{"id":"CVE-2025-21664","summary":"dm thin: make get_first_thin use rcu-safe list first function","details":"In the Linux kernel, the following vulnerability has been resolved:\n\ndm thin: make get_first_thin use rcu-safe list first function\n\nThe documentation in rculist.h explains the absence of list_empty_rcu()\nand cautions programmers against relying on a list_empty() -\u003e\nlist_first() sequence in RCU safe code.  This is because each of these\nfunctions performs its own READ_ONCE() of the list head.  This can lead\nto a situation where the list_empty() sees a valid list entry, but the\nsubsequent list_first() sees a different view of list head state after a\nmodification.\n\nIn the case of dm-thin, this author had a production box crash from a GP\nfault in the process_deferred_bios path.  This function saw a valid list\nhead in get_first_thin() but when it subsequently dereferenced that and\nturned it into a thin_c, it got the inside of the struct pool, since the\nlist was now empty and referring to itself.  The kernel on which this\noccurred printed both a warning about a refcount_t being saturated, and\na UBSAN error for an out-of-bounds cpuid access in the queued spinlock,\nprior to the fault itself.  When the resulting kdump was examined, it\nwas possible to see another thread patiently waiting in thin_dtr's\nsynchronize_rcu.\n\nThe thin_dtr call managed to pull the thin_c out of the active thins\nlist (and have it be the last entry in the active_thins list) at just\nthe wrong moment which lead to this crash.\n\nFortunately, the fix here is straight forward.  Switch get_first_thin()\nfunction to use list_first_or_null_rcu() which performs just a single\nREAD_ONCE() and returns NULL if the list is already empty.\n\nThis was run against the devicemapper test suite's thin-provisioning\nsuites for delete and suspend and no regressions were observed.","modified":"2026-04-16T04:38:07.753867040Z","published":"2025-01-21T12:18:19.015Z","related":["SUSE-SU-2025:0428-1","SUSE-SU-2025:0499-1","SUSE-SU-2025:0557-1","SUSE-SU-2025:0564-1","SUSE-SU-2025:0565-1","SUSE-SU-2025:20165-1","SUSE-SU-2025:20166-1","SUSE-SU-2025:20248-1","SUSE-SU-2025:20249-1","USN-7379-2","USN-7380-1"],"database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/21xxx/CVE-2025-21664.json","cna_assigner":"Linux"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/12771050b6d059eea096993bf2001da9da9fddff"},{"type":"WEB","url":"https://git.kernel.org/stable/c/6b305e98de0d225ccebfb225730a9f560d28ecb0"},{"type":"WEB","url":"https://git.kernel.org/stable/c/802666a40c71a23542c43a3f87e3a2d0f4e8fe45"},{"type":"WEB","url":"https://git.kernel.org/stable/c/80f130bfad1dab93b95683fc39b87235682b8f72"},{"type":"WEB","url":"https://git.kernel.org/stable/c/cbd0d5ecfa390ac29c5380200147d09c381b2ac6"},{"type":"WEB","url":"https://git.kernel.org/stable/c/cd30a3960433ec2db94b3689752fa3c5df44d649"},{"type":"WEB","url":"https://git.kernel.org/stable/c/ec037fe8c0d0f6140e3d8a49c7b29cb5582160b8"},{"type":"WEB","url":"https://lists.debian.org/debian-lts-announce/2025/03/msg00001.html"},{"type":"WEB","url":"https://lists.debian.org/debian-lts-announce/2025/03/msg00002.html"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/21xxx/CVE-2025-21664.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2025-21664"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"b10ebd34cccae1b431caf1be54919aede2be7cbe"},{"fixed":"ec037fe8c0d0f6140e3d8a49c7b29cb5582160b8"},{"fixed":"cd30a3960433ec2db94b3689752fa3c5df44d649"},{"fixed":"802666a40c71a23542c43a3f87e3a2d0f4e8fe45"},{"fixed":"12771050b6d059eea096993bf2001da9da9fddff"},{"fixed":"6b305e98de0d225ccebfb225730a9f560d28ecb0"},{"fixed":"cbd0d5ecfa390ac29c5380200147d09c381b2ac6"},{"fixed":"80f130bfad1dab93b95683fc39b87235682b8f72"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2025-21664.json"}}],"schema_version":"1.7.5"}