CVE 2011-1898
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."
See the 
CVE page on Mitre.org
for more details.
