arch/x86/x86_emulate/x86_emulate.c in Xen 3.2.1 through 4.4.x does not properly check privileges, which allows local HVM guest users to gain privileges or cause a denial of service (crash) via a crafted (1) CALL, (2) JMP, (3) RETF, (4) LCALL, (5) LJMP, or (6) LRET far branch instruction.
https://security.gentoo.org/glsa/201504-04
https://exchange.xforce.ibmcloud.com/vulnerabilities/98768
http://xenbits.xen.org/xsa/advisory-110.html
http://www.securityfocus.com/bid/71151
http://www.debian.org/security/2015/dsa-3140
http://support.citrix.com/article/CTX201794
http://support.citrix.com/article/CTX200288
http://secunia.com/advisories/62672
http://secunia.com/advisories/62537
http://lists.opensuse.org/opensuse-security-announce/2015-02/msg00010.html
http://lists.opensuse.org/opensuse-security-announce/2015-02/msg00005.html