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Advisory

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             Xen Security Advisory CVE-2013-1432 / XSA-58
                            version 2

        Page reference counting error due to XSA-45/CVE-2013-1918 fixes

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

Public release.  Credits section added.

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

The XSA-45/CVE-2013-1918 patch making error handling paths preemptible broke
page reference counting by not retaining a reference on pages stored for
deferred cleanup. This would lead to the hypervisor prematurely attempting to
free the page, generally crashing upon finding the page still in use.

CREDITS
=======

Thanks to Andrew Cooper and the Citrix XenServer team for discovering
and reporting this vulnerability, and helping investigate it.

IMPACT
======

Malicious or buggy PV guest kernels can mount a denial of service attack
affecting the whole system. It can't be excluded that this could also be
exploited to mount a privilege escalation attack.

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

All Xen versions having the XSA-45/CVE-2013-1918 fixes applied are vulnerable.

The vulnerability is only exposed by PV guests.

MITIGATION
==========

Running only HVM guests, or PV guests with trusted kernels, will avoid this
vulnerability.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa58-4.1.patch             Xen 4.1.x
xsa58-4.2.patch             Xen 4.2.x
xsa58-unstable.patch        xen-unstable

$ sha256sum xsa58*.patch
3623ec87e5a2830f0d41de19a8e448d618954973c3264727a1f3a095f15a8641  xsa58-4.1.patch
194d6610fc38b767d643e5d58a1268f45921fb35e309b47aca6a388b861311c2  xsa58-4.2.patch
2c94b099d7144d03c0f7f44e892a521537fc040d11bc46f84a2438eece46a0f5  xsa58-unstable.patch
$
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