The xrstor function in arch/x86/xstate.c in Xen 4.x does not properly handle writes to the hardware FSW.ES bit when running on AMD64 processors, which allows local guest OS users to obtain sensitive register content information from another guest by leveraging pending exception and mask bits. NOTE: this vulnerability exists because of an incorrect fix for CVE-2013-2076.
http://xenbits.xen.org/xsa/xsa172.patch
http://xenbits.xen.org/xsa/xsa172-4.3.patch
http://xenbits.xen.org/xsa/advisory-172.html
http://www.securitytracker.com/id/1035435
http://www.securityfocus.com/bid/85714
http://www.oracle.com/technetwork/topics/security/ovmbulletinjul2016-3090546.html
http://www.debian.org/security/2016/dsa-3554
http://support.citrix.com/article/CTX209443
http://lists.fedoraproject.org/pipermail/package-announce/2016-April/181729.html
http://lists.fedoraproject.org/pipermail/package-announce/2016-April/181699.html