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            Xen Security Advisory CVE-2017-12136 / XSA-228
                               version 3

     grant_table: Race conditions with maptrack free list handling

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

Public release.

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

The grant table code in Xen has a bespoke semi-lockfree allocator for
recording grant mappings ("maptrack" entries).  This allocator has a
race which allows the free list to be corrupted.

Specifically: the code for removing an entry from the free list, prior
to use, assumes (without locking) that if inspecting head item shows
that it is not the tail, it will continue to not be the tail of the
list if it is later found to be still the head and removed with
cmpxchg.  But the entry might have been removed and replaced, with the
result that it might be the tail by then.  (The invariants for the
semi-lockfree data structure were never formally documented.)

Additionally, a stolen entry is put on the free list with an incorrect
link field, which will very likely corrupt the list.

IMPACT
======

A malicious guest administrator can crash the host, and can probably
escalate their privilege to that of the host.

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

Xen 4.6 and later are vulnerable.

Xen 4.5 and earlier are not vulnerable.

MITIGATION
==========

There is no mitigation for this vulnerability.

CREDITS
=======

This issue was discovered by Ian Jackson of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa228.patch           xen-unstable, Xen 4.9.x
xsa228-4.8.patch       Xen 4.8.x, Xen 4.7.x, Xen 4.6.x

$ sha256sum xsa228*
35a1a7f8905770fa64da0756fe3e0400bb8c28ecae0b7cf80e749cb7962018db  xsa228.meta
1979e111442517891b483e316a15a760a4c992ac4440f95e361ff12f4bebff62  xsa228.patch
5a7416f15ac9cd7cace354b6102ff58199fe0581f65a36a36869650c71784e48  xsa228-4.8.patch
$

(The .meta file is a prototype machine-readable file for describing
which patches are to be applied how.)

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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