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.
lists.opensuse.org/opensuse-security-announce/2015-02/msg00005.html
lists.opensuse.org/opensuse-security-announce/2015-02/msg00010.html
secunia.com/advisories/62537
secunia.com/advisories/62672
support.citrix.com/article/CTX200288
support.citrix.com/article/CTX201794
www.debian.org/security/2015/dsa-3140
www.securityfocus.com/bid/71151
xenbits.xen.org/xsa/advisory-110.html
exchange.xforce.ibmcloud.com/vulnerabilities/98768
security.gentoo.org/glsa/201504-04