The set_debugreg hypercall in include/asm-x86/debugreg.h in Xen 4.0, 4.1, and 4.2, and Citrix XenServer 6.0.2 and earlier, when running on x86-64 systems, allows local OS guest users to cause a denial of service (host crash) by writing to the reserved bits of the DR7 debug control register.
https://security.gentoo.org/glsa/201604-03
https://exchange.xforce.ibmcloud.com/vulnerabilities/78265
https://bugzilla.redhat.com/show_bug.cgi?id=851139
http://www.securitytracker.com/id?1027479
http://www.securityfocus.com/bid/55400
http://www.openwall.com/lists/oss-security/2012/09/05/5
http://www.debian.org/security/2012/dsa-2544
http://wiki.xen.org/wiki/Security_Announcements#XSA-12_hypercall_set_debugreg_vulnerability
http://support.citrix.com/article/CTX134708
http://security.gentoo.org/glsa/glsa-201309-24.xml
http://secunia.com/advisories/55082
http://secunia.com/advisories/51413
http://secunia.com/advisories/50530
http://secunia.com/advisories/50472
http://lists.xen.org/archives/html/xen-announce/2012-09/msg00000.html
http://lists.opensuse.org/opensuse-security-announce/2012-11/msg00018.html
http://lists.opensuse.org/opensuse-security-announce/2012-11/msg00017.html
http://lists.opensuse.org/opensuse-security-announce/2012-09/msg00018.html
http://lists.opensuse.org/opensuse-security-announce/2012-09/msg00017.html
http://lists.opensuse.org/opensuse-security-announce/2012-09/msg00012.html
http://lists.opensuse.org/opensuse-security-announce/2012-09/msg00005.html
http://lists.opensuse.org/opensuse-security-announce/2012-09/msg00004.html
http://lists.opensuse.org/opensuse-security-announce/2012-09/msg00003.html
http://lists.opensuse.org/opensuse-security-announce/2012-09/msg00001.html