The fixuppagefault function in arch/x86/traps.c in Xen 4.0.1 and earlier on 64-bit platforms, when paravirtualization is enabled, does not verify that kernel mode is used to call the handlegdtldtmappingfault function, which allows guest OS users to cause a denial of service (host OS BUGON) via a crafted memory access.

Problem description: Intel VT-d chipsets without interrupt remapping do not prevent a guest which owns a PCI device from using DMA to generate MSI interrupts by writing to the interrupt injection registers. This can be exploited to inject traps and gain control of the host.

References: http://lists.xensource.com/archives/html/xen-devel/2011-05/msg00687.html http://theinvisiblethings.blogspot.com/2011/05/following-white-rabbit-software-attacks.html http://www.invisiblethingslab.com/resources/2011/Software%20Attacks%20on%20Intel%20VT-d.pdf