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            Xen Security Advisory CVE-2018-12893 / XSA-265
                               version 3

      x86: #DB exception safety check can be triggered by a guest

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

Public release.

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

One of the fixes in XSA-260 added some safety checks to help prevent Xen
livelocking with debug exceptions.  Unfortunately, due to an oversight, at
least one of these safety checks can be triggered by a guest.

IMPACT
======

A malicious PV guest can crash Xen, leading to a Denial of Service.

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

All Xen systems which have applied the XSA-260 fix are vulnerable.

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

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

An attacker needs to be able to control hardware debugging facilities to
exploit the vulnerability, but such permissions are typically available
to unprivileged users.

MITIGATION
==========

Running only x86 HVM or PVH guests will avoid the vulnerability.

CREDITS
=======

This issue was discovered by Andrew Cooper of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa265.patch           xen-unstable, Xen 4.10.x, 4.9.x, 4.8.x
xsa265-4.7.patch       Xen 4.7.x, 4.6.x

$ sha256sum xsa265*
3eb66ed7251dcc4259eeffe608b2747857e269307d894a1cb950973420184aa7  xsa265.patch
00faf2a4159698b6540565ece06de103c3547855e2084324ca44772b8a24aa18  xsa265-4.7.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is then no longer applicable.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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