The xenlight library (libxl) in Xen 4.0.x through 4.2.x, when IOMMU is disabled, provides access to a busmastering-capable PCI passthrough device before the IOMMU setup is complete, which allows local HVM guest domains to gain privileges or cause a denial of service via a DMA instruction.
http://www.openwall.com/lists/oss-security/2013/09/10/4
http://www.debian.org/security/2014/dsa-3006
http://security.gentoo.org/glsa/glsa-201407-03.xml
http://lists.xen.org/archives/html/xen-devel/2013-07/msg00066.html
http://lists.opensuse.org/opensuse-security-announce/2014-03/msg00021.html