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            Xen Security Advisory CVE-2021-26932 / XSA-361
                               version 4

                Linux: grant mapping error handling issues

UPDATES IN VERSION 4
====================

Public release.

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

Grant mapping operations often occur in batch hypercalls, where a
number of operations are done in a single hypercall, the success or
failure of each one reported to the backend driver, and the backend
driver then loops over the results, performing follow-up actions based
on the success or failure of each operation.

Unfortunately, when running in PV mode, the Linux backend drivers
mishandle this: Some errors are ignored, effectively implying their
success from the success of related batch elements.  In other cases,
errors resulting from one batch element lead to further batch elements
not being inspected, and hence successful ones to not be possible to
properly unmap upon error recovery.

IMPACT
======

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

A malicious or buggy frontend driver may be able to cause resource
leaks in the domain running the corresponding backend driver, leading
to a denial of service.

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

All Linux versions back to at least 3.2 are vulnerable, when running in
PV mode on x86 or when running on Arm.

On x86, only systems with Linux backends running in PV mode are
vulnerable.  Linux backends run in HVM / PVH modes are not vulnerable.

MITIGATION
==========

On x86, running the backends in HVM or PVH domains will avoid the
vulnerability.

For protocols where other, e.g. non-kernel-based backends are available,
reconfiguring guests to use alternative (e.g. qemu-based) backends may
allow to avoid the vulnerability as long as these backends don't rely on
similar functionality provided by the xen-gntdev (/dev/gntdev) driver.

In all other cases there is no known mitigation.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the attached patches resolves this issue.

xsa361-linux-1.patch           Linux 5.11-rc - 3.19
xsa361-linux-2.patch           Linux 5.11-rc - 3.15
xsa361-linux-3.patch           Linux 5.11-rc - 4.19
xsa361-linux-4.patch           Linux 5.11-rc - 4.19
xsa361-linux-5.patch           Linux 5.11-rc - 4.4

$ sha256sum xsa361*
bb00ab6319b4fc536566af50c73e064f10f8b99eaa6b0f0b35a8d174c285a905  xsa361-linux-1.patch
73b6a54aa3773ce11f0de6b9aa1d80dd7f4c297dc71924b1a3886bc3b99ac859  xsa361-linux-2.patch
8e554cfab8cdb4fe1b74601a9432ea4c570f74a952ad757f9294ba1666cbeaea  xsa361-linux-3.patch
8c290895d10fc148f99e2a6587811b3037f29c3a0201d69d448ff520cea6f96d  xsa361-linux-4.patch
231ae3e1b9bec1b75dbbbee4b5acff620ef7ac2853332aa7b3c4957c6ca7f341  xsa361-linux-5.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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