The hypercall_create_continuation function in arch/arm/domain.c in Xen 4.4.x through 4.6.x allows local guest users to cause a denial of service (host crash) via a preemptible hypercall to the multicall interface.
lists.fedoraproject.org/pipermail/package-announce/2015-November/171082.html
lists.fedoraproject.org/pipermail/package-announce/2015-November/171185.html
lists.fedoraproject.org/pipermail/package-announce/2015-November/171249.html
www.debian.org/security/2015/dsa-3414
www.securitytracker.com/id/1034031
xenbits.xen.org/xsa/advisory-145.html
security.gentoo.org/glsa/201604-03