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            Xen Security Advisory CVE-2020-27670 / XSA-347
                              version 3

                  unsafe AMD IOMMU page table updates

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

CVE assigned.

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

AMD IOMMU page table entries are updated in a step by step manner,
without regard to them being potentially in use by the IOMMU.  Therefore
it was possible that the IOMMU would read and then use a half-updated
entry.  Furthermore, updates to Device Table entries lacked suitable
ordering enforcement for certain steps involved in these updates.

In both case the specific outcome heavily depends on how exactly the
compiler translated the affected pieces of code.

IMPACT
======

A malicious guest might be able to cause data corruption and data
leaks.  Host or guest Denial of Service (DoS), and privilege
escalation, cannot be ruled out.

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

All Xen versions are potentially vulnerable.

Only x86 systems with AMD, Hygon, or compatible IOMMU hardware are
vulnerable.  Arm systems as well as x86 systems with VT-d hardware or
without any IOMMUs in use are not vulnerable.

Only x86 guests which have physical devices passed through to them can
leverage the vulnerability.

MITIGATION
==========

Not passing through physical devices to untrusted guests will avoid
the vulnerability.

CREDITS
=======

This issue was discovered by Paul Durrant of Amazon and Jan Beulich of
SUSE.

RESOLUTION
==========

Applying the appropriate set of attached patches resolves this issue.

Note that patches for released versions are generally prepared to
apply to the stable branches, and may not apply cleanly to the most
recent release tarball.  Downstreams are encouraged to update to the
tip of the stable branch before applying these patches.

xsa347/xsa347-?.patch           xen-unstable
xsa347/xsa347-4.14-?.patch      Xen 4.14
xsa347/xsa347-4.13-?.patch      Xen 4.13
xsa347/xsa347-4.12-?.patch      Xen 4.12
xsa347/xsa347-4.11-?.patch      Xen 4.10 - 4.11

$ sha256sum xsa347* xsa347*/*
f16e1a348b0e45601c96b2bd08afc4202bbccc92c8af8344b3c8286ca819acef  xsa347.meta
82e14d0507ec94f8cfac2b4d5d1b60681b925218ab927332bee338e6b6c679c9  xsa347/xsa347-1.patch
1bc6018c3685727ba4035bf0b5cea95940a1b9c4746fa9bddfd41507482d68a1  xsa347/xsa347-2.patch
f1bd8eba268300f564837ac37fe43b774ace885c9cbf8fcacae457128730bc80  xsa347/xsa347-3.patch
5aec8f3b15aa799e1ff7ec0dfe53523cb91aa5fd88033f7f034cb74ebaa6abe4  xsa347/xsa347-4.11-1.patch
4ab3a6fa181ce486b4c9943f6629b7c1a4337c7ccb92701ae6e40108533778ca  xsa347/xsa347-4.11-2.patch
fec82340dc65fc1001358de51d0639b2b401818fa1e831f8715cb1780b17dc7b  xsa347/xsa347-4.12-1.patch
be89e976fe03464ce3a73b162c07927128f41a8a03466e903ebfa4ea0dc46116  xsa347/xsa347-4.12-2.patch
5dc0abf73d1a9d21f2b57e6c57ee5c15cc3febbb783123c0946f3e5778671929  xsa347/xsa347-4.13-1.patch
6d2b6ea7a373fb1c4cce63db349bbafa8603b5e7c6b74fc6d029954075f2268d  xsa347/xsa347-4.13-2.patch
4e154bfca5101569c8260e307eb6439783bc99547b7dfb5aba2bafebbde46190  xsa347/xsa347-4.13-3.patch
6a70c2afba0d3ad73b12743a6808ba8002e9ee573d7c460397355e40de3b553f  xsa347/xsa347-4.14-1.patch
1bc6018c3685727ba4035bf0b5cea95940a1b9c4746fa9bddfd41507482d68a1  xsa347/xsa347-4.14-2.patch
f1bd8eba268300f564837ac37fe43b774ace885c9cbf8fcacae457128730bc80  xsa347/xsa347-4.14-3.patch
$

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

Deployment of the patches 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 mitigation 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 removal of pass-through devices or their replacement by
emulated devices is a guest visible configuration change, which may lead
to re-discovery of the issue.

Deployment of this mitigation is permitted only AFTER the embargo ends.

AND: 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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