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	     Xen Security Advisory CVE-2013-0153 / XSA-36
			      version 4

  interrupt remap entries shared and old ones not cleared on AMD IOMMUs

UPDATES IN VERSION 4
====================

Updated patches, to deal with a boot time crash resulting from the earlier
changes on systems with firmware broken in a way not previously accounted
for.

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

To avoid an erratum in early hardware, the Xen AMD IOMMU code by
default chooses to use a single interrupt remapping table for the
whole system.  This sharing implies that any guest with a passed
through PCI device that is bus mastering capable can inject interrupts
into other guests, including domain 0.

Furthermore, regardless of whether a shared interrupt remapping table
is in use, old entries are not always cleared, providing opportunities
(which accumulate over time) for guests to inject interrupts into
other guests, again including domain 0.

In a typical Xen system many devices are owned by domain 0 or driver
domains, leaving them vulnerable to such an attack. Such a DoS is
likely to have an impact on other guests running in the system.

IMPACT
======

A malicious domain which is given access to a physical PCI device can
mount a denial of service attack affecting the whole system.

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

Xen versions 3.3 onwards are vulnerable.  Earlier Xen versions do not
implement interrupt remapping, and hence do not support secure AMD-Vi
PCI passthrough in any case.

Only systems using AMD-Vi for PCI passthrough are vulnerable.

Any domain which is given access to a PCI device can take advantage of
this vulnerability.

MITIGATION
==========

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

In Xen versions 4.1.3 and above the sharing of the interrupt remapping
table (and hence the more severe part of this problem) can be avoided
by passing "iommu=amd-iommu-perdev-intremap" as a command line option
to the hypervisor.  This option is not fully functional on earlier
hypervisors.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

Note that on certain systems (SP5100 chipsets with erratum 28 present,
or such with broken IVRS ACPI table) these patches will result in the
IOMMU not being enabled anymore.  This should be dealt with by a BIOS
update, if available.  Alternatively the check can be overridden by
specifying "iommu=no-amd-iommu-perdev-intremap" on the Xen command
line ("iommu=amd-iommu-global-intremap" on 4.1.x), at the price of
re-opening the security hole addressed by these patches.

xsa36-unstable.patch              Xen unstable
xsa36-4.2.patch                   Xen 4.2.x
xsa36-4.1.patch                   Xen 4.1.x

$ sha256sum xsa36*.patch
4bdc0f1f94f82c6bc6c777971f22ef915215b72b98b29f9064e4df65c0efc6f4  xsa36-4.1.patch
dd32ecaa84edbf6d11241045f40ba53ec4a3bc6c24f719bc21204067c4eb8964  xsa36-4.2.patch
7c0b3a1b332a24a830c7a436b065943f60c54cd5b7e746c440e2992a7b5cfe41  xsa36-unstable.patch
$

Incremental patches on top of what was provided in version 3 can also be
taken from the respective mercurial trees:

http://xenbits.xen.org/hg/xen-unstable.hg/rev/e68f14b9e739
http://xenbits.xen.org/hg/staging/xen-4.2-testing.hg/rev/6a03b38b9cd6
http://xenbits.xen.org/hg/staging/xen-4.1-testing.hg/rev/4d522221fa77
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