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Advisory

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            Xen Security Advisory CVE-2020-15563 / XSA-319
                               version 3

            inverted code paths in x86 dirty VRAM tracking

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

Public release.

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

An inverted conditional in x86 HVM guests' dirty video RAM tracking
code allows such guests to make Xen de-reference a pointer guaranteed
to point at unmapped space.

IMPACT
======

A malicious or buggy HVM guest may cause the hypervisor to crash,
resulting in Denial of Service (DoS) affecting the entire host.

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

Xen versions from 4.8 onwards are affected.  Xen versions 4.7 and
earlier are not affected.

Only x86 systems are affected.  Arm systems are not affected.

Only x86 HVM guests using shadow paging can leverage the vulnerability.
In addition there needs to be an entity actively monitoring a guest's
video frame buffer (typically for display purposes) in order for such a
guest to be able to leverage the vulnerability.  x86 PV guests as well
as x86 HVM guest using hardware assisted paging (HAP) cannot leverage
the vulnerability.

MITIGATION
==========

Running only PV guests will avoid the vulnerability.

For HVM guest explicitly configured to use shadow paging (e.g. via the
`hap=0' xl domain configuration file parameter), changing to HAP (e.g.
by setting `hap=1') will avoid exposing the vulnerability to those
guests.  HAP is the default (in upstream Xen), where the hardware
supports it; so this mitigation is only applicable if HAP has been
disabled by configuration.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

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.

xsa319.patch           xen-unstable, 4.13 - 4.9

$ sha256sum xsa319*
1fe0dc2e274776b8e1275f85129280f280f94ca4eabe6a8166113283dad93ed8  xsa319.meta
c145f394f8ac7d8838c376a97e1850c4125c12e478fc66ebe025ae397b27e6ea  xsa319.patch
$

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

Deployment of the patch 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 "use HAP mode" mitigation described above 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 in that case the configuration change can be observed
by guests, which could lead to the rediscovery of the vulnerability.

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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