Information

Advisory

XSA-362

Public release

2021-02-16 12:00

Updated

2021-02-16 12:35

Version

3

CVE(s)

CVE-2021-26931

Title

Linux: backends treating grant mapping errors as bugs

Filesadvisory-362.txt (signed advisory file)
xsa362-linux-1.patch
xsa362-linux-2.patch
xsa362-linux-3.patchAdvisory

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256

        Xen Security Advisory CVE-2021-26931 / XSA-362
                           version 3

     Linux: backends treating grant mapping errors as bugs

UPDATES IN VERSION 3

Public release.

ISSUE DESCRIPTION

Block, net, and SCSI backends consider certain errors a plain bug, deliberately causing a kernel crash. For errors potentially being at least under the influence of guests, like out of memory conditions, it isn’t correct to assume so. Memory allocations potentially causing such crashes occur only when Linux is running in PV mode, though.

IMPACT

A malicious or buggy frontend driver may be able to crash the corresponding backend driver, potentially affecting the entire domain running the backend driver.

VULNERABLE SYSTEMS

Linux versions from at least 2.6.39 onwards are vulnerable, when run in PV mode. Earlier versions differ significantly in behavior and may therefore instead surface other issues under the same conditions. Linux run in HVM / PVH modes is not vulnerable.

MITIGATION

For Linux, running the backends in HVM or PVH domains will avoid the vulnerability.

For protocols where non-Linux-kernel based backends are available, reconfiguring guests to use alternative (e.g. qemu-based) backends may allow to avoid the vulnerability.

In all other cases there is no known mitigation.

CREDITS

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION

Applying the appropriate attached patches resolves this issue.

Applying the attached patches resolves this issue.

xsa362-linux-1.patch Linux 5.11-rc - 5.10 xsa362-linux-2.patch Linux 5.11-rc - 3.16 xsa362-linux-3.patch Linux 5.11-rc - 4.1

$ sha256sum xsa362* d64334807f16ff9909503b3cc9b8b93fd42d2c36e1fb0e508b89a765a53071a8 xsa362-linux-1.patch b6d02952e7fbede55b868cb2dc4d8853284996883dc72518a0cd5b14d6c7fdd4 xsa362-linux-2.patch 0a2661380d8f786fefe12e5a8b1528d4a79f1ad058c26b417c52449a7e16a302 xsa362-linux-3.patch $

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 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 -----BEGIN PGP SIGNATURE-----

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Xenproject.org Security Team