Xen 4.4.x, 4.5.x, and 4.6.x does not limit the number of printk console messages when reporting unimplemented hypercalls, which allows local guests to cause a denial of service via a sequence of (1) HYPERVISOR_physdev_op hypercalls, which are not properly handled in the do_physdev_op function in arch/arm/physdev.c, or (2) HYPERVISOR_hvm_op hypercalls, which are not properly handled in the do_hvm_op function in arch/arm/hvm.c.
https://security.gentoo.org/glsa/201604-03
http://xenbits.xen.org/xsa/advisory-146.html
http://www.securitytracker.com/id/1034029
http://www.debian.org/security/2015/dsa-3414
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