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            Xen Security Advisory CVE-2015-2751 / XSA-127
                              version 3

     Certain domctl operations may be abused to lock up the host

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

Fix patch name.

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

XSA-77 put the majority of the domctl operations on a list excepting
them from having security advisories issued for them if any effects
their use might have could hamper security. Subsequently some of them
got declared disaggregation safe, but for a small subset this was not
really correct: Their (mis-)use may result in host lockups.

As a result, the potential security benefits of toolstack
disaggregation are not always fully realised.

IMPACT
======

Domains deliberately given partial management control may be able to
deny service to the entire host.

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
==================

Xen versions 4.3 onwards are vulnerable.
Xen versions 4.2 and earlier do not have the described disaggregation
functionality and hence are not vulnerable.

MITIGATION
==========

The issues discussed in this advisory are themselves bugs in features
used for a security risk mitigation.

There is no further mitigation available, beyond general measures to
try to avoid parts of the system management becoming controlled by
attackers.  Those are the kind of measures which we expect any users
of radical disaggregation to have already deployed.

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.  The robustness benefits of disaggregation are
unaffected, and (depending on system design) security benefits are
likely to remain despite the vulnerabilities.

CREDITS
=======

This issue was discovered by Andrew Cooper of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa127.patch                 xen-unstable
xsa127-4.x.patch             Xen 4.5.x, Xen 4.4.x, Xen 4.3.x

$ sha256sum xsa127*.patch
5b98280738a205c40f56d0a7feb6ea6cd867da7ac1e0d9f4fc4620bae2c09171  xsa127.patch
e5fd3c126ae10fe45283e6eb1a4216b75057f1772d869d2b3a26398b0984c7bd  xsa127-4.x.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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