Date: Tue, 15 Dec 2020 12:20:18 +0000 From: Xen.org security team <security@....org> To: xen-announce@...ts.xen.org, xen-devel@...ts.xen.org, xen-users@...ts.xen.org, oss-security@...ts.openwall.com CC: Xen.org security team <security-team-members@....org> Subject: Xen Security Advisory 324 v3 (CVE-2020-29484) - Xenstore: guests can crash xenstored via watchs -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2020-29484 / XSA-324 version 3 Xenstore: guests can crash xenstored via watchs UPDATES IN VERSION 3 ==================== Public release. ISSUE DESCRIPTION ================= When a Xenstore watch fires, the xenstore client which registered the watch will receive a Xenstore message containing the path of the modified Xenstore entry which triggered the watch, and the tag which was specified when registering the watch. Any communication with xenstored is done via Xenstore messages, consisting of a message header and the payload. The payload length is limited to 4096 bytes. Any request to xenstored resulting in a response with a payload longer than 4096 bytes will result in an error. When registering a watch the payload length limit applies to the combined length of the watched path and the specified tag. As watches for a specific path are also triggered for all nodes below that path, the payload of a watch event message can be longer than the payload needed to register the watch. A malicious guest which registers a watch using a very large tag (ie with a registration operation payload length close to the 4096 byte limit) can cause the generation of watch events with a payload length larger than 4096 bytes, by writing to Xenstore entries below the watched path. This will result in an error condition in xenstored. This error can result in a NULL pointer dereference leading to a crash of xenstored. IMPACT ====== A malicious guest administrator can cause xenstored to crash, leading to a denial of service. Following a xenstored crash, domains may continue to run, but management operations will be impossible. VULNERABLE SYSTEMS ================== All Xen versions are affected. Only C xenstored is affected, oxenstored is not affected. MITIGATION ========== There are no mitigations. Changing to use of Ocaml xenstored would avoid this vulnerability. However, given the other vulnerabilities in both versions of xenstored being reported at this time, changing xenstored implementation is not a recommended approach to mitigation of individual issues. CREDITS ======= This issue was discovered by Jürgen Groß of SUSE. RESOLUTION ========== 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. xsa324.patch xen-unstable - 4.10 $ sha256sum xsa324* 78932f0a83b479902553b1acdf601f7625b383497c03c6e834a0a2b847f1a72e xsa324.meta 8dba79842fa913290c7043d065a50abb0efe27fa5a173e421c21c544cc1e264c xsa324.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----- iQFABAEBCAAqFiEEI+MiLBRfRHX6gGCng/4UyVfoK9kFAl/Yqd4MHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZBoIH/ir2NdOiUg6JFoa/DXgtMBosLXRkRRjikvlaMJTY krz3r/aBZ0nLn8wsF5u+BctJYdHrIQDrt3N7GGv1wyvnLA18HrtupsxqrHj+CCMD pogl6QxRmmqRina7+EzRTt8N8qe6fhi8tuVmH3TYlsL1PeHyqNurwwTZizHL9BFx uCY10qNUV0FTY05tUhdP0FD3yiNfN8QwytARo/LRhELbUMx7D+N/CmUtCKh5uklr KfBBHy3Vb4MDlGPN7pa5vdEjZGFVj4xHWxUP+72C+bdhvLEiDi+IKkvy/TVbjoAN eQEfFVjBpj21MeQV+3mHJMJGknaJ8NTc00txrLM5D+WscHM= =KypE -----END PGP SIGNATURE----- Download attachment "xsa324.meta" of type "application/octet-stream" (2010 bytes) Download attachment "xsa324.patch" of type "application/octet-stream" (1753 bytes)
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