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AUSCERT External Security Bulletin Redistribution
Denial of Service vulnerability found in Xen
24 November 2021
AusCERT Security Bulletin Summary
Operating System: Virtualisation
Impact/Access: Denial of Service -- Existing Account
CVE Names: CVE-2021-28706
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Xen Security Advisory CVE-2021-28706 / XSA-385
guests may exceed their designated memory limit
UPDATES IN VERSION 2
Add CVE numbers to patches.
When a guest is permitted to have close to 16TiB of memory, it may be
able to issue hypercalls to increase its memory allocation beyond the
administrator established limit. This is a result of a calculation
done with 32-bit precision, which may overflow. It would then only
be the overflowed (and hence small) number which gets compared against
the established upper bound.
A guest may be able too allocate unbounded amounts of memory to itself.
This may result in a Denial of Service (DoS) affecting the entire host.
All Xen versions from at least 3.2 onwards are affected.
On x86, only Xen builds with the BIGMEM configuration option enabled are
affected. (This option is off by default.)
Only hosts with more than 16 TiB of memory are affected.
Setting the maximum amount of memory a guest may allocate to strictly
less than 1023 GiB will avoid the vulnerability.
This issue was discovered by Julien Grall of Amazon.
Applying the appropriate first attached patch resolves this specific
issue. The second patch in addition documents altered support status of
Xen on huge memory systems.
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.
xsa385-4.15.patch Xen 4.15.x - 4.14.x
xsa385-4.13.patch Xen 4.13.x
xsa385-4.12.patch Xen 4.12.x
$ sha256sum xsa385*
DEPLOYMENT DURING EMBARGO
Deployment of the patches described above (or others which are
substantially similar) is permitted during the embargo, even on public-
facing systems with untrusted guest users and administrators.
HOWEVER, deployment of the mitigation described above is NOT permitted
during the embargo on public-facing systems with untrusted guest users
and administrators. This is because such a configuration change is
recognizable by the affected guests.
AND: 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
(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:
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