Xen 4.1.x and 4.2.x, when the XSA-45 patch is in place, does not properly maintain references on pages stored for deferred cleanup, which allows local PV guest kernels to cause a denial of service (premature page free and hypervisor crash) or possibly gain privileges via unspecified vectors.
http://www.openwall.com/lists/oss-security/2013/06/26/5
http://www.debian.org/security/2014/dsa-3006
http://support.citrix.com/article/CTX138134
http://security.gentoo.org/glsa/glsa-201309-24.xml
http://secunia.com/advisories/55082
http://lists.opensuse.org/opensuse-security-announce/2014-03/msg00021.html