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Advisory

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            Xen Security Advisory CVE-2020-11739 / XSA-314
                               version 3

          Missing memory barriers in read-write unlock paths

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

Public release.

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

The read-write unlock paths don't contain a memory barrier.  On Arm, this
means a processor is allowed to re-order the memory access with the
preceding ones.

In other words, the unlock may be seen by another processor before all the
memory accesses within the "critical" section.

As a consequence, it may be possible to have a writer executing a critical
section at the same time as readers or another writer. In other words,
many of the assumptions (e.g a variable cannot be modified after a check)
in the critical sections are not safe anymore.

The read-write locks are used in hypercalls (such as grant-table ones), so
a malicious guest could exploit the race.  For instance, there is a small
window where Xen can leak memory if XENMAPSPACE_grant_table is used
concurrently.

IMPACT
======

A malicous guest may be able to leak memory, or cause a hypervisor crash
resulting in a Denial of Service (DoS). Information leak and privilege
escalation cannot be excluded.

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

Systems running all versions of Xen are affected.

Whether an individual Arm-based CPU is vulnerable depends on its memory
re-ordering properties.  Consult your CPU vendor.

x86 systems are not vulnerable.

MITIGATION
==========

There is no known mitigation.

CREDITS
=======

This issue was discovered by Julien Grall of Amazon.

RESOLUTION
==========

Applying the appropriate attached patch 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.

xsa314.patch           xen-unstable
xsa314-4.13.patch      Xen 4.13 - Xen 4.9

$ sha256sum xsa314*
ff6e03780766d0358699ed0c5b0154be9ccbbc80796650f7568c295c5451ba0a  xsa314.meta
7c507e7b46568e94aa9595a549bd3020b16d1eca97b8bfc3bb1f5d96eb338cc1  xsa314.patch
a13e6a9cd531859882d1b0ef38245441d363d1ead1fa2a5ae5da7a0fce27e072  xsa314-4.13.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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