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Advisory

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            Xen Security Advisory CVE-2020-25602 / XSA-333
                               version 3

      x86 pv: Crash when handling guest access to MSR_MISC_ENABLE

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

Public release.

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

When a guest accesses certain Model Specific Registers, Xen first reads
the value from hardware to use as the basis for auditing the guest
access.

For the MISC_ENABLE MSR, which is an Intel specific MSR, this MSR read
is performed without error handling for a #GP fault, which is the
consequence of trying to read this MSR on non-Intel hardware.

IMPACT
======

A buggy or malicious PV guest administrator can crash Xen, resulting in
a host Denial of Service.

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

Only x86 systems are vulnerable.  ARM systems are not vulnerable.

Only Xen versions 4.11 and onwards are vulnerable.  4.10 and earlier are
not vulnerable.

Only x86 systems which do not implement the MISC_ENABLE MSR (0x1a0) are
vulnerable.  AMD and Hygon systems do not implement this MSR and are
vulnerable.  Intel systems do implement this MSR and are not vulnerable.
Other manufacturers have not been checked.

Only x86 PV guests can exploit the vulnerability.  x86 HVM/PVH guests
cannot exploit the vulnerability.

MITIGATION
==========

Running only HVM/PVH guests avoids the vulnerability.

CREDITS
=======

This issue was discovered by Andrew Cooper of Citrix.

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.

xsa333.patch           Xen 4.11 - xen-unstable

$ sha256sum xsa333*
3f3d974ede9fe80f4eb63640dce058cf9e2073cd79e4c085c944f3ca5e454e26  xsa333.meta
8edec914fbdf036fba8cb54a75d3a9b025fac936e0af35512954a2dc2b12a26f  xsa333.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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