CVE-2021-28706

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2021-28706 / XSA-385 version 2 guests may exceed their designated memory limit UPDATES IN VERSION 2 ==================== Add CVE numbers to patches. Public release. ISSUE DESCRIPTION ================= When a guest is permitted to have close to 16TiB of memory, it may be able to issue hypercalls to increase its memory allocation beyond the administrator established limit. This is a result of a calculation done with 32-bit precision, which may overflow. It would then only be the overflowed (and hence small) number which gets compared against the established upper bound. IMPACT ====== A guest may be able too allocate unbounded amounts of memory to itself. This may result in a Denial of Service (DoS) affecting the entire host. VULNERABLE SYSTEMS ================== All Xen versions from at least 3.2 onwards are affected. On x86, only Xen builds with the BIGMEM configuration option enabled are affected. (This option is off by default.) Only hosts with more than 16 TiB of memory are affected. MITIGATION ========== Setting the maximum amount of memory a guest may allocate to strictly less than 1023 GiB will avoid the vulnerability. CREDITS ======= This issue was discovered by Julien Grall of Amazon. RESOLUTION ========== Applying the appropriate first attached patch resolves this specific issue. The second patch in addition documents altered support status of Xen on huge memory systems. 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. xsa385-?.patch xen-unstable xsa385-4.15.patch Xen 4.15.x - 4.14.x xsa385-4.13.patch Xen 4.13.x xsa385-4.12.patch Xen 4.12.x $ sha256sum xsa385* b278902e293730a117605200910180bb842cf95db4bdedfd54b42b7314041d8c xsa385-1.patch 46a5ccfbb763b857f6cd0df46a9b7eed155b9de399ca4c68c9925faf4d1d9adb xsa385-2.patch 69ebe63dc7dca71f74260af19205a6387be56c7dc67b97fa7695ab1acd3c4da4 xsa385-4.12.patch 858eaad715e7cc62c4ab9784360f4ec77df70b2636b0755afe780d5c618cf9b4 xsa385-4.13.patch 831e86c3adfec532b1a48a0b967b7c58c37db3733aee8d78216eb9d535b34f12 xsa385-4.15.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. HOWEVER, deployment of the 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 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/4UyVfoK9kFAmGc2jYMHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZEd4IAMwrHHAqFvSHgZ8Uw+DzMeT54db9nowudP9i/kYy +KobbVlGkxwLAU3mvh5lRkOLYzoIonrcA99cajZQNIcOKt3Mfi/8qzGGUN+hWZvh 6EZo3m7+7vx9mhtAeDBUbjkcZBLiVyxRAWALMS67ScBEX9lZTvbyj9nGkdQJmmfR pKt98z2Da2uR9YF521KWobuPYC0AFXujYBoavaTQpU/M8SiM+Wp1A2Fc6ZG+9ZKo frMeqFbHvwj94Hbqpn6CoLu2d/XnykMvttuLlqCKTccQc3puHXdQRz14W8IxxGYx gqYaIShZCFw/bUCu8mYHroDUlELJI3PIWQ1nJxy02bd5+N0= =7E6A -----END PGP SIGNATURE-----