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            Xen Security Advisory CVE-2017-14319 / XSA-234
                               version 3

          insufficient grant unmapping checks for x86 PV guests

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

Added metadata file

Public release.

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.

CREDITS
=======

This issue was discovered by Andrew Cooper of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa234.patch           xen-unstable
xsa234-4.9.patch       Xen 4.9.x
xsa234-4.8.patch       Xen 4.8.x, Xen 4.7.x
xsa234-4.6.patch       Xen 4.6.x
xsa234-4.5.patch       Xen 4.5.x

$ sha256sum xsa234*
efbcc7eac0f010281c5651d191076ac08cc7dd22a1945e88e92ba8a03ae8cc40  xsa234.meta
08ffa79e5c2a77db0b91b3bfcf9fa5c50f174fe842b7418e2e1549d47e0aec4d  xsa234.patch
4b74f3c85a98bc6f40c6a448b068bf45e71f7cce887b7cb1481aca0e8746d990  xsa234-4.5.patch
3df4ce173196111c1ff849039ea4927c0b4bd632b08a501fb26f64e31b951fba  xsa234-4.6.patch
169e4e0eaa6b27e58ff0f4ce50e8fcc3f81b1e0a10210decf22d1b4cac7501fb  xsa234-4.8.patch
213f9d81a4ab785db67b9f579c9e88c9c8586c46b93f466a309060750df2df32  xsa234-4.9.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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