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            Xen Security Advisory CVE-2021-28706 / XSA-385
                               version 2

             guests may exceed their designated memory limit

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

Add CVE numbers to patches.

Public release.

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

When a guest is permitted to have close to 16TiB of memory, it may be
able to issue hypercalls to increase its memory allocation beyond the
administrator established limit.  This is a result of a calculation
done with 32-bit precision, which may overflow.  It would then only
be the overflowed (and hence small) number which gets compared against
the established upper bound.

IMPACT
======

A guest may be able too allocate unbounded amounts of memory to itself.
This may result in a Denial of Service (DoS) affecting the entire host.

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

All Xen versions from at least 3.2 onwards are affected.

On x86, only Xen builds with the BIGMEM configuration option enabled are
affected.  (This option is off by default.)

Only hosts with more than 16 TiB of memory are affected.

MITIGATION
==========

Setting the maximum amount of memory a guest may allocate to strictly
less than 1023 GiB will avoid the vulnerability.

CREDITS
=======

This issue was discovered by Julien Grall of Amazon.

RESOLUTION
==========

Applying the appropriate first attached patch resolves this specific
issue.  The second patch in addition documents altered support status of
Xen on huge memory systems.

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.

xsa385-?.patch           xen-unstable
xsa385-4.15.patch        Xen 4.15.x - 4.14.x
xsa385-4.13.patch        Xen 4.13.x
xsa385-4.12.patch        Xen 4.12.x

$ sha256sum xsa385*
b278902e293730a117605200910180bb842cf95db4bdedfd54b42b7314041d8c  xsa385-1.patch
46a5ccfbb763b857f6cd0df46a9b7eed155b9de399ca4c68c9925faf4d1d9adb  xsa385-2.patch
69ebe63dc7dca71f74260af19205a6387be56c7dc67b97fa7695ab1acd3c4da4  xsa385-4.12.patch
858eaad715e7cc62c4ab9784360f4ec77df70b2636b0755afe780d5c618cf9b4  xsa385-4.13.patch
831e86c3adfec532b1a48a0b967b7c58c37db3733aee8d78216eb9d535b34f12  xsa385-4.15.patch
$

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.

HOWEVER, deployment of the 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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