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Advisory

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            Xen Security Advisory CVE-2017-17566 / XSA-248
                              version 3

         x86 PV guests may gain access to internally used pages

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

CVE assigned.

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

Memory management for PV guests builds on page ownership and page
attributes.  A domain can always map, at least r/o, pages of which it
is the owner.  Certain fields in the control structure of a page are
used for different purposes in the main PV memory management code and
in code handling shadow paging.

When a guest is running in shadow mode (which for PV guests is necessary
e.g. for live migration), certain auxiliary pages used by Xen internally
had their owner set to the guest itself.  When the PV guest maps such a
page, shadow code and PV memory management code will disagree on the
meaning of said multi-purpose fields, generally leading to a crash of
the hypervisor.

IMPACT
======

A malicious or buggy PV guest may cause a hypervisor crash, resulting in
a Denial of Service (DoS) affecting the entire host, or cause hypervisor
memory corruption.  We cannot rule out a guest being able to escalate
its privilege.

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

All versions of Xen are vulnerable.

Only x86 systems are affected.  ARM systems are not vulnerable.

x86 HVM guests cannot exploit this vulnerability.

Only x86 PV guests can exploit this vulnerability, and only when being
run in shadow mode.  PV guests are typically run in shadow mode for live
migration, as well as for features like VM snapshot.

Note that save / restore does *not* use shadow mode, and so does not
expose this vulnerability.  Some downstreams also include a "non-live
migration" feature, which also does not use shadow mode (and thus does
not expose this vulnerability).

MITIGATION
==========

Running only HVM guests avoids the vulnerability.

Avoiding live migration of x86 PV guests also avoids the vulnerability.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa248.patch           xen-unstable, Xen 4.9.x
xsa248-4.8.patch       Xen 4.8.x, Xen 4.7.x, Xen 4.6.x
xsa248-4.5.patch       Xen 4.5.x

$ sha256sum xsa248*
f0ac5c5ff956118f52821e111c6e27416f788cea6e98cc54cb051c42b793357e  xsa248.meta
20bcfb1890d90bd74f52e45a1e8aa020a8991e3a0db37eecf53ce48b16e602bf  xsa248.patch
ec4227633df18f76fbd8cb12e367879470b63fb5236f10b2a971dccef9f83172  xsa248-4.5.patch
3bbd9fd92e5ffab1ddd7ff804bfbab09c1c654af3aa7f80f742f321da120b715  xsa248-4.8.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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