The qdisk PV disk backend in qemu-xen in Xen 4.2.x and 4.3.x before 4.3.1, and qemu 1.1 and other versions, allows local HVM guests to cause a denial of service (domain grant reference consumption) via unspecified vectors.
http://xenproject.org/downloads/xen-archives/supported-xen-43-series/xen-431.html
http://www.ubuntu.com/usn/USN-2092-1