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            Xen Security Advisory CVE-2021-28688 / XSA-371
                               version 3

           Linux: blkback driver may leak persistent grants

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

Public release.

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

The fix for XSA-365 includes initialization of pointers such that
subsequent cleanup code wouldn't use uninitialized or stale values.
This initialization went too far and may under certain conditions also
overwrite pointers which are in need of cleaning up.  The lack of
cleanup would result in leaking persistent grants.  The leak in turn
would prevent fully cleaning up after a respective guest has died,
leaving around zombie domains.

IMPACT
======

A malicious or buggy frontend driver may be able to cause resource leaks
from the corresponding backend driver.  This can result in a host-wide
Denial of Sevice (DoS).

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

All Linux versions having the fix for XSA-365 applied are vulnerable.
XSA-365 was classified to affect versions back to at least 3.11.

MITIGATION
==========

Reconfiguring guests to use alternative (e.g. qemu-based) backends may
avoid the vulnerability.

Avoiding the use of persistent grants will also avoid the vulnerability.
This can be achieved by passing the "feature_persistent=0" module option
to the xen-blkback driver.

CREDITS
=======

This issue was discovered by Nicolai Stange of SUSE.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa371-linux.patch           Linux 5.12-rc, 5.11.1 onwards, 5.10.18 onwards
<not directly affected>      Linux 5.10.0 - 5.10.17, 5.11.0
<applicability unknown>      Linux 4.4 - 5.9
<no fix available>           Linux 3.11 - 4.3

$ sha256sum xsa371*
1b2472253aa82385b3eff280fa4adf52742f06813fc093f5f86cd4a3021f736c  xsa371-linux.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 mitigations described above is NOT permitted
during the embargo on public-facing systems with untrusted guest users
and administrators.  This is because such configuration changes may be
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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