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            Xen Security Advisory CVE-2021-28697 / XSA-379
                               version 2

 grant table v2 status pages may remain accessible after de-allocation

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

Patches updated to fix a typo in a comment.

Public release.

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

Guest get permitted access to certain Xen-owned pages of memory.  The
majority of such pages remain allocated / associated with a guest for
its entire lifetime.  Grant table v2 status pages, however, get
de-allocated when a guest switched (back) from v2 to v1.  The freeing
of such pages requires that the hypervisor know where in the guest
these pages were mapped.  The hypervisor tracks only one use within
guest space, but racing requests from the guest to insert mappings of
these pages may result in any of them to become mapped in multiple
locations.  Upon switching back from v2 to v1, the guest would then
retain access to a page that was freed and perhaps re-used for other
purposes.

IMPACT
======

A malicious guest may be able to elevate its privileges to that of the
host, cause host or guest Denial of Service (DoS), or cause information
leaks.

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

All Xen versions from 4.0 onwards are affected.  Xen versions 3.4 and
older are not affected.

Only x86 HVM and PVH guests permitted to use grant table version 2
interfaces can leverage this vulnerability.  x86 PV guests cannot
leverage this vulnerability.  On Arm, grant table v2 use is explicitly
unsupported.

MITIGATION
==========

Running only PV guests will avoid this vulnerability.

Suppressing use of grant table v2 interfaces for HVM or PVH guests will
also avoid this vulnerability.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate 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.

xsa379.patch           xen-unstable
xsa379-4.15.patch      Xen 4.15.x
xsa379-4.14.patch      Xen 4.14.x - 4.13.x
xsa379-4.12.patch      Xen 4.12.x - 4.11.x

$ sha256sum xsa379*
bdda4cb431301551336388ff7300a6ae95bb75af8fcae09cfb12c22a91d399d9  xsa379.meta
508dbfcac7420ec780df39402116bf7d3f497c4a9d883a369df7bf5340778e6c  xsa379.patch
2a1db918f1fa387a97d7bcb525eaa928fd71a9967e6ced4e7ac6e39a79ab5b80  xsa379-4.12.patch
c57b72078460f45a5e003db5c4c3669f27310420e04eb16e4413318dfee54fa1  xsa379-4.14.patch
3154869b12fcde70ce845df723aae4bbb2eb9576d90267c1be01eb6d3c5196e9  xsa379-4.15.patch
$

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

Deployment of the patches and/or PV-guest-only 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.

HOWEVER, deployment of the grant table v2 disabling mitigation described
above is NOT permitted during the embargo on public-facing systems with
untrusted guest users and administrators.  This is because such a
configuration change is recognizable by the affected guests.

AND: 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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