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            Xen Security Advisory CVE-2021-26931 / XSA-362
                               version 3

         Linux: backends treating grant mapping errors as bugs

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

Public release.

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

Block, net, and SCSI backends consider certain errors a plain bug,
deliberately causing a kernel crash.  For errors potentially being at
least under the influence of guests, like out of memory conditions, it
isn't correct to assume so.  Memory allocations potentially causing
such crashes occur only when Linux is running in PV mode, though.

IMPACT
======

A malicious or buggy frontend driver may be able to crash the
corresponding backend driver, potentially affecting the entire domain
running the backend driver.

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

Linux versions from at least 2.6.39 onwards are vulnerable, when run in
PV mode.  Earlier versions differ significantly in behavior and may
therefore instead surface other issues under the same conditions.  Linux
run in HVM / PVH modes is not vulnerable.

MITIGATION
==========

For Linux, running the backends in HVM or PVH domains will avoid the
vulnerability.

For protocols where non-Linux-kernel based backends are available,
reconfiguring guests to use alternative (e.g. qemu-based) backends may
allow to avoid the vulnerability.

In all other cases there is no known mitigation.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate attached patches resolves this issue.

Applying the attached patches resolves this issue.

xsa362-linux-1.patch           Linux 5.11-rc - 5.10
xsa362-linux-2.patch           Linux 5.11-rc - 3.16
xsa362-linux-3.patch           Linux 5.11-rc - 4.1

$ sha256sum xsa362*
d64334807f16ff9909503b3cc9b8b93fd42d2c36e1fb0e508b89a765a53071a8  xsa362-linux-1.patch
b6d02952e7fbede55b868cb2dc4d8853284996883dc72518a0cd5b14d6c7fdd4  xsa362-linux-2.patch
0a2661380d8f786fefe12e5a8b1528d4a79f1ad058c26b417c52449a7e16a302  xsa362-linux-3.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.

Deployment of the mitigation to switch to HVM / PVH backend domains
is also permitted during the embargo, even on public-facing systems with
untrusted guest users and administrators.

HOWEVER, deployment of the non-kernel-based backends 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 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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