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             Xen Security Advisory CVE-2016-4480 / XSA-176
                               version 3

           x86 software guest page walk PS bit handling flaw

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

Public release.

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

The Page Size (PS) page table entry bit exists at all page table levels
other than L1.  Its meaning is reserved in L4, and conditionally
reserved in L3 and L2 (depending on hardware capabilities).  The
software page table walker in the hypervisor, however, so far ignored
that bit in L4 and (on respective hardware) L3 entries, resulting in
pages to be treated as page tables which the guest OS may not have
designated as such.  If the page in question is writable by an
unprivileged user, then that user will be able to map arbitrary guest
memory.

IMPACT
======

On vulnerable OSes, guest user mode code may be able to establish
mappings of arbitrary memory inside the guest, allowing it to elevate
its privileges inside the guest.

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

All Xen versions expose the vulnerability.

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

To be vulnerable, a system must have both a vulnerable hypervisor, and
a vulnerable guest operating system, i.e. ones which make non-standard
use of the PS bit.  We are not aware of any vulnerable guest operating
systems, but we cannot rule it out.  We have checked with maintainers
of the following operating systems, all of whom have said that to the
best of their knowledge their operating system is not vulnerable:
Linux, FreeBSD, NetBSD, OpenBSD, and Solaris.  Nor has it been observed
in common proprietary operating systems.

MITIGATION
==========

Running only PV guests will avoid this issue.

CREDITS
=======

This issue was discovered by Jan Beulich from SUSE.

RESOLUTION
==========

Applying the attached patch resolves this issue.

Note, however, that on hosts supporting 1Gb page mappings, for guests
which get this capability hidden via CPUID override in their config
file, fully correct behavior cannot be provided when using HAP paging.
This is a result of hardware behavior, which software cannot mitigate.
If that is a concern, such guests would need to be run in shadow paging
mode.

xsa176.patch      xen-unstable, Xen 4.6.x, Xen 4.5.x, Xen 4.4.x, Xen 4.3.x

$ sha256sum xsa176*
e61c52477a8d8aa79111d686b103202ff8a558d8b3356635288c1290789b7eb3  xsa176.patch
$

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

Deployment of the patches and/or mitigations 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: 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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