The kvm_arch_vcpu_ioctl_set_sregs function in the KVM in Linux kernel 2.6 before 2.6.30, when running on x86 systems, does not validate the page table root in a KVM_SET_SREGS call, which allows local users to cause a denial of service (crash or hang) via a crafted cr3 value, which triggers a NULL pointer dereference in the gfn_to_rmap function.
http://www.ubuntu.com/usn/usn-807-1
http://www.openwall.com/lists/oss-security/2009/06/30/1
http://www.debian.org/security/2009/dsa-1845
http://sourceforge.net/tracker/?func=detail&atid=893831&aid=2687641&group_id=180599