The hypervisor_callback function in Xen, possibly before 3.4.0, as applied to the Linux kernel 2.6.30-rc4, 2.6.18, and probably other versions allows guest user applications to cause a denial of service (kernel oops) of the guest OS by triggering a segmentation fault in "certain address ranges."
Debian Security Advisory 1809-1 - Several vulnerabilities have been discovered in the Linux kernel that may lead to a denial of service, or privilege escalation.