The fbld instruction emulation in Xen 3.3.x through 4.3.x does not use the correct variable for the source effective address, which allows local HVM guests to obtain hypervisor stack information by reading the values used by the instruction.
http://www.openwall.com/lists/oss-security/2013/09/30/3
http://www.debian.org/security/2014/dsa-3006
http://security.gentoo.org/glsa/glsa-201407-03.xml
http://lists.opensuse.org/opensuse-updates/2013-11/msg00009.html
http://lists.opensuse.org/opensuse-security-announce/2014-03/msg00021.html