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             Xen Security Advisory CVE-2013-4368 / XSA-67
                              version 2

         Information leak through outs instruction emulation

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

Public release.

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

The emulation of the outs instruction for 64-bit PV guests uses an
uninitialized variable as the segment base for the source data if an FS: or
GS: segment override is used, and if the segment descriptor the respective
non-null selector in the corresponding selector register points to cannot be
read by the emulation code (this is possible if the segment register was
loaded before a more recent GDT or LDT update, i.e. the segment register
contains stale data).

A malicious guest might be able to get hold of contents of the hypervisor
stack, through the fault address passed to the page fault handler if the outs
raises such a fault (which is mostly under guest control).  Other methods for
indirectly deducing information also exist.

IMPACT
======

A malicious 64-bit PV guest might conceivably gain access to sensitive data
relating to other guests.

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

Xen 3.1.x and later are vulnerable.

Only 64-bit PV guests can take advantage of this vulnerability.

MITIGATION
==========

Running only HVM or 32-bit PV guests will avoid this issue.

CREDITS
=======

This issue was discovered by Coverity Scan and Matthew Daley.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa67.patch             Xen 4.2.x, Xen 4.3.x, xen-unstable

$ sha256sum xsa67*.patch
7de3ac9baa6cd9fead46e68912dfa0189e900095317645d0e33d85346fc8a028  xsa67.patch
$
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