Xen 3.1 through 4.x, when running 64-bit hosts on Intel CPUs, does not clear the NT flag when using an IRET after a SYSENTER instruction, which allows PV guest users to cause a denial of service (hypervisor crash) by triggering a #GP fault, which is not properly handled by another IRET instruction.
lists.fedoraproject.org/pipermail/package-announce/2013-May/104537.html
lists.opensuse.org/opensuse-security-announce/2014-03/msg00015.html
lists.opensuse.org/opensuse-security-announce/2014-03/msg00021.html
lists.opensuse.org/opensuse-security-announce/2014-04/msg00000.html
lists.opensuse.org/opensuse-updates/2013-06/msg00049.html
secunia.com/advisories/55082
security.gentoo.org/glsa/glsa-201309-24.xml
www.debian.org/security/2012/dsa-2662
www.openwall.com/lists/oss-security/2013/04/18/8
www.securitytracker.com/id/1028455