The kvm_emulate_hypercall function in arch/x86/kvm/x86.c in KVM in the Linux kernel 2.6.25-rc1, and other versions before 2.6.31, when running on x86 systems, does not prevent access to MMU hypercalls from ring 0, which allows local guest OS users to cause a denial of service (guest kernel crash) and read or write guest kernel memory via unspecified "random addresses."
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A11328
https://bugzilla.redhat.com/show_bug.cgi?id=524124
http://www.ubuntu.com/usn/USN-852-1
http://www.redhat.com/support/errata/RHSA-2009-1465.html
http://www.openwall.com/lists/oss-security/2009/09/22/8
http://www.openwall.com/lists/oss-security/2009/09/21/1
http://www.openwall.com/lists/oss-security/2009/09/18/1
http://secunia.com/advisories/37105