Xen Security Advisory 356 v3 (CVE-2020-29567) - infinite loop when cleaning up IRQ vectors

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

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            Xen Security Advisory CVE-2020-29567 / XSA-356
                               version 3

              infinite loop when cleaning up IRQ vectors

UPDATES IN VERSION 3
====================

Public release.

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

When moving IRQs between CPUs to distribute the load of IRQ handling,
IRQ vectors are dynamically allocated and de-allocated on the relevant
CPUs.  De-allocation has to happen when certain constraints are met.
If these conditions are not met when first checked, the checking CPU
may send an interrupt to itself, in the expectation that this IRQ will
be delivered only after the condition preventing the cleanup has
cleared.  For two specific IRQ vectors this expectation was violated,
resulting in a continuous stream of self-interrupts, which renders the
CPU effectively unusable.

IMPACT
======

A domain with a passed through PCI device can cause lockup of a
physical CPU, resulting in a Denial of Service (DoS) to the entire
host.

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

Only Xen 4.14 is affected.  Xen versions 4.13 and older are not
affected.

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

Only guests with physical PCI devices passed through to them can exploit
the vulnerability.

MITIGATION
==========

There is no known mitigation.

CREDITS
=======

This issue was discovered by Roger Pau Monné of Citrix.

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.

xsa356.patch           xen-unstable - Xen 4.14.x

$ sha256sum xsa356*
77316e3b86e2482ee9741db7484d323a399028762af1c88734f8c83e78069fb3  xsa356.meta
21c217e41549bf74d5fcc26f1d23b6d902c5c72de5e2c8490842aea9f999b036  xsa356.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).

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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