Xen 4.3.x and earlier does not properly handle certain errors, which allows local HVM guests to obtain hypervisor stack memory via a (1) port or (2) memory mapped I/O write or (3) other unspecified operations related to addresses without associated memory.
http://www.openwall.com/lists/oss-security/2013/09/30/1
http://www.debian.org/security/2014/dsa-3006
http://security.gentoo.org/glsa/glsa-201407-03.xml
http://rhn.redhat.com/errata/RHSA-2013-1790.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