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

Both writes to the FS and GS register base MSRs as well as the
WRFSBASE and WRGSBASE instructions require their input values to be
canonical, or a #GP fault will be raised.  When the use of those
instructions by the hypervisor was enabled, the previous guard against
#GP faults (having recovery code attached) was accidentally removed.

IMPACT
======

A malicious guest administrator can crash the host, leading to a DoS.

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

Xen versions 4.4 and onwards are affected.  Xen versions 4.3 and
earlier are not affected.

The vulnerability is only exposed to x86 PV guests.

The vulnerability is NOT exposed to x86 HVM guests.

ARM systems are NOT vulnerable.

MITIGATION
==========

Running only HVM guests will avoid this vulnerability.

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-193.html

Acknowledgements:

Name: the Xen project
Upstream: Andrew Cooper (Citrix)