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Xen Security Advisory CVE-2022-26362 / XSA-401
version 2

x86 pv: Race condition in typeref acquisition

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

Update 4.16 and 4.15 baselines.

Public release.

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

Xen maintains a type reference count for pages, in addition to a regular
reference count. This scheme is used to maintain invariants required
for Xen's safety, e.g. PV guests may not have direct writeable access to
pagetables; updates need auditing by Xen.

Unfortunately, the logic for acquiring a type reference has a race
condition, whereby a safely TLB flush is issued too early and creates a
window where the guest can re-establish the read/write mapping before
writeability is prohibited.

IMPACT
======

Malicious x86 PV guest administrators may be able to escalate privilege
so as to control the whole system.

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

All versions of Xen are vulnerable.

Only x86 PV guests can trigger this vulnerability.

To exploit the vulnerability, there needs to be an undue delay at just
the wrong moment in _get_page_type(). The degree to which an x86 PV
guest can practically control this race condition is unknown.

MITIGATION
==========

Not running x86 PV guests will avoid the vulnerability.

CREDITS
=======

This issue was discovered by Jann Horn of Google Project Zero.

RESOLUTION
==========

Applying the appropriate attached patches 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.

xsa401/xsa401-?.patch xen-unstable
xsa401/xsa401-4.16-?.patch Xen 4.16.x - Xen 4.14.x
xsa401/xsa401-4.13-?.patch Xen 4.13.x

$ sha256sum xsa401* xsa401*/*
d442bc0946eaa4c325226fd0805ab81eba6a68b68cffb9b03d9552edea86b118 xsa401.meta
074b57204f828cbd004c2d024b02a41af5d5bf3547d407af27249dca95eca13a xsa401/xsa401-1.patch
a095b39b203d501f9c9d4974638cd4d5e2d7a18daee7a7a61e2010dea477e212 xsa401/xsa401-2.patch
99af3efc91d2dbf4fd54cc9f454b87bd76edbc85abd1a20bdad0bd22acabf466 xsa401/xsa401-4.13-1.patch
bb997094052edbbbdd0dc9f3a0454508eb737556e2449ec6a0bc649deb921e4f xsa401/xsa401-4.13-2.patch
d336b31cb91466942e4fb8b44783bb2f0be4995076e70e0e78cdf992147cf72a xsa401/xsa401-4.16-1.patch
b380a76d67957b602ff3c9a3faaa4d9b6666422834d6ee3ab72432a6d07ddbc6 xsa401/xsa401-4.16-2.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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