Bug 1936565 (CVE-2021-28038)

Xen Security Advisory CVE-2021-28038 / XSA-367
                              version 2

          Linux: netback fails to honor grant mapping errors

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

CVE assigned.

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

XSA-362 tried to address issues here, but in the case of the netback
driver the changes were insufficient: It left the relevant function
invocation with, effectively, no error handling at all.  As a result,
memory allocation failures there could still lead to frontend-induced
crashes of the backend.

IMPACT
======

A malicious or buggy networking frontend driver may be able to crash
the corresponding backend driver, potentially affecting the entire
domain running the backend driver.  In a typical (non-disaggregated)
system that is a host-wide denial of service (DoS).

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 example, by running the dom0 in PVH mode.

In all other cases there is no known mitigation.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa367-linux.patch           Linux 5.12-rc

$ sha256sum xsa367*
b0244bfddee91cd7986172893e70664b74e698c5d44f25865870f179f80f9a92  xsa367-linux.patch
$

CREDITS
=======

This issue was reported by Intel's kernel test robot and recognized as a
security issue by Jan Beulich of SUSE.

NOTE REGARDING LACK OF EMBARGO
==============================

This issue was reported publicly, before the XSA could be issued.