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            Xen Security Advisory CVE-2015-4104 / XSA-129
                              version 2

            PCI MSI mask bits inadvertently exposed to guests

UPDATES IN VERSION 2
====================

Public release.

CVE assigned.

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

The mask bits optionally available in the PCI MSI capability structure
are used by the hypervisor to occasionally suppress interrupt
delivery.  Unprivileged guests were, however, nevertheless allowed
direct control of these bits.

IMPACT
======

Interrupts may be observed by Xen at unexpected times, which may lead
to a host crash and therefore a Denial of Service.

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

Xen versions 3.3 and onwards are vulnerable due to supporting PCI
pass-through.

Only x86 systems are vulnerable.  ARM systems are not vulnerable.

Only HVM guests with their device model run in Dom0 can take advantage
of this vulnerability.

Only HVM guests which have been granted access to physical PCI devices
(`PCI passthrough') can take advantage of this vulnerability.

Furthermore, the vulnerability is only applicable when the
passed-through PCI devices are MSI-capable.  (Most modern devices
are.)

MITIGATION
==========

This issue can be avoided by not assigning MSI capable PCI devices to
untrusted HVM guests.

This issue can also be avoided by only using PV guests.

It can also be avoided by configuring HVM guests with their device
model run in a separate (stub) domain.  (When using xl, this can be
requested with "device_model_stubdomain_override=1" in the domain
configuration file.)

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa129-qemuu.patch           qemu-upstream-unstable, Xen 4.5.x, Xen 4.4.x
xsa129-qemuu-4.3.patch       Xen 4.3.x
xsa129-qemut.patch           qemu-xen-unstable, Xen 4.5.x, Xen 4.4.x, Xen 4.3.x, Xen 4.2.x

$ sha256sum xsa129*.patch
3c6b5a085eec3a528b18207ca65222300911fd25501a9ffaffa76a5d85d23992  xsa129-qemut.patch
314808fbaa97d06bc4bb6cb6644dca1ae2da55534661c662c6e442d5b91e6061  xsa129-qemuu-4.3.patch
9f0658e197c539306118723d63b468d09fe3a1d9f9364f6d06e53b7be8268bdc  xsa129-qemuu.patch
$

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

Deployment of patches or migitations is NOT permitted (except on
systems used and administered only by organisations which are members
of the Xen Project Security Issues Predisclosure List).  Specifically,
deployent on public cloud systems is NOT permitted.

This is because the altered PCI config space access behavior is visible
to guests.

Deployment is permitted only AFTER the embargo ends.

(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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