--- id: CVE-2026-89708 title: |- In the Linux kernel, the following vulnerability has been resolved: nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown After a DESTROY_SESSION the per-session teardown path can free a session while rpciod still h… summary: |- In the Linux kernel, the following vulnerability has been resolved: nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown After a DESTROY_SESSION the per-session teardown path can free a session 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: 0.0045 epssPercentile: 0.3849 scores: nvd: 9.8 cna: 9.8 vendor: 7 ingestedAt: '2026-09-14T15:23:07.472Z' --- ## Overview In the Linux kernel, the following vulnerability has been resolved: nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown After a DESTROY_SESSION the per-session teardown path can free a session while rpciod still holds an inflight callback rpc_task that dereferences clp->cl_cb_session. nfsd4_probe_callback_sync() flushes cl_callback_wq, but once nfsd4_run_cb_work() has called rpc_call_async() the rpc_task lives on rpciod; flushing the workqueue does not wait for it. rpc_shutdown_client() does drain rpciod tasks, but uses a 1-second wait_event_timeout — tasks stuck in rpc_delay() (e.g. 2-second NFS4ERR_DELAY retries) can outlive the drain. destroy path rpciod ------------ ------ unhash_session(ses) nfsd4_probe_callback_sync(clp) flush_workqueue(cl_callback_wq) /* returns; rpc_task still live */ nfsd4_put_session_locked(ses) free_session(ses) -> kfree(ses) nfsd4_cb_sequence_done() reads cb_clp->cl_cb_session /* freed slab */ A second window exists in nfsd4_process_cb_update(). When __nfsd4_find_backchannel() returns NULL because unhash_session() has already removed the destroyed session from cl_sessions, setup_callback_client() takes the v4.1 early return so clp->cl_cb_session = ses never fires and the field retains a pointer to the about-to-be-freed session. Fix both by converting cl_cb_session to an RCU-protected pointer: - Move the cl_cb_session = ses assignment in setup_callback_client() to after rpc_create() succeeds, so it is only published when a working backchannel exists. Clear cl_cb_session on the error return in nfsd4_process_cb_update(). Both stores use rcu_assign_pointer(). - Annotate cl_cb_session with __rcu. All rpciod-side readers use rcu_read_lock()/rcu_dereference() and check for NULL, bailing to the appropriate error or requeue path: encode_cb_sequence4args(), decode_cb_sequence4resok(), nfsd41_cb_get_slot(), nfsd41_cb_release_slot(), nfsd4_cb_prepare(), and nfsd4_cb_sequence_done(). - Switch __free_session() from kfree() to kfree_rcu() so the session slab is not reclaimed until after an RCU grace period, guaranteeing that rpciod readers inside rcu_read_lock() never dereference freed memory. - Pass the session pointer to the nfsd_cb_seq_status and nfsd_cb_free_slot tracepoints instead of having them re-read cl_cb_session. - nfsd4_cb_prepare() calls rpc_exit() when the session is NULL, routing through the done/release path to requeue the callback. ## Remediation Refer to the linked advisories for vendor-supplied fixes and affected version ranges. ## Vendor advisories - **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)