FreeBSD -- bhyve privilege escalation via VMCS access

Problem Description:
AMD and Intel CPUs support hardware virtualization using specialized data
structures that control various aspects of guest operation. These are the
Virtual Machine Control Structure (VMCS) on Intel CPUs, and the Virtual
Machine Control Block (VMCB) on AMD CPUs. Insufficient access controls allow
root users, including those running in a jail, to change these data
structures.
Impact:
An attacker with host root access (including to a jailed bhyve instance) can
use this vulnerability to achieve kernel code execution.