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

The PV pagetable code has fast-paths for making updates to pre-existing
pagetable entries, to skip expensive re-validation in safe cases
(e.g. clearing only Access/Dirty bits).  The bits considered safe were too
broad, and not actually safe.

IMPACT
======

A malicous PV guest administrator can escalate their privilege to that
of the host.

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

All versions of Xen are vulnerable.

The vulnerability is only exposed to PV guests on x86 hardware.

The vulnerability is not exposed to x86 HVM guests, or ARM guests.

MITIGATION
==========

Running only HVM guests will avoid this vulnerability.

External References:

http://xenbits.xen.org/xsa/advisory-182.html

Acknowledgements:

Name: the Xen project