In the Linux kernel, the following vulnerability has been resolved: nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardo

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.

 1destroy path                       rpciod
 2------------                       ------
 3unhash_session(ses)
 4nfsd4_probe_callback_sync(clp)
 5  flush_workqueue(cl_callback_wq)
 6  /* returns; rpc_task still live */
 7nfsd4_put_session_locked(ses)
 8free_session(ses) -> kfree(ses)
 9                                   nfsd4_cb_sequence_done()
10                                     reads cb_clp->cl_cb_session
11                                     /* 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.