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            Xen Security Advisory CVE-2017-12855 / XSA-230
                              version 3

 grant_table: possibly premature clearing of GTF_writing / GTF_reading

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

CVE assigned.

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

Xen maintains the _GTF_{read,writ}ing bits as appropriate, to inform the
guest that a grant is in use.  A guest is expected not to modify the
grant details while it is in use, whereas the guest is free to
modify/reuse the grant entry when it is not in use.

Under some circumstances, Xen will clear the status bits too early,
incorrectly informing the guest that the grant is no longer in use.

IMPACT
======

A guest may prematurely believe that a granted frame is safely private
again, and reuse it in a way which contains sensitive information, while
the domain on the far end of the grant is still using the grant.

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

All systems are vulnerable.

MITIGATION
==========

There are no mitigations.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa230.patch           xen-unstable, 4.9, 4.8, 4.7, 4.6, 4.5

$ sha256sum xsa230*
912c24771dc9e9b305be630b7771505abb3db735564c5574fc30b58a5da0139e  xsa230.meta
77a73f1c32d083e315ef0b1bbb119cb8840ceb5ada790cad76cbfb9116f725cc  xsa230.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is then no longer applicable.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


NOTE REGARDING SHORT EMBARGO
============================

This issue was discovered while investigating problems with the initial
version of XSA-226.  Accordingly, XSA-230 is embargoed and the embargo
will end at the same time as that of XSA-226.
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