CVE-2026-89546

In the Linux kernel, the following vulnerability has been resolved:

SUNRPC: close backchannel before destroying callback service

A backchannel receive can complete a request while the NFS callback
service is being torn down.  xprt_complete_bc_request() removes the
request from bc_pa_list, drops bc_alloc_count, marks the request in use,
and then asks xprt_enqueue_bc_request() to hand it to the callback
service.

If teardown has already cleared xprt->bc_serv, xprt_enqueue_bc_request()
currently returns without enqueueing or freeing the committed request.
The xprt_get() taken on entry is leaked as well.  If the producer wins
the race before bc_serv is cleared, it can also enqueue onto sv_cb_list
after nfs_callback_down() has stopped the callback threads, leaving the
request linked to a svc_serv that is about to be freed.

Close the producer side before callback threads are stopped.  Add
xprt_svc_shutdown_bc() to clear xprt->bc_serv under bc_pa_lock, and call
it on callback shutdown and callback-start failure before stopping the
service threads.  Requests that lose the NULL transition in
xprt_enqueue_bc_request() are released through the normal backchannel
free path after balancing bc_slot_count.  Finally, drain any remaining
sv_cb_list requests after the callback threads have stopped and before
svc_destroy() frees the service.