CVE-2026-89708
moderateUse-After-Free in Linux Kernel nfsd NFSv4.1 Session Teardown
The Linux kernel's NFS server (nfsd) contains a use-after-free in the NFSv4.1 callback path: after a client sends DESTROY_SESSION, the teardown path can free the session slab while an rpciod callback task still dereferences clp->cl_cb_session, because flushing cl_callback_wq does not wait for tasks already handed to rpciod, and a 2-second NFS4ERR_DELAY retry can outlive rpc_shutdown_client()'s 1-second drain. A second window in nfsd4_process_cb_update() leaves cl_cb_session pointing at the about-to-be-freed session when the backchannel lookup fails. A remote NFSv4.1 client that can open a session against the server can race session destruction with in-flight callbacks, causing the kernel to read freed memory — rated CVSS 9.8 with high impact to confidentiality, integrity, and availability (realistic outcomes include kernel crashes and potential memory corruption). Any Linux host acting as an NFSv4.1+ server with backchannel callbacks (e.g., delegations) is affected; specific kernel version ranges were not enumerated in the source data. No public PoC exists, the CVE is not in CISA's KEV catalog, and no exploitation in the wild is known.
What to do: Patch to a kernel release containing the nfsd cl_cb_session RCU fix as soon as your distribution ships it in stable backports. Until then, restrict NFS exports and firewall TCP/2049 to trusted client networks, since triggering the race requires an attacker-controlled or compromised NFSv4.1 client session. Monitor NFS servers for kernel oops, KASAN use-after-free reports, or unexplained crashes correlated with nfsd4_cb_* callback activity.
| Linux kernel (nfsd, NFSv4.1 backchannel callback path) | — |
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
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.
- Vector
- CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
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