The p2m_pod_emergency_sweep function in arch/x86/mm/p2m-pod.c in Xen 3.4.x, 3.5.x, and 3.6.x is not preemptible, which allows local x86 HVM guest administrators to cause a denial of service (CPU consumption and possibly reboot) via crafted memory contents that triggers a "time-consuming linear scan," related to Populate-on-Demand.
https://security.gentoo.org/glsa/201604-03
http://xenbits.xen.org/xsa/advisory-150.html
http://www.securitytracker.com/id/1034034
http://www.securityfocus.com/bid/77362
http://www.debian.org/security/2015/dsa-3414
http://support.citrix.com/article/CTX202404
http://lists.opensuse.org/opensuse-updates/2015-12/msg00053.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-November/171249.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-November/171185.html
http://lists.fedoraproject.org/pipermail/package-announce/2015-November/171082.html