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Advisory

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            Xen Security Advisory CVE-2016-5242 / XSA-181
                              version 2

               arm: Host crash caused by VMID exhaustion

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

CVE assigned.

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

VMIDs are a finite hardware resource, and allocated as part of domain
creation.  If no free VMIDs are available when trying to create a new domain,
a bug in the error path causes a NULL pointer to be used, resulting in a Data
Abort and host crash.

IMPACT
======

Attempting to create too many concurrent domains causes a host crash rather
than a graceful error.  A malicious device driver domain can hold references
to domains, preventing its VMID being released.

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

Xen versions 4.4 and later are affected.  Older Xen versions are unaffected.

x86 systems are not affected.

Only arm systems with less-privileged device driver domains can expose this
vulnerability.

MITIGATION
==========

There is no mitigation.  Not using driver domains reclassifies the problem,
but does not fix it.

NOTE REGARDING LACK OF EMBARGO
==============================

The crash was discussed publicly on xen-devel, before it was appreciated
that there was a security problem.

CREDITS
=======

This issue was discovered by Aaron Cornelius of DornerWorks.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa181.patch           xen-unstable, Xen 4.6.x, 4.5.x
xsa181-4.4.patch       Xen 4.4.x

$ sha256sum xsa181*
6756fcf44446675e5277f6d6c0e8a0aaa51a7909ad9a55af89a09367fded8733  xsa181.patch
97a90c7cb42466647622cb2ed98de531b7ba2e174a1bc639a32a6f1b626d503f  xsa181-4.4.patch
$
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