CVE-2026-89708

Status: UPDATED  |  Advisory ID: CVE-2026-89708

Key Details

Affected Products, Subsystems & Sectors

What to Know

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. (NVD)

What to Do

Monitor Linux's web page for any future patch releases.

References