{"id":"CVE-2026-89708","title":"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 h…","summary":"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 h…","severity":"critical","cvss":9.8,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H","cwe":["CWE-825"],"vendor":"Linux","product":"Linux","affected":["Linux >= dcbeaa68dbbdacbbb330a86c7fc95a28473fc209 < 2775ec1d617e665eab71ce0f13ab1f7959713a2b","Linux >= dcbeaa68dbbdacbbb330a86c7fc95a28473fc209 < f164eb52b6f3cbf40f07fe379f9f421f88e02f76","Linux >= dcbeaa68dbbdacbbb330a86c7fc95a28473fc209 < 13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25","Linux >= dcbeaa68dbbdacbbb330a86c7fc95a28473fc209 < 01c5d5f58a5db9b0ee5afba2e49d3157788687b2","Linux 2.6.38"],"published":"2026-09-11","updated":"2026-09-21","sourceUpdated":"2026-09-21T14:17:25.690","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-89708","references":[{"url":"https://git.kernel.org/stable/c/01c5d5f58a5db9b0ee5afba2e49d3157788687b2","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/2775ec1d617e665eab71ce0f13ab1f7959713a2b","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/f164eb52b6f3cbf40f07fe379f9f421f88e02f76","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89708.json"},{"url":"https://access.redhat.com/security/cve/CVE-2026-89708"},{"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2532446"},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-89708"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-89708"},{"url":"https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-89708.mbox"}],"tags":["nvd","cve.org","csaf","vex","red-hat","score-dispute"],"epss":0.00435,"epssPercentile":0.37191,"scores":{"nvd":9.8,"cna":9.8,"vendor":7},"ingestedAt":"2026-09-14T15:23:07.472Z","slug":"CVE-2026-89708","body":"## Overview\n\nIn 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.\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.\n\n## Vendor advisories\n\n- **Red Hat VEX** · Important · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4, Red Hat Enterprise Linux 6 · no fix planned: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, … · updated 2026-09-15 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89708.json)","depth":"midnight","depthScore":54,"depthScoreParts":{"impact":53.9,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[{"seq":208369,"id":"CVE-2026-89708","ts":1789997912611,"field":"cvss","old":"7","new":"9.8"},{"seq":208368,"id":"CVE-2026-89708","ts":1789997912611,"field":"severity","old":"high","new":"critical"},{"seq":203874,"id":"CVE-2026-89708","ts":1789490210503,"field":"cvss","old":"5.3","new":"7"},{"seq":203873,"id":"CVE-2026-89708","ts":1789490210503,"field":"severity","old":"medium","new":"high"},{"seq":202970,"id":"CVE-2026-89708","ts":1789403733163,"field":"cvss","old":"9.8","new":"5.3"},{"seq":202969,"id":"CVE-2026-89708","ts":1789403733163,"field":"severity","old":"critical","new":"medium"},{"seq":197714,"id":"CVE-2026-89708","ts":1789384318517,"field":"cvss","old":"5.3","new":"9.8"},{"seq":197713,"id":"CVE-2026-89708","ts":1789384318517,"field":"severity","old":"medium","new":"critical"},{"seq":183659,"id":"CVE-2026-89708","ts":1789356676818,"field":"cvss","old":"9.8","new":"5.3"},{"seq":183658,"id":"CVE-2026-89708","ts":1789356676818,"field":"severity","old":"critical","new":"medium"},{"seq":153596,"id":"CVE-2026-89708","ts":1789285351428,"field":"cvss","old":null,"new":"9.8"},{"seq":153595,"id":"CVE-2026-89708","ts":1789285351428,"field":"severity","old":"none","new":"critical"},{"seq":147197,"id":"CVE-2026-89708","ts":1789270200531,"field":"cvss","old":null,"new":"5.3"},{"seq":147196,"id":"CVE-2026-89708","ts":1789270200531,"field":"severity","old":"none","new":"medium"},{"seq":108944,"id":"CVE-2026-89708","ts":1789183729666,"field":"cvss","old":null,"new":"5.3"},{"seq":108943,"id":"CVE-2026-89708","ts":1789183729666,"field":"severity","old":"none","new":"medium"}]}