Xen Security Advisory 363 v3 (CVE-2021-26934) - Linux: display frontend "be-alloc" mode is unsupported

Xen Security Advisory 363 v3 (CVE-2021-26934) - Linux: display frontend "be-alloc" mode is unsupported

[Xen.org security team]

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            Xen Security Advisory CVE-2021-26934 / XSA-363
                               version 3

        Linux: display frontend "be-alloc" mode is unsupported

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

Public release.

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

The backend allocation mode of Linux'es drm_xen_front drivers was
not meant to be a supported configuration, but this wasn't stated
accordingly in its support status entry.

IMPACT
======

Use of the feature may have unknown effects.

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

Linux versions from 4.18 onwards are affected.  Earlier Linux versions
do not provide the affected driver.

MITIGATION
==========

Not using the driver or its backend allocation mode will avoid the
vulnerability.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the attached patch documents the situation.  The patch does
not fix any security issues.

xsa363.patch           xen-unstable

$ sha256sum xsa363*
cf2f2eff446aec625b19d9d01301ec66098b58b792d74012235f10c62a21bb68  xsa363.patch
$

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From: Jan Beulich <jbeulich@suse.com>
Subject: SUPPORT.md: PV display frontend is unsupported in "backend allocation" mode

This wasn't meant to be supported, but wasn't stated this way.

This is XSA-363.

Reported-by: Jan Belich <jbeulich@suse.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/SUPPORT.md
+++ b/SUPPORT.md
@@ -414,7 +414,8 @@ Guest-side driver capable of speaking th
 
 Guest-side driver capable of speaking the Xen PV display protocol
 
-    Status, Linux: Supported
+    Status, Linux: Supported (outside of "backend allocation" mode)
+    Status, Linux: Experimental (in "backend allocation" mode)
 
 ### PV Console (frontend)