Xen Security Advisory 277 v2 - x86: incorrect error handling for guest p2m page removals

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Xen Security Advisory 277 v2 - x86: incorrect error handling for guest p2m page removals

Xen.org security team
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                    Xen Security Advisory XSA-277
                              version 2

       x86: incorrect error handling for guest p2m page removals

UPDATES IN VERSION 2
====================

Public release.

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

The internal function querying a domain's p2m table grabs the p2m lock
by default, so that the answer to the query remains true until the
caller can act on that information; it is up to the caller then to
release the lock.  Unfortunately, certain failure paths don't release
the lock.

IMPACT
======

A malicious or buggy guest may cause a deadlock, resulting in a DoS
(Denial of Service) affecting the entire host.

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

Xen 4.11 and onward are vulnerable.

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

Only systems running untrusted HVM or PVH guests are vulnerable.
Systems running only PV guests are not vulnerable.

MITIGATION
==========

Running only PV guests will avoid this vulnerability.

CREDITS
=======

This issue was discovered by Paul Durrant of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa277.patch           xen-unstable, Xen 4.11.x

$ sha256sum xsa277*
576cdc05975e43698624b88f7290119dd702b3db8f30f3219754d992d7fef0c6  xsa277.meta
c9025e1daaec4081a61f1ed7b96e69cfe8e35bdd5b4fcc0fadc98f71c2e243e2  xsa277.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is then no longer applicable.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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xsa277.meta (918 bytes) Download Attachment
xsa277.patch (2K) Download Attachment