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            Xen Security Advisory CVE-2021-28691 / XSA-374
                               version 2

          Guest triggered use-after-free in Linux xen-netback

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

Public release.

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

A malicious or buggy network PV frontend can force Linux netback to
disable the interface and terminate the receive kernel thread
associated with queue 0 in response to the frontend sending a
malformed packet.

Such kernel thread termination will lead to a use-after-free in Linux
netback when the backend is destroyed, as the kernel thread associated
with queue 0 will have already exited and thus the call to
kthread_stop will be performed against a stale pointer.

IMPACT
======

A malicious or buggy frontend driver can trigger a dom0 crash.
Privilege escalation and information leaks cannot be ruled out.

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

Systems using Linux version 5.5 or newer are vulnerable.

MITIGATION
==========

On x86 running only HVM guests with emulated network cards will avoid the
issue.  There's however no option in the upstream toolstack to offer only
emulated network cards to guests.

CREDITS
=======

This issue was discovered by Michael Brown of iPXE and diagnosed by
Olivier Benjamin, Michael Kurth and Martin Mazein of AWS.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa374-linux.patch     Linux 5.5.0 - 5.12.2

$ sha256sum xsa374*
156cee65022359a5901cce97714d2abb16fef786246b1c4bf509083d21e085d6  xsa374-linux.patch
$

DEPLOYMENT DURING EMBARGO
=========================

Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
administrators.

But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).

Deployment of the mitigation to disable PV network interfaces is NOT
permitted (except where all the affected systems and VMs are
administered and used only by organisations which are members of the
Xen Project Security Issues Predisclosure List).  Specifically,
deployment on public cloud systems is NOT permitted.

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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