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Advisory

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             Xen Security Advisory CVE-2013-4375 / XSA-71
                              version 2

               qemu disk backend (qdisk) resource leak

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

Public release

Fix patch header corruption in xsa71-qemu-xen-unstable.patch.

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

The qdisk PV disk backend in the qemu-xen flavour of qemu ("upstream
qemu") can be influenced by a malicious frontend to leak mapped grant
references.

IMPACT
======

A malicious HVM guest can cause the backend domain to run out of grant
references, leading to a DoS for any other domain which shares that
driver domain.

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

Any system which is using the qemu-xen qdisk backend for HVM guests is
vulnerable.

qemu-xen and qdisk are exposed by systems using libxl from Xen 4.2.0
onwards. In Xen 4.2.0 qemu-xen was a non-default option, from Xen
4.3.0 onwards qemu-xen is the default.

Xen 4.1.0 exposes qdisk via libxl but does not support qemu-xen and
therefore is not vulnerable.

The xend toolstack has never supported qdisk as a disk backend and
therefore such systems are not vulnerable.

Upstream qemu is vulnerable from version 1.1 onwards.

MITIGATION
==========

This vulnerability can be avoided by using a different block backend
(e.g. blkback or blktap2) or by using the qemu-xen-traditional version
of qemu.

Users of the xl toolstack, see docs/misc/xl-disk-configuration.txt for
information on forcing the use of a particular disk backend and
xl.cfg(5) for information on forcing the use of qemu-xen-traditional.

Systems which only run PV guests and/or run HVM guests without PV
drivers are not vulnerable.

CREDITS
=======

This issue was discovered by Coverity Scan and Matthew Daley.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa71-qemu-xen-unstable.patch        xen-unstable, Xen 4.3.x
xsa71-qemu-xen-4.2.patch             Xen 4.2.x


$ sha256sum xsa71*.patch
a3f667e251a32fa5eff4a78eae49acd020b2f340fb203dc08a033d43841b0a2a  xsa71-qemu-xen-4.2.patch
f5ec607babb01dc8f8065dfe121882af4c3d93c035bafbfed48825dea684d6d9  xsa71-qemu-xen-unstable.patch
$
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