{"id":"CVE-2026-89708","revision":"1e8d2f49538f4a4e9594ca22dce3fdf3","records":[{"revision":"1e8d2f49538f4a4e9594ca22dce3fdf3","observedAt":"2026-10-23T15:48:27.903+00:00","statement":{"source":"cve-program","id":"CVE-2026-89708","aliases":[],"title":"nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown","description":"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->cl_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) -> kfree(ses)\n nfsd4_cb_sequence_done()\n reads cb_clp->cl_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->cl_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.","state":0,"publishedAt":"2026-09-11T19:46:24.061+00:00","modifiedAt":"2026-10-19T08:51:21.242+00:00","affected":[{"ecosystem":"Vendor product","name":"Linux","ranges":[],"versions":[],"rawVersionEvidence":"{\n \"product\": \"Linux\",\n \"vendor\": \"Linux\",\n \"defaultStatus\": \"unaffected\",\n \"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\n \"programFiles\": [\n \"fs/nfsd/nfs4callback.c\",\n \"fs/nfsd/nfs4state.c\",\n \"fs/nfsd/state.h\",\n \"fs/nfsd/trace.h\"\n ],\n \"versions\": [\n {\n \"version\": \"dcbeaa68dbbdacbbb330a86c7fc95a28473fc209\",\n \"lessThan\": \"2775ec1d617e665eab71ce0f13ab1f7959713a2b\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"dcbeaa68dbbdacbbb330a86c7fc95a28473fc209\",\n \"lessThan\": \"f164eb52b6f3cbf40f07fe379f9f421f88e02f76\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"dcbeaa68dbbdacbbb330a86c7fc95a28473fc209\",\n \"lessThan\": \"13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n },\n {\n \"version\": \"dcbeaa68dbbdacbbb330a86c7fc95a28473fc209\",\n \"lessThan\": \"01c5d5f58a5db9b0ee5afba2e49d3157788687b2\",\n \"status\": \"affected\",\n \"versionType\": \"git\"\n }\n ]\n }"},{"ecosystem":"Vendor product","name":"Linux","ranges":[],"versions":[],"rawVersionEvidence":"{\n \"product\": \"Linux\",\n \"vendor\": \"Linux\",\n \"defaultStatus\": \"affected\",\n \"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\n \"programFiles\": [\n \"fs/nfsd/nfs4callback.c\",\n \"fs/nfsd/nfs4state.c\",\n \"fs/nfsd/state.h\",\n \"fs/nfsd/trace.h\"\n ],\n \"versions\": [\n {\n \"version\": \"2.6.38\",\n \"status\": \"affected\"\n },\n {\n \"version\": \"0\",\n \"lessThan\": \"2.6.38\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.12.111\",\n \"lessThanOrEqual\": \"6.12.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"6.18.50\",\n \"lessThanOrEqual\": \"6.18.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"7.2.4\",\n \"lessThanOrEqual\": \"7.2.*\",\n \"status\": \"unaffected\",\n \"versionType\": \"semver\"\n },\n {\n \"version\": \"7.3\",\n \"lessThanOrEqual\": \"*\",\n \"status\": \"unaffected\",\n \"versionType\": \"original_commit_for_fix\"\n }\n ]\n }"}],"metrics":[{"system":"CVSS 3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H","score":9.8,"severity":"CRITICAL"}],"references":["https://git.kernel.org/stable/c/2775ec1d617e665eab71ce0f13ab1f7959713a2b","https://git.kernel.org/stable/c/f164eb52b6f3cbf40f07fe379f9f421f88e02f76","https://git.kernel.org/stable/c/13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25","https://git.kernel.org/stable/c/01c5d5f58a5db9b0ee5afba2e49d3157788687b2"],"sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-89708","license":"CVE Program Terms of Use","key":"cve-program:CVE-2026-89708","identifiers":["CVE-2026-89708"]},"retained":false,"historicalAffected":[]}],"coverage":[]}