From $URL : A flaw was found in the way xen_failsafe_callback() handled failed iret, which causes the stack pointer to be wrong when entering the iret_exc error path. An unprivileged local guest user in the 32-bit PV Xen domain could use this flaw to crash the guest. References: http://www.openwall.com/lists/oss-security/2013/01/16/6
There are no longer any 2.x kernels available in the repository with the exception of sys-kernel/xbox-sources which is unsupported by security.