The outs instruction emulation in Xen 3.1.x, 4.2.x, 4.3.x, and earlier, when using FS: or GS: segment override, uses an uninitialized variable as a segment base, which allows local 64-bit PV guests to obtain sensitive information (hypervisor stack content) via unspecified vectors related to stale data in a segment register.
https://exchange.xforce.ibmcloud.com/vulnerabilities/87799
http://www.openwall.com/lists/oss-security/2013/10/10/10
http://www.debian.org/security/2014/dsa-3006
http://security.gentoo.org/glsa/glsa-201407-03.xml
http://rhn.redhat.com/errata/RHSA-2013-1449.html
http://lists.opensuse.org/opensuse-updates/2013-11/msg00009.html
http://lists.opensuse.org/opensuse-security-announce/2014-04/msg00000.html
http://lists.opensuse.org/opensuse-security-announce/2014-03/msg00021.html
http://lists.opensuse.org/opensuse-security-announce/2014-03/msg00015.html