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Advisory

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            Xen Security Advisory CVE-2017-10917 / XSA-221
                               version 4

               NULL pointer deref in event channel poll

UPDATES IN VERSION 4
====================

Normalize version tags.

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

When polling event channels, in general arbitrary port numbers can be
specified.  Specifically, there is no requirement that a polled event
channel ports has ever been created.  When the code was generalised
from an earlier implementation, introducing some intermediate
pointers, a check should have been made that these intermediate
pointers are non-NULL.  However, that check was omitted.

IMPACT
======

A malicious or buggy guest may cause the hypervisor to access
addresses it doesn't control, usually leading to a host crash (Denial
of Service).  Information leaks cannot be excluded.

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

Xen versions 4.4 and newer are vulnerable.  Xen versions 4.3 and
earlier are not affected.

Both x86 and ARM systems are vulnerable.

While all guest kinds can cause a Denial of Service, only x86 PV guests
may be able to leverage the possible information leaks.

MITIGATION
==========

There is no known mitigation.

CREDITS
=======

This issue was discovered by Ankur Arora of Oracle.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa221.patch           xen-unstable - Xen 4.4.x

$ sha256sum xsa221*
2425396a713466808b0f75f91337be4dd20a4dee7733972b04489773c6e97655  xsa221.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is then no longer applicable.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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