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                                      CERT-Renater

                          Note d'Information No. 2015/VULN165
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DATE                : 04/09/2015

HARDWARE PLATFORM(S): /

OPERATING SYSTEM(S): Systems running Xen versions 4.4.x, 4.5.x,
                                          xen-unstable.

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http://xenbits.xen.org/xsa/advisory-141.html
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             Xen Security Advisory CVE-2015-6654 / XSA-141
                               version 3

          printk is not rate-limited in xenmem_add_to_physmap_one

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

Public release.

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

XENMAPSPACE_gmfn_foreign dumps the p2m, on ARM, when it fails to get a
reference on the foreign page.  However, dump_p2m_lookup does not use
rate-limited printk.

A malicious infrastructure domain, which is allowed to map memory of
a foreign guest, would be able to flood the Xen console.

IMPACT
======

Domains deliberately given partial management control may be able to
deny service to other parts of the system.

As a result, in a system designed to enhance security by radically
disaggregating the management, the security may be reduced.  But, the
security will be no worse than a non-disaggregated design.

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

This issue is only relevant to systems which intend to increase
security through the use of advanced disaggregated management
techniques.

This does not include systems using libxl, libvirt, xm/xend,
XCP/XenServer, OpenStack or CloudStack (unless substantially modified
or supplemented, as compared to versions supplied by the respective
upstreams).

This issue is not relevant to stub device models, driver domains, or
stub xenstored.  Those disaggregation techniques do not rely on
granting the semi-privileged support domains access to the affected
hypercall, and are believed to provide the intended security benefits.

Only ARM systems are potentially affected.  All Xen versions which
support ARM are potentially affected.

MITIGATION
==========

Reducing the hypervisor log level can be used to suppress messages.

Switching from disaggregated to a non-disaggregated operation does NOT
mitigate these vulnerabilities.  Rather, it simply recategorises the
vulnerability to hostile management code, regarding it "as designed";
thus it merely reclassifies these issues as "not a bug".  Users and
vendors of disaggregated systems should not change their
configuration.

CREDITS
=======

This issue was discovered by Julien Grall of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa141.patch        Xen 4.4.x, 4.5.x, xen-unstable

$ sha256sum xsa141*.patch
12358565dc443e1855a1b5776fa9008c5ea5e5854bd4e93b88ab4178c698fc2a 
xsa141.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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https://services.renater.fr/ssi/
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