arch/x86/kvm/vmx.c in the Linux kernel through 4.6.3 mishandles the APICv on/off state, which allows guest OS users to obtain direct APIC MSR access on the host OS, and consequently cause a denial of service (host OS crash) or possibly execute arbitrary code on the host OS, via x2APIC mode.
https://github.com/torvalds/linux/commit/3ce424e45411cf5a13105e0386b6ecf6eeb4f66f
https://bugzilla.redhat.com/show_bug.cgi?id=1337806