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            Xen Security Advisory CVE-2019-17342 / XSA-287
                              version 3

         x86: steal_page violates page_struct access discipline

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

CVE assigned.

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

Xen's reference counting rules were designed to allow pages to change
owner and state without requiring a global lock.  Each page has a page
structure, and a very specific set of access disciplines must be
observed to ensure that pages are freed properly, and that no writable
mappings exist for PV pagetable pages.

Unfortunately, when the XENMEM_exchange hypercall was introduced,
these access disciplines were violated, opening up several potential
race conditions.

IMPACT
======

A single PV guest can leak arbitrary amounts of memory, leading to a
denial of service.

A cooperating pair of PV and HVM/PVH guests can get a writable
pagetable entry, leading to information disclosure or privilege
escalation.

Privilege escalation attacks using only a single PV guest or a pair of
PV guests have not been ruled out.

Note that both of these attacks require very precise timing, which may
be difficult to exploit in practice.

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

Only x86 systems are vulnerable.

Only systems which run PV guests are vulnerable.  Systems which run
only HVM/PVH guests are not vulnerable.

MITIGATION
==========

Running only HVM or PVH guests will avoid these vulnerabilities.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa287.patch           xen-unstable
xsa287-4.11.patch      Xen 4.11.x
xsa287-4.10.patch      Xen 4.10.x
xsa287-4.9.patch       Xen 4.9.x
xsa287-4.8.patch       Xen 4.8.x
xsa287-4.7.patch       Xen 4.7.x

$ sha256sum xsa287*
ae2b9261e26df871693478629c63970ba30817ee1dcb2266b89d8b067833c1b3  xsa287.meta
7de1b886d69dd7c497f88d41adf9a6f7cf9a305fd8ae9d714e1125e2a22208ab  xsa287.patch
55f40f2f9bb41c85ac80dac775352e28b25fada80dae574e9d10300d5e2b91ce  xsa287-4.7.patch
57312ff131eb6b51235723e862adf42ad3529ed13135375875c054fa0b55f80b  xsa287-4.8.patch
34f4b835766a38bcf4066ccbab74676eda176e15ed2a6bd7884678a64507f89a  xsa287-4.9.patch
c7eaf8a325011dda84b02ee097ddbc7b5f2f4d3399de545a3a7b14e2d23f4278  xsa287-4.10.patch
6793315f714a249a4fad12b36559640b2f97f19f5b85f0d58694c6e78aa3d567  xsa287-4.11.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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