Headline
CVE-2020-29571: 359 - Xen Security Advisories
An issue was discovered in Xen through 4.14.x. A bounds check common to most operation time functions specific to FIFO event channels depends on the CPU observing consistent state. While the producer side uses appropriately ordered writes, the consumer side isn’t protected against re-ordered reads, and may hence end up de-referencing a NULL pointer. Malicious or buggy guest kernels can mount a Denial of Service (DoS) attack affecting the entire system. Only Arm systems may be vulnerable. Whether a system is vulnerable depends on the specific CPU. x86 systems are not vulnerable.
Information
Advisory
XSA-359
Public release
2020-12-15 12:00
Updated
2020-12-15 12:19
Version
3
CVE(s)
CVE-2020-29571
Title
FIFO event channels control structure ordering
Filesadvisory-359.txt (signed advisory file)
xsa359.meta
xsa359.patchAdvisory
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256
Xen Security Advisory CVE-2020-29571 / XSA-359
version 3
FIFO event channels control structure ordering
UPDATES IN VERSION 3
Public release.
ISSUE DESCRIPTION
A bounds check common to most operation time functions specific to FIFO event channels depends on the CPU observing consistent state. While the producer side uses appropriately ordered writes, the consumer side isn’t protected against re-ordered reads, and may hence end up de-referencing a NULL pointer.
IMPACT
Malicious or buggy guest kernels can mount a Denial of Service (DoS) attack affecting the entire system.
VULNERABLE SYSTEMS
All Xen versions from 4.4 onwards are vulnerable. Xen versions 4.3 and earlier are not vulnerable.
Only Arm systems may be vulnerable. Whether a system is vulnerable will depend on the specific CPU. x86 systems are not vulnerable.
MITIGATION
There is no known mitigation.
CREDITS
This issue was discovered by Julien Grall of Amazon.
RESOLUTION
Applying the 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.
xsa359.patch xen-unstable - 4.10
$ sha256sum xsa359* cb009ad77d1a3d8044431b2af568dd9dffefe07fc9f537fb6b53c2ec57aa77b7 xsa359.meta 3126d9304b68be84a89c42c223227c8f96ecbb96a0385a7e1bdc65ae5e0f344f xsa359.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 -----BEGIN PGP SIGNATURE-----
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Xenproject.org Security Team