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            Xen Security Advisory CVE-2012-3515 / XSA-17
                           version 2

               Qemu VT100 emulation vulnerability

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

Public release.

Update version tag format.

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

The device model used by fully virtualised (HVM) domains, qemu, does
not properly handle escape VT100 sequences when emulating certain
devices with a virtual console backend.

IMPACT
======

An attacker who has sufficient privilege to access a vulnerable device
within a guest can overwrite portions of the device model's address
space. This can allow them to escalate their privileges to that of the
device model process.

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

All Xen systems running HVM guests are potentially 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.

MITIGATION
==========

This issue can be avoided by only running PV guests or by configuring
HVM guests to not use the virtual console('vc') backend for any device.

For serial devices specify in your guest configuration:
     serial = 'none'
in your guest configuration.

For parallel port devices the syntax is toolstack specific.
For xend specify in your guest configuration:
     parallel = 'none'
For xl specify in your guest configuration:
     xl: device_model_args = ['-parallel', 'none']

In both cases the default is to use the vulnerable 'vc' mode.

You can confirm whether or not you are vulnerable by pressing
Ctrl-Alt-<N> (for digit N) while connected to either the VNC or SDL
console. If you are able to switch to a window displaying "serial" or
"parallel" then you are vulnerable.

The issue can also be mitigated by enabling the stub domain device
model. In this case the attacked can only potentially gain control of
the stub domain and not of the entire system.

To enable stub domains specify in your guest configuration:
    device_model = "stubdom-dm"

RESOLUTION
==========

Applying the appropriate attached patch(es) will resolve the issue.

PATCH INFORMATION
=================

The attached patches resolve this issue

xsa17-qemu-xen-traditional-all.patch  qemu-xen-traditional master, 4.0, 4.1
xsa17-qemu-xen-unstable.patch         qemu-xen master

$ sha256sum xsa17-*.patch
60215322d3fbbc2054dfc160a20d9e0811af88487c4edc2f6ea81dcd5cedf039  xsa17-qemu-xen-traditional-all.patch
7b4bb59e7757080e7806a8b8eeb6b78fa0ffdfbfb28a7a379f7edff285bffd88  xsa17-qemu-xen-unstable.patch
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