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Advisory

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            Xen Security Advisory CVE-2016-9385 / XSA-193
                              version 3

   x86 segment base write emulation lacking canonical address checks

UPDATES IN VERSION 3
====================

Public release.

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.

CREDITS
=======

This issue was discovered by Andrew Cooper of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa193.patch           xen-unstable
xsa193-4.7.patch       Xen 4.7.x, Xen 4.6.x
xsa193-4.5.patch       Xen 4.5.x, Xen 4.4.x

$ sha256sum xsa193*
401df29b462a3430403a4f5bb36fd7824e692c9b5bac650e1a9d70bd440a55a1  xsa193.patch
b3494b1fe5fefc0d032bd603340e364c880ec0d3ae3fb8aa3a773038e956f955  xsa193-4.5.patch
f1b0092c585ebffe83d6ed7df94885ec5dfcb4227bdb33f421bad9febb8135a1  xsa193-4.7.patch
$

DEPLOYMENT DURING EMBARGO
=========================

Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
administrators.

But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).

Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
Team.

(Note: this during-embargo deployment notice is retained in
post-embargo publicly released Xen Project advisories, even though it
is then no longer applicable.  This is to enable the community to have
oversight of the Xen Project Security Team's decisionmaking.)

For more information about permissible uses of embargoed information,
consult the Xen Project community's agreed Security Policy:
  http://www.xenproject.org/security-policy.html
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