com/lists/oss-security/2013/01/16/6
http://www.openwall.com/lists/oss-security/2013/01/16/8
http://www.securityfocus.com/bid/57433
http://www.ubuntu.com/usn/USN-1725-1