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            Xen Security Advisory CVE-2015-7812 / XSA-145
                              version 3

             arm: Host crash when preempting a multicall

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

Public release.

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

Early versions of Xen on ARM did not support "multicall" functionality
(the ability to perform multiple operations via a single hypercall)
and therefore stubbed out the functionality needed to support
preemption of multicalls in a manner which crashed the host.

When multicall support was subsequently added these stubs were not
replaced with the correct functionality and therefore exposed to
guests a code path which crashes the host.

Any guest can issue a preemptable hypercall via the multicall interface
to exploit this vulnerability.

IMPACT
======

A malicious guest can crash the host.

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

Both 32- and 64-bit ARM systems are vulnerable from Xen 4.4 onward.

x86 systems are not vulnerable.

MITIGATION
==========

On systems where the guest kernel is controlled by the host rather
than guest administrator, running only kernels which do not make use
of multicall functionality 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 Citrix.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa145.patch        xen-unstable, Xen 4.6.x, Xen 4.5.x, Xen 4.4.x

$ sha256sum xsa145*.patch
4d4a4724e4d367ddfc9ac1b43dfe81bce873c65fe9bb13f443266dd12c002db1  xsa145.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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