Xen 4.1 before 4.1.1 and 4.0 before 4.0.2, when using PCI passthrough on Intel VT-d chipsets that do not have interrupt remapping, allows guest OS users to gain host OS privileges by "using DMA to generate MSI interrupts by writing to the interrupt injection registers."
http://xen.org/download/index_4.0.2.html
http://theinvisiblethings.blogspot.com/2011/05/following-white-rabbit-software-attacks.html
http://lists.opensuse.org/opensuse-security-announce/2011-08/msg00018.html
http://lists.opensuse.org/opensuse-security-announce/2011-08/msg00017.html
http://lists.fedoraproject.org/pipermail/package-announce/2011-June/062139.html
http://lists.fedoraproject.org/pipermail/package-announce/2011-June/062112.html