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            Xen Security Advisory CVE-2015-3456 / XSA-133
                              version 3

          Privilege escalation via emulated floppy disk drive

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

Normalize version tags.

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

The code in qemu which emulates a floppy disk controller did not
correctly bounds check accesses to an array and therefore was
vulnerable to a buffer overflow attack.

IMPACT
======

A guest which has access to an emulated floppy device can exploit this
vulnerability to take over the qemu process elevating its privilege to
that of the qemu process.

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

All Xen systems running x86 HVM guests without stubdomains are
vulnerable to this depending on the specific guest configuration. The
default configuration is vulnerable.

Guests using either the traditional "qemu-xen" or upstream qemu device
models are vulnerable.

Guests using a qemu-dm stubdomain to run the device model are only
vulnerable to takeover of that service domain.

Systems running only x86 PV guests are not vulnerable.

ARM systems are not vulnerable.

MITIGATION
==========

Enabling stubdomains will mitigate this issue, by reducing the
escalation to only those privileges accorded to the service domain.

qemu-dm stubdomains are only available with the traditional "qemu-xen"
version.

CREDITS
=======

This issue was discovered by Jason Geffner, Senior Security Researcher
at CrowdStrike.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa133-qemuu.patch           qemu-xen master, 4.5.x, 4.4.x
xsa133-qemuu-4.3-4.2.patch   qemu-xen 4.3.x, 4.2.x
xsa133-qemut.patch           qemu-xen-traditional master, 4.5.x, 4.4.x, 4.3.x, 4.2.x

$ sha256sum xsa133*.patch
e7ca0106a9d4bfe472b3b52bbed8646b47305634ff16c3e17ed6185296a7e7ff  xsa133-qemut.patch
0cbc0415ef63bc195a0338441f3770d9fe6741e894879e35d1a6609ad028e42f  xsa133-qemuu.patch
cf735c1ecb6a40ca57d408e5c01725eca5b9b0a14b1d31b4362dc3f036bdeb28  xsa133-qemuu-4.3-4.2.patch
$

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

Deployment of the patches 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: Deployment of the mitigation by enabling stubdomains is NOT
permitted (except on systems used and administered 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 this configuration change may be
visible to the guest.

Also, 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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