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            Xen Security Advisory CVE-2016-2270 / XSA-154
                              version 3

          x86: inconsistent cachability flags on guest mappings

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

Clarify cumbersome Resolution wording.

The patch now adds a command line option to overcome the possible
performance regression.  Add patch backports.

Clarify origin of assertion (at start of patch description) that
inconsistent cacheability is a problem only for mmio pages.

Public release.

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

Multiple mappings of the same physical page with different cachability
setting can cause problems.  While one category (risk of using stale
data) affects only guests themselves (and hence avoiding this can be
left for them to control), the other category being Machine Check
exceptions can be fatal to entire hosts.  According to the information
we were able to gather, only mappings of MMIO pages may surface this
second category, but even for them there were cases where the
hypervisor did not properly enforce consistent cachability.

IMPACT
======

A malicious guest administrator might be able to cause a reboot,
denying service to the entire host.

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

All Xen versions are affected.

Only x86 guests given control over some physical device can trigger
this vulnerability.

x86 systems are vulnerable.  ARM systems are not vulnerable.

The vulnerability depends on the system response to mapping the same
memory with different cacheability.  On some systems this is harmless;
on others, depending on CPU and chipset, it may be fatal.

MITIGATION
==========

Not handing physical devices to guests will also avoid this issue.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

We believe that the attached patch fixes the issue.  However, no
formal description of CPU behaviour in particular use cases has been
provided by Intel.  There has been no response from AMD.

We are aware of a potential performance regression with this patch on
some systems - even if no hardware passthrough is configured.  This is
due to the behaviour of some drivers and peripherals that is beyond
the scope of this security fix.  The patch adds a command line option
"mmio-relax" to overcome this possible regression for Domain 0 or all
para-virtual guests.  Note however that enabling this workaround will
reinstate the security issue these patches aim to address.

xsa154.patch        xen-unstable
xsa154-4.6.patch    Xen 4.6.x
xsa154-4.5.patch    Xen 4.5.x
xsa154-4.4.patch    Xen 4.4.x
xsa154-4.3.patch    Xen 4.3.x

$ sha256sum xsa154*
bbe7fba38ee30c00ef850fa6419c769e88b5669164d447f50b1ebbe333573152  xsa154.patch
011a4e33c0e476c52fe44253d50e01a1185948fd1b2a8e645274b25da6030d71  xsa154-4.3.patch
92d475bbc344127faa4f0183a9ccca9e975c7d24eb5772bf0a0a0a2e019144c6  xsa154-4.4.patch
b13737e71f22185b94ab25c07afd521add1a7e3886326c719d5df4d42f3f87f4  xsa154-4.5.patch
eec88c2a57466f83a81844cb7025f70c2b671d07a75d85487d4ed73cdabbb020  xsa154-4.6.patch
$

DEPLOYMENT DURING EMBARGO
=========================

Deployment of the patch described above (or others which are
substantially similar) is permitted during the embargo, even on
public-facing systems with untrusted guest users and administrators.

However deployment of the mitigations described above is NOT permitted
(except where all the affected systems and VMs are administered and
used only by organisations which are members of the Xen Project
Security Issues Predisclosure List).  Specifically, deployment on
public cloud systems is NOT permitted.

This is because the configuration change would be visible to the guest,
which could lead to the rediscovery of the vulnerability.

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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