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.
https://bugzilla.redhat.com/show_bug.cgi?id=896038
http://www.ubuntu.com/usn/USN-1728-1
http://www.ubuntu.com/usn/USN-1725-1
http://www.securityfocus.com/bid/57433
http://www.openwall.com/lists/oss-security/2013/01/16/8