{"id":"CVE-2026-89708","summary":"nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nnfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown\n\nAfter a DESTROY_SESSION the per-session teardown path can free a\nsession while rpciod still holds an inflight callback rpc_task that\ndereferences clp-\u003ecl_cb_session.  nfsd4_probe_callback_sync() flushes\ncl_callback_wq, but once nfsd4_run_cb_work() has called\nrpc_call_async() the rpc_task lives on rpciod; flushing the workqueue\ndoes not wait for it.  rpc_shutdown_client() does drain rpciod tasks,\nbut uses a 1-second wait_event_timeout — tasks stuck in rpc_delay()\n(e.g. 2-second NFS4ERR_DELAY retries) can outlive the drain.\n\n    destroy path                       rpciod\n    ------------                       ------\n    unhash_session(ses)\n    nfsd4_probe_callback_sync(clp)\n      flush_workqueue(cl_callback_wq)\n      /* returns; rpc_task still live */\n    nfsd4_put_session_locked(ses)\n    free_session(ses) -\u003e kfree(ses)\n                                       nfsd4_cb_sequence_done()\n                                         reads cb_clp-\u003ecl_cb_session\n                                         /* freed slab */\n\nA second window exists in nfsd4_process_cb_update().  When\n__nfsd4_find_backchannel() returns NULL because unhash_session() has\nalready removed the destroyed session from cl_sessions,\nsetup_callback_client() takes the v4.1 early return so\nclp-\u003ecl_cb_session = ses never fires and the field retains a pointer\nto the about-to-be-freed session.\n\nFix both by converting cl_cb_session to an RCU-protected pointer:\n\n  - Move the cl_cb_session = ses assignment in setup_callback_client()\n    to after rpc_create() succeeds, so it is only published when a\n    working backchannel exists.  Clear cl_cb_session on the error\n    return in nfsd4_process_cb_update().  Both stores use\n    rcu_assign_pointer().\n\n  - Annotate cl_cb_session with __rcu.  All rpciod-side readers use\n    rcu_read_lock()/rcu_dereference() and check for NULL, bailing to\n    the appropriate error or requeue path:\n    encode_cb_sequence4args(), decode_cb_sequence4resok(),\n    nfsd41_cb_get_slot(), nfsd41_cb_release_slot(),\n    nfsd4_cb_prepare(), and nfsd4_cb_sequence_done().\n\n  - Switch __free_session() from kfree() to kfree_rcu() so the\n    session slab is not reclaimed until after an RCU grace period,\n    guaranteeing that rpciod readers inside rcu_read_lock() never\n    dereference freed memory.\n\n  - Pass the session pointer to the nfsd_cb_seq_status and\n    nfsd_cb_free_slot tracepoints instead of having them re-read\n    cl_cb_session.\n\n  - nfsd4_cb_prepare() calls rpc_exit() when the session is NULL,\n    routing through the done/release path to requeue the callback.","modified":"2026-09-13T03:47:20.289599836Z","published":"2026-09-11T19:46:24.061Z","database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/89xxx/CVE-2026-89708.json","cna_assigner":"Linux"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/01c5d5f58a5db9b0ee5afba2e49d3157788687b2"},{"type":"WEB","url":"https://git.kernel.org/stable/c/13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25"},{"type":"WEB","url":"https://git.kernel.org/stable/c/f164eb52b6f3cbf40f07fe379f9f421f88e02f76"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/89xxx/CVE-2026-89708.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-89708"},{"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":"dcbeaa68dbbdacbbb330a86c7fc95a28473fc209"},{"fixed":"f164eb52b6f3cbf40f07fe379f9f421f88e02f76"},{"fixed":"13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25"},{"fixed":"01c5d5f58a5db9b0ee5afba2e49d3157788687b2"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-89708.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"2.6.38"},{"fixed":"6.18.50"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.2.4"}]}],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-89708.json"}}],"schema_version":"1.9.0"}