Xen Security Advisory 408 v2 (CVE-2022-33745) - insufficient TLB flush for x86 PV guests in shadow mode

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

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            Xen Security Advisory CVE-2022-33745 / XSA-408
                               version 2

        insufficient TLB flush for x86 PV guests in shadow mode

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

Added metadata

Public release.

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

For migration as well as to work around kernels unaware of L1TF (see
XSA-273), PV guests may be run in shadow paging mode.  To address
XSA-401, code was moved inside a function in Xen.  This code movement
missed a variable changing meaning / value between old and new code
positions.  The now wrong use of the variable did lead to a wrong TLB
flush condition, omitting flushes where such are necessary.

IMPACT
======

The known (observed) impact would be a Denial of Service (DoS) affecting
the entire host, due to running out of memory.  Privilege escalation and
information leaks cannot be ruled out.

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

All versions of Xen with the XSA-401 fixes applied are vulnerable.

Only x86 PV guests can trigger this vulnerability, and only when running
in shadow mode.  Shadow mode would be in use when migrating guests or as
a workaround for XSA-273 (L1TF).

MITIGATION
==========

Not running x86 PV guests will avoid the vulnerability.

CREDITS
=======

This issue was discovered by Charles Arnold of SUSE.

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.

xsa408.patch           xen-unstable - Xen 4.14.x
xsa408-4.13.patch      Xen 4.13.x

$ sha256sum xsa408*
7349445d53b68bc8e2be2aea9fa20409a9b87e0d6b78fc2515093a65668444a0  xsa408.meta
f49cb67842c7576f1d59b965331956a9fa1f529a8e2da3531d7ebc4eb3f079b3  xsa408.patch
26871efbd3f834dd4af4fbab6f2cb09a83c509e49894f025ee656071419ed995  
xsa408-4.13.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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