Xen Security Advisory 359 v3 (CVE-2020-29571) - FIFO event channels control structure ordering

[Xen.org security team <security@xxxxxxx>]

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            Xen Security Advisory CVE-2020-29571 / XSA-359
                               version 3

            FIFO event channels control structure ordering

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

Public release.

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

A bounds check common to most operation time functions specific to FIFO
event channels depends on the CPU observing consistent state.  While the
producer side uses appropriately ordered writes, the consumer side isn't
protected against re-ordered reads, and may hence end up de-referencing
a NULL pointer.

IMPACT
======

Malicious or buggy guest kernels can mount a Denial of Service (DoS)
attack affecting the entire system.

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

All Xen versions from 4.4 onwards are vulnerable.  Xen versions 4.3 and
earlier are not vulnerable.

Only Arm systems may be vulnerable.  Whether a system is vulnerable will
depend on the specific CPU.  x86 systems are not vulnerable.

MITIGATION
==========

There is no known mitigation.

CREDITS
=======

This issue was discovered by Julien Grall of Amazon.

RESOLUTION
==========

Applying the 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.

xsa359.patch           xen-unstable - 4.10

$ sha256sum xsa359*
cb009ad77d1a3d8044431b2af568dd9dffefe07fc9f537fb6b53c2ec57aa77b7  xsa359.meta
3126d9304b68be84a89c42c223227c8f96ecbb96a0385a7e1bdc65ae5e0f344f  xsa359.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is then no longer applicable.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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