Xen Security Advisory 223 - ARM guest disabling interrupt may crash Xen

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Xen Security Advisory 223 - ARM guest disabling interrupt may crash Xen

Xen.org security team
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                    Xen Security Advisory XSA-223
                              version 2

              ARM guest disabling interrupt may crash Xen

UPDATES IN VERSION 2
====================

Public release.

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

Virtual interrupt injection could be triggered by a guest when sending
an SGI (e.g IPI) to any vCPU or by configuring timers. When the virtual
interrupt is masked, a missing check in the injection path may result in
reading invalid hardware register or crashing the host.

IMPACT
======

A guest may cause a hypervisor crash, resulting in a Denial of Service
(DoS).

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

All Xen versions which support ARM are affected.

x86 systems are not affected.

MITIGATION
==========

On systems where the guest kernel is controlled by the host rather than
guest administrator, running only kernels which do not disable SGI and
PPI (i.e IRQ < 32) will prevent untrusted guest users from exploiting
this issue. However untrusted guest administrators can still trigger it
unless further steps are taken to prevent them from loading code into
the kernel (e.g by disabling loadable modules etc) or from using other
mechanisms which allow them to run code at kernel privilege.

CREDITS
=======

This issue was discovered by Julien Grall of ARM.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa223.patch           xen-unstable, Xen 4.8.x, Xen 4.7.x, Xen 4.6.x, Xen 4.5.x

$ sha256sum xsa223*
b5c8d8e8dac027069bec7dd812cff3f6f99e5949dd4a8ee729255c38274958b1  xsa223.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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xsa223.patch (2K) Download Attachment