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AUSCERT External Security Bulletin Redistribution
Linux: grant mapping error handling issues
17 February 2021
AusCERT Security Bulletin Summary
Product: Linux Kernel
Operating System: Xen
Impact/Access: Denial of Service -- Unknown/Unspecified
CVE Names: CVE-2021-26932
Comment: This advisory references vulnerabilities in the Linux kernel that
also affect distributions other than Xen. It is recommended that
administrators running Linux check for an updated version of the
kernel for their system.
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Xen Security Advisory CVE-2021-26932 / XSA-361
Linux: grant mapping error handling issues
UPDATES IN VERSION 4
Grant mapping operations often occur in batch hypercalls, where a
number of operations are done in a single hypercall, the success or
failure of each one reported to the backend driver, and the backend
driver then loops over the results, performing follow-up actions based
on the success or failure of each operation.
Unfortunately, when running in PV mode, the Linux backend drivers
mishandle this: Some errors are ignored, effectively implying their
success from the success of related batch elements. In other cases,
errors resulting from one batch element lead to further batch elements
not being inspected, and hence successful ones to not be possible to
properly unmap upon error recovery.
A malicious or buggy frontend driver may be able to crash the
corresponding backend driver, causing a denial of service potentially
affecting the entire domain running the backend driver.
A malicious or buggy frontend driver may be able to cause resource
leaks in the domain running the corresponding backend driver, leading
to a denial of service.
All Linux versions back to at least 3.2 are vulnerable, when running in
PV mode on x86 or when running on Arm.
On x86, only systems with Linux backends running in PV mode are
vulnerable. Linux backends run in HVM / PVH modes are not vulnerable.
On x86, running the backends in HVM or PVH domains will avoid the
For protocols where other, e.g. non-kernel-based backends are available,
reconfiguring guests to use alternative (e.g. qemu-based) backends may
allow to avoid the vulnerability as long as these backends don't rely on
similar functionality provided by the xen-gntdev (/dev/gntdev) driver.
In all other cases there is no known mitigation.
This issue was discovered by Jan Beulich of SUSE.
Applying the attached patches resolves this issue.
xsa361-linux-1.patch Linux 5.11-rc - 3.19
xsa361-linux-2.patch Linux 5.11-rc - 3.15
xsa361-linux-3.patch Linux 5.11-rc - 4.19
xsa361-linux-4.patch Linux 5.11-rc - 4.19
xsa361-linux-5.patch Linux 5.11-rc - 4.4
$ sha256sum xsa361*
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.
Deployment of the mitigation to switch to HVM / PVH backend domains is
also permitted during the embargo, even on public-facing systems with
untrusted guest users and administrators.
HOWEVER, deployment of the non-kernel-based backends 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 may be 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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