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Advisory

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            Xen Security Advisory CVE-2016-3961 / XSA-174
                              version 3

                hugetlbfs use may crash PV Linux guests

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

Public release.

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

Huge (2Mb) pages are generally unavailable to PV guests.  Since x86
Linux pvops-based kernels are generally multi purpose, they would
normally be built with hugetlbfs support enabled.  Use of that
functionality by an application in a PV guest would cause an
infinite page fault loop, and an OOPS to occur upon an attempt to
terminate the hung application.

IMPACT
======

Depending on the guest kernel configuration, the OOPS could result
in a kernel crash (guest DoS).

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

All upstream x86 Linux versions operating as PV Xen guests are
vulnerable.

ARM systems are not vulnerable.  x86 HVM guests are not vulnerable.

x86 Linux versions derived from linux-2.6.18-xen.hg (XenoLinux) are not
vulnerable.

Oracle Unbreakable Enterprise Kernels are not vulnerable.

We believe that non-Linux guests are not vulnerable, as we are not
aware of any with an analogous bug.

MITIGATION
==========

Running only HVM guests will avoid this issue.

Not enabling hugetlbfs use, by not altering the boot time default value
of zero in /proc/sys/vm/nr_hugepages (which can only be written by the
root user) will avoid this issue.

It is possible that disabling (or not enabling) the "panic on OOPS"
behavior (via use of the "oops=panic" command line option or the
"panic_on_oops" sysctl) will also avoid this issue, by limiting the
effect to an application crash.  We are not currently sure whether
this is an effective mitigation, as we are not sure whether any locks
or mutexes are held at the point of the crash.

CREDITS
=======

This issue was discovered by Vitaly Kuznetsov from Red Hat.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa174.patch           Linux 4.5.x ... 3.10.x

$ sha256sum xsa174*
cbec70e183f76b4081ebba05c0a8105bd4952d164a2e5c40528c05bf8861ddef  xsa174.patch
$

DEPLOYMENT DURING EMBARGO
=========================

Deployment of patches or mitigations is NOT permitted (except where
all the affected systems and VMs are administered and used only by
organisations which are members of the Xen Project Security Issues
Predisclosure List).  Specifically, deployment on public cloud systems
is NOT permitted.

This is because such host configuration changes would be user mode
visible, which could lead to the rediscovery of the vulnerability.

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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