Date: Mon, 25 Sep 2023 16:05:37 +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 439 v1 (CVE-2023-20588) - x86/AMD: Divide speculative information leak -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2023-20588 / XSA-439 x86/AMD: Divide speculative information leak ISSUE DESCRIPTION ================= In the Zen1 microarchitecure, there is one divider in the pipeline which services uops from both threads. In the case of #DE, the latched result from the previous DIV to execute will be forwarded speculatively. This is a covert channel that allows two threads to communicate without any system calls. In also allows userspace to obtain the result of the most recent DIV instruction executed (even speculatively) in the core, which can be from a higher privilege context. For more information, see: * https://www.amd.com/en/resources/product-security/bulletin/amd-sb-7008.html IMPACT ====== An attacker might be able to infer data from a different execution context on the same CPU core. VULNERABLE SYSTEMS ================== All versions of Xen are vulnerable. Only AMD Zen1 CPUs are believed to be vulnerable. MITIGATION ========== There is no mitigation. RESOLUTION ========== The patches for Xen overwrite the buffer in the divider on the return-to-guest path. However, as with some prior speculative vulnerabilities, the fix is only effective in combination with disabling SMT. For the same reasons as before, Xen does not disable SMT by default. The system administrator is required to risk-assess their workload, and choose whether to enable or disable SMT. Xen will issue a warning if SMT is active and the user has not provided an explicit choice via the smt=<bool> command line option. Details of the vulnerability became public before the Xen patches were complete. Hence the patches are already applied to the appropriate trees. They are: Xen-unstable: 1c18d7377453^..b5926c6ecf05 Xen 4.17: d2d2dcae879c^..9ac2f49f5fa3 Xen 4.16: 08539e8315fd^..de751c3d906d Xen 4.15: db3386e6cad6^..d7b78041dc81 -----BEGIN PGP SIGNATURE----- iQFABAEBCAAqFiEEI+MiLBRfRHX6gGCng/4UyVfoK9kFAmURr2UMHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZA1QH/RNSR1O6QJjd7z2gSGA9Yka7VWyYOMB2J01AaIl7 69zCRkpqg+baF1aQaAVR0fj39aF7M7xXrd/LSk+E4BBiCRSxxRzbWUGYn9qTLR9w srbpGXqy0aWod9MiwfbTuEzf9uG8XpwOGoRg6p6YBRYE3WrQxIVnYY+KjeeToTEs +UXZ0iZPrjaGaqKnF+PpkX4CMsqHhxk3iJw+ZFX2V4fVNRYgCOpjejmMjbWM4ABr eSsCjTU92/YZvFOsTeIzu74h5yM6SH+XTPW2S8Ve5j3mk7sM8nIiYbIyTMWNCJID HXeodt6eHjhZzV2z7f+/zEngnoITIqz+X3tRcTkHB9+H5jU= =AtsG -----END PGP SIGNATURE-----
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