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            Xen Security Advisory CVE-2023-34324 / XSA-441
                               version 4

           Possible deadlock in Linux kernel event handling

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

Public release.

Modified advisory again to state that Arm32 guests are NOT affected.

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

Closing of an event channel in the Linux kernel can result in a deadlock.
This happens when the close is being performed in parallel to an unrelated
Xen console action and the handling of a Xen console interrupt in an
unprivileged guest.

The closing of an event channel is e.g. triggered by removal of a
paravirtual device on the other side. As this action will cause console
messages to be issued on the other side quite often, the chance of
triggering the deadlock is not neglectable.

Note that 32-bit Arm-guests are not affected, as the 32-bit Linux kernel
on Arm doesn't use queued-RW-locks, which are required to trigger the
issue (on Arm32 a waiting writer doesn't block further readers to get
the lock).

IMPACT
======

A (malicious) guest administrator could cause a denial of service (DoS)
in a backend domain (other than dom0) by disabling a paravirtualized
device.

A malicious backend could cause DoS in a guest running a Linux kernel by
disabling a paravirtualized device.

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

All unprivileged guests running a Linux kernel of version 5.10 and later,
or with the fixes for XSA-332, are vulnerable.

All guest types are vulnerable.

Only x86- and 64-bit Arm-guests are vulnerable.

Arm-guests running in 32-bit mode are not vulnerable.

Guests not using paravirtualized drivers are not vulnerable.

MITIGATION
==========

There is no known mitigation.

CREDITS
=======

This issue was discovered as a bug by Marek Marczykowski-Górecki of
Invisible Things Lab; the security impact was recognised by Jürgen
Groß of SUSE.

RESOLUTION
==========

Applying the attached patch resolves this issue.

Note that patches for released versions are generally prepared to
apply to the stable branches, and may not apply cleanly to the most
recent release tarball.  Downstreams are encouraged to update to the
tip of the stable branch before applying these patches.

xsa441-linux.patch     Linux

$ sha256sum xsa441*
937406d86dd6dd3e389fdae726a25e5f0e960f7004c314e370cb2369d6715c24  xsa441-linux.patch
$

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

Deployment of the patches and/or mitigations described above (or
others which are substantially similar) on the host and on VMs being
administered and used only by organisations which are members of the Xen
Project Security Issue Predisclosure List is permitted during the embargo,
even on public-facing systems with other untrusted guest users and
administrators.

But: 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.

Deployment of patches or mitigations is NOT permitted on VMs being
administered or used by organisations which are not members of the Xen
Project Security Issue Predisclosure List. On those VMs deployment is
permitted only AFTER the embargo ends.

(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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