Xen Security Advisory 449 v2 (CVE-2023-46839) - pci: phantom functions assigned to incorrect contexts

[Xen.org security team <security@xxxxxxx>]

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            Xen Security Advisory CVE-2023-46839 / XSA-449
                               version 2

         pci: phantom functions assigned to incorrect contexts

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

Public release.

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

PCI devices can make use of a functionality called phantom functions,
that when enabled allows the device to generate requests using the IDs
of functions that are otherwise unpopulated.  This allows a device to
extend the number of outstanding requests.

Such phantom functions need an IOMMU context setup, but failure to
setup the context is not fatal when the device is assigned.  Not
failing device assignment when such failure happens can lead to the
primary device being assigned to a guest, while some of the phantom
functions are assigned to a different domain.

IMPACT
======

Under certain circumstances a malicious guest assigned a PCI device
with phantom functions may be able to access memory from a previous
owner of the device.

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

Systems running all version of Xen are affected.

Only x86 systems are vulnerable.  Arm systems are not vulnerable.

Only systems using PCI passthrough of devices with phantom functions
are affected.

MITIGATION
==========

There is no mitigation (other than not passing through PCI devices
with phantom functions to guests).

CREDITS
=======

This issue was discovered by Roger Pau Monné of XenServer.

RESOLUTION
==========

Applying the appropriate 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.

xsa449.patch           xen-unstable - Xen 4.17.x
xsa449-4.16.patch      Xen 4.16.x - Xen 4.15.x

$ sha256sum xsa449*
f77914aae8f917952f66d863d26314875ff96a0d8178f64c94b95825eabbc8a8  xsa449.patch
8f0302c24535ad4c7379469f33afcfdce08ba6db970e0ca1a1bfdd788af6fc6c  
xsa449-4.16.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.

HOWEVER, deployment of the mitigation 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.

This is because removing/replacing of pass-through devices or their
replacement by emulated devices is a guest visible configuration
change, which may lead to re-discovery of the issue.

Deployment of this mitigation is permitted only AFTER the embargo ends.

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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Attachment: xsa449.patch
Description: Binary data

Attachment: xsa449-4.16.patch
Description: Binary data