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

When removing or replacing a grant mapping, the x86 PV specific path
needs to make sure page table entries remain in sync with other
accounting done.  Although the identity of the page frame was
validated correctly, neither the presence of the mapping nor page
writability were taken into account.

IMPACT
======

A malicious or buggy x86 PV guest could escalate its privileges or
crash the hypervisor.

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

All Xen versions are affected.

Only x86 PV guests can leverage the vulnerability.  x86 HVM guests as
well as ARM guests cannot leverage the vulnerability.

MITIGATION
==========

Running only HVM guests will avoid this vulnerability.  However, the
vulnerability is exposed to PV stub qemu serving as the device model
for HVM guests.  Our default assumption is that an HVM guest has
compromised its PV stub qemu.  By extension, it is likely that the
vulnerability is exposed to HVM guests which are served by a PV stub
qemu.

For PV guests, the vulnerability can be avoided if the guest kernel is
controlled by the host rather than guest administrator, provided that
further steps are taken to prevent the guest administrator from loading
code into the kernel (e.g. by disabling loadable modules etc) or from
using other mechanisms which allow them to run code at kernel privilege.

External References:

http://xenbits.xen.org/xsa/advisory-234.html