ISSUE DESCRIPTION
=================

So far, x86 PVH guests can be configured with or without Local APICs.
Configurations with Local APICs are identical to x86 HVM guests, and
will use as much hardware acceleration support as possible.
Configurations without Local APICs try to turn off all hardware
acceleration, and disable all software emulation.

Multiple paths in Xen assume the presence of a Local APIC without
sufficient checks, and can fall over a NULL pointer.  On Intel hardware,
the logic to turn off hardware acceleration is incomplete and leaves the
guest with full control of the real Task Priority Register.

IMPACT
======

A malicious or buggy guest may cause a hypervisor crash, resulting in
a Denial of Service (DoS) affecting the entire host.

VULNERABLE SYSTEMS
==================

Xen version 4.8 and onwards are vulnerable.

Only x86 systems are vulnerable.  ARM systems are not vulnerable.

Only x86 PVH guests can exploit the vulnerability.  x86 PV and HVM
guests cannot exploit the vulnerability.

MITIGATION
==========

Running only PV or HVM guests avoids the vulnerability.

Running all PVH guests with "apic=1" in the guest configuration file
(or equivalent thereof) also avoids the vulnerability.

External References:

http://xenbits.xen.org/xsa/advisory-256.html