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Advisory

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            Xen Security Advisory CVE-2026-23554 / XSA-480
                               version 3

              Use after free of paging structures in EPT

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

Public release.

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

The Intel EPT paging code uses an optimization to defer flushing of any cached
EPT state until the p2m lock is dropped, so that multiple modifications done
under the same locked region only issue a single flush.

Freeing of paging structures however is not deferred until the flushing is
done, and can result in freed pages transiently being present in cached state.
Such stale entries can point to memory ranges not owned by the guest, thus
allowing access to unintended memory regions.

IMPACT
======

Privilege escalation, Denial of Service (DoS) affecting the entire host,
and information leaks.

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

Xen 4.17 and onwards are vulnerable.  Xen 4.16 and older are not vulnerable.

Only x86 Intel systems with EPT support are vulnerable.

Only x86 HVM/PVH guests using HAP can leverage the vulnerability on affected
systems.

MITIGATION
==========

There are no mitigations.

CREDITS
=======

This issue was discovered by Roger Pau Monné of XenServer.

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.

xsa480.patch           xen-unstable - Xen 4.17.x

$ sha256sum xsa480*
578f8fec3f34656e085419f6376d43987ffd6ed32e067b4024d3c83ce03a5901  xsa480.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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