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AUSCERT External Security Bulletin Redistribution
New Xen bulletin: grant table v2 status pages may remain
accessible after de-allocation
26 August 2021
AusCERT Security Bulletin Summary
Operating System: Xen
Impact/Access: Increased Privileges -- Existing Account
Access Confidential Data -- Existing Account
Denial of Service -- Existing Account
CVE Names: CVE-2021-28697
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Xen Security Advisory CVE-2021-28697 / XSA-379
grant table v2 status pages may remain accessible after de-allocation
UPDATES IN VERSION 2
Patches updated to fix a typo in a comment.
Guest get permitted access to certain Xen-owned pages of memory. The
majority of such pages remain allocated / associated with a guest for
its entire lifetime. Grant table v2 status pages, however, get
de-allocated when a guest switched (back) from v2 to v1. The freeing
of such pages requires that the hypervisor know where in the guest
these pages were mapped. The hypervisor tracks only one use within
guest space, but racing requests from the guest to insert mappings of
these pages may result in any of them to become mapped in multiple
locations. Upon switching back from v2 to v1, the guest would then
retain access to a page that was freed and perhaps re-used for other
A malicious guest may be able to elevate its privileges to that of the
host, cause host or guest Denial of Service (DoS), or cause information
All Xen versions from 4.0 onwards are affected. Xen versions 3.4 and
older are not affected.
Only x86 HVM and PVH guests permitted to use grant table version 2
interfaces can leverage this vulnerability. x86 PV guests cannot
leverage this vulnerability. On Arm, grant table v2 use is explicitly
Running only PV guests will avoid this vulnerability.
Suppressing use of grant table v2 interfaces for HVM or PVH guests will
also avoid this vulnerability.
This issue was discovered by Jan Beulich of SUSE.
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.
xsa379-4.15.patch Xen 4.15.x
xsa379-4.14.patch Xen 4.14.x - 4.13.x
xsa379-4.12.patch Xen 4.12.x - 4.11.x
$ sha256sum xsa379*
DEPLOYMENT DURING EMBARGO
Deployment of the patches and/or PV-guest-only mitigations described
above (or others which are substantially similar) is permitted during
the embargo, even on public-facing systems with untrusted guest users
HOWEVER, deployment of the grant table v2 disabling 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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