Xen Security Advisory 318 v3 (CVE-2020-11742) - Bad continuation handling in GNTTABOP_copy

[Xen.org security team <security@xxxxxxx>]

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            Xen Security Advisory CVE-2020-11742 / XSA-318
                               version 3

              Bad continuation handling in GNTTABOP_copy

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

Public release.

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

Grant table operations are expected to return 0 for success, and a
negative number for errors.  The fix for CVE-2017-12135 / XSA-226
introduced a path through grant copy handling where success may be
returned to the caller without any action taken.

In particular the status fields of individual operations are left
uninitialised, and may result in errant behaviour in the caller of
GNTTABOP_copy.

IMPACT
======

A buggy or malicious guest can construct its grant table in such a way
that, when a backend domain tries to copy a grant, it hits the incorrect
exit path.

This returns success to the caller without doing anything, which may
cause in crashes or other incorrect behaviour.

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

Systems running any version of Xen are vulnerable.

MITIGATION
==========

Only guests with access to transitive grants can exploit the
vulnerability.  In particular, this means that:

 * ARM systems which have taken the XSA-268 fix are not vulnerable, as
   Grant Table v2 was disabled for other security reasons.

 * All systems with the XSA-226 fixes, and booted with
   `gnttab=max-ver:1` or `gnttab=no-transitive` are not vulnerable.

CREDITS
=======

This issue was discovered by Pawel Wieczorkiewicz of Amazon and Jürgen
Groß of SUSE.

RESOLUTION
==========

Applying the attached patch resolves this issue.

Note that patches for released versions are generally prepared to
apply to the stable branches, and may not apply cleanly to the most
recent release tarball.  Downstreams are encouraged to update to the
tip of the stable branch before applying these patches.

xsa318.patch       Xen 4.9 - xen-unstable

$ sha256sum xsa318*
4618c2609ab08178977c2b2a3d13f380ccfddd0168caca5ced708dd76a8e547c  xsa318.patch
$

NOTE CONCERNING SHORT EMBARGO
=============================

This issue was discovered in response to the XSA-316 predisclosure.

DEPLOYMENT DURING EMBARGO
=========================

Deployment of the patches 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.

However, deployment of the mitigations is NOT permitted (except where
all the affected systems and VMs are administered and used only by
organisations which are members of the Xen Project Security Issues
Predisclosure List).  Specifically, deployment on public cloud systems
is NOT permitted.

This is because it is a guest visible change which will draw attention
to the issue.
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Attachment: xsa318.patch
Description: Binary data