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."