Xen 4.5.x through 4.7.x do not implement Supervisor Mode Access Prevention (SMAP) whitelisting in 32-bit exception and event delivery, which allows local 32-bit PV guest OS kernels to cause a denial of service (hypervisor and VM crash) by triggering a safety check.
http://xenbits.xen.org/xsa/xsa183-unstable.patch
http://xenbits.xen.org/xsa/xsa183-4.6.patch
http://xenbits.xen.org/xsa/advisory-183.html
http://www.securitytracker.com/id/1036447