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Advisory

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            Xen Security Advisory CVE-2016-9384 / XSA-194
                              version 3

           guest 32-bit ELF symbol table load leaking host data

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

Public release.

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

Along with their main kernel binary, unprivileged guests may arrange
to have their Xen environment load (kernel) symbol tables for their
use.  The ELF image metadata created for this purpose has a few unused
bytes when the symbol table binary is in 32-bit ELF format.  These
unused bytes were not properly cleared during symbol table loading.

IMPACT
======

A malicious unprivileged guest may be able to obtain sensitive
information from the host.

The information leak is small and not under the control of the guest,
so effectively exploiting this vulnerability is probably difficult.

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

Only Xen version 4.7 is affected.  Xen versions 4.6 and earlier are not
affected.

The vulnerability is not exposed to x86 HVM guests, unless the host
toolstack has configured to load the guest with a non-default loader,
rather than hvmloader.

MITIGATION
==========

There is no known mitigation.

CREDITS
=======

This issue was discovered by Roger Pau Monné of Citrix.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa194.patch           xen-unstable, Xen 4.7.x

$ sha256sum xsa194*
4dad65417d9ff3c86e763d3c88cf8de79b58a9981d531f641ae0dd0dcedce911  xsa194.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is then no longer applicable.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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