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            Xen Security Advisory CVE-2017-15589 / XSA-239
                              version 3

            hypervisor stack leak in x86 I/O intercept code

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

CVE assigned.

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

Intercepted I/O operations may deal with less than a full machine
word's worth of data.  While read paths had been the subject of earlier
XSAs (and hence have been fixed), at least one write path was found
where the data stored into an internal structure could contain bits
from an uninitialized hypervisor stack slot.  A subsequent emulated
read would then be able to retrieve these bits.

IMPACT
======

A malicious unprivileged x86 HVM guest may be able to obtain sensitive
information from the host or other guests.

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

All Xen versions are vulnerable.

Only x86 systems are affected.  ARM systems are not affected.

Only HVM guests can leverage this vulnerability.  PV guests cannot
leverage this vulnerability.

MITIGATION
==========

Running only PV guests will avoid this issue.

CREDITS
=======

This issue was discovered by Roger Pau Monné of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa239.patch           xen-unstable, Xen 4.9.x, Xen 4.8.x, Xen 4.7.x, Xen 4.6.x
xsa239-4.5.patch       Xen 4.5.x

$ sha256sum xsa239*
eb7971be89199eb3ff510f4f5650fd5a8ec588b9fcb8f89230216fac4214ef21  xsa239.meta
087a8b3cf7ecbdbde593033c127cbcf6c37f532bf33d90f72c19e493970a799c  xsa239.patch
b91a68fe67240f2a5bb9460c5b650e9595364afa180f8702aef783815e3d7dcd  xsa239-4.5.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is then no longer applicable.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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