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."
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A10313
http://www.securityfocus.com/bid/34957
http://www.openwall.com/lists/oss-security/2009/05/14/2
http://www.debian.org/security/2009/dsa-1809