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.

External References:

http://xenbits.xen.org/xsa/advisory-223.html

Acknowledgements:

Name: the Xen project
Upstream: Julien Grall (ARM)