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Xen Security Advisory CVE-2021-28693 / XSA-372
version 3

xen/arm: Boot modules are not scrubbed

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

Public release.

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

The bootloader will load boot modules (e.g. kernel, initramfs...) in a
temporary area before they are copied by Xen to each domain memory.
To ensure sensitive data is not leaked from the modules, Xen must
"scrub" them before handing the page over to the allocator.

Unfortunately, it was discovered that modules will not be scrubbed on
Arm.

IMPACT
======

Sensitive information from the boot modules might be visible to another
domain after boot.

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

Only Arm systems are vulnerable. System running with "bootscrub=off"
(disabling boot scrubbing) are not vulnerable.

All versions of Xen since 4.12 are vulnerable.

MITIGATION
==========

There is no mitigation available.

CREDITS
=======

This issue was discovered by Julien Grall of Amazon.

RESOLUTION
==========

Applying the appropriate set of 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.

xsa372/*.patch xen-unstable
xsa372-4.15/*.patch Xen 4.15.x
xsa372-4.14/*.patch Xen 4.14.x - Xen 4.13.x
xsa372-4.12/*.patch Xen 4.12.x

$ sha256sum xsa372* xsa372*/*
06e43684c2d8a3085d55b8b40f57e1b9f1ee47519fac844dcbc21b57fb039915 xsa372.meta
8f872c7abe6c795dbef2e401f2223fda0dbb9d7c57dfebd8047eef37e1caf952 xsa372-4.12/0001-xen-arm-Create-dom0less-domUs-earlier.patch
a43c6c11481cc3f13900908cee79cc6c5401921f6f4e8858c0796cf301cfe923 xsa372-4.12/0002-xen-arm-Boot-modules-should-always-be-scrubbed-if-bo.patch
6d1fad53795ebd251520022b6be901215426ba78ccbbc075841698973b74d2a2 xsa372-4.14/0001-xen-arm-Create-dom0less-domUs-earlier.patch
2ceb5d4d8d4f8a18046721daa3bb29633a620c4794b54e1265f5d4d69a314c3b xsa372-4.14/0002-xen-arm-Boot-modules-should-always-be-scrubbed-if-bo.patch
7feae5f9f7f2df0ec38c0b9358dc32671a9955f966b3120e17bb3fd820ce33ff xsa372-4.15/0001-xen-arm-Create-dom0less-domUs-earlier.patch
0cc73b4751fa49f68c6584b1c7882606c6e1f18561d8a6547017ab068de4eb4b xsa372-4.15/0002-xen-arm-Boot-modules-should-always-be-scrubbed-if-bo.patch
950672405c695ebf6ae59eebeb454bc0738b7afc3efa35ef9680d76eef4d4ec0 xsa372/0001-xen-arm-Create-dom0less-domUs-earlier.patch
9ceccd39c795e7756052a2f00256e043c8dda42e2c691df30e3f8b59190d6e8e xsa372/0002-xen-arm-Boot-modules-should-always-be-scrubbed-if-bo.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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