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Xen Security Advisory CVE-2019-19583 / XSA-308
version 3

VMX: VMentry failure with debug exceptions and blocked states

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

Public release.

Updated metadata to add 4.13, update StableRef's

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

Please see XSA-260 for background on the MovSS shadow:
http://xenbits.xen.org/xsa/advisory-260.html

Please see XSA-156 for background on the need for #DB interception:
http://xenbits.xen.org/xsa/advisory-156.html

The VMX VMEntry checks does not like the exact combination of state
which occurs when #DB in intercepted, Single Stepping is active, and
blocked by STI/MovSS is active, despite this being a legitimate state to
be in. The resulting VMEntry failure is fatal to the guest.

IMPACT
======

HVM/PVH guest userspace code may be able to crash the guest, resulting
in a guest Denial of Service.

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

All versions of Xen are affected.

Only systems supporting VMX hardware virtual extensions (Intel, Cyrix or
Zhaoxin CPUs) are affected. Arm and AMD systems are unaffected.

Only HVM/PVH guests are affected. PV guests cannot leverage the
vulnerability.

MITIGATION
==========

Running only PV guests will avoid this vulnerability.

Running HVM guests on only AMD hardware will also avoid this
vulnerability.

CREDITS
=======

This issue was discovered by Håkon Alstadheim and diagnosed as a
security issue by Andrew Cooper of Citrix.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa308.patch xen-unstable, Xen 4.13.x .. Xen 4.8.x

$ sha256sum xsa308*
4aa06d21478d9debb12388ff14d8abc31982e18895db40d0cec78fcc9fe68ef2 xsa308.meta
7e782b09b16f7534c8db52042f7bb3bd730d108571c8b10af184ae0b02fdae9d xsa308.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.
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