Revert "NFSD: Defer sub-object cleanup in export put callbacks"

Revert "NFSD: Defer sub-object cleanup in export put callbacks"

This reverts commit 48db892356d6cb80f6942885545de4a6dd8d2a29.

Commit 48db892356d6 ("NFSD: Defer sub-object cleanup in export
put callbacks") moved path_put() and auth_domain_put() out of
svc_export_put() and expkey_put() and behind queue_rcu_work() to
close a claimed use-after-free in e_show() and c_show() against
ex_path and ex_client->name. Discussion in [1] shows neither
the diagnosis nor the remedy survives review.

The downstream teardown of both sub-objects is already RCU-deferred.
auth_domain_put() reaches svcauth_unix_domain_release(), which frees
the unix_domain and its ->name through call_rcu(). path_put()
reaches dentry_free(), which frees the dentry through call_rcu(),
and prepend_path() is already structured to tolerate concurrent
dentry teardown. A reader in cache_seq_start_rcu() therefore
observes both sub-objects through the next grace period regardless
of whether svc_export_put() runs synchronously, so the synchronous
form was never unsafe.

The crash signature in the report cited by commit 48db892356d6
("NFSD: Defer sub-object cleanup in export put callbacks") has a
different root cause: a /proc/net/rpc cache file held open across
network-namespace exit lets cache_destroy_net() free cd->hash_table
while a reader is still walking it. The correct fix pins cd->net for
the open fd's lifetime and does not require any deferral inside
svc_export_put().

Meanwhile, deferring path_put() out of svc_export_put() reintroduces
the regression that commit 69d803c40ede ("nfsd: Revert "nfsd:
release svc_expkey/svc_export with rcu_work"") repaired: after
"exportfs -r" drops the last cache reference, the mount reference
held through ex_path lingers in the workqueue, so a subsequent
umount fails with EBUSY.

Restore the synchronous path_put() and auth_domain_put() in
svc_export_put() and expkey_put() and the call_rcu()/kfree_rcu()
free of the containing structures. The unrelated fix for
ex_uuid/ex_stats from commit 2530766492ec ("nfsd: fix UAF when
access ex_uuid or ex_stats") is preserved.

Link: https://lore.kernel.org/all/10019b42-4589-4f9f-8d5b-d8197db1ce3c@huawei.com/
Fixes: 48db892356d6 ("NFSD: Defer sub-object cleanup in export put callbacks")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Alexandr Alexandrov <alexandr.alexandrov@oracle.com>
Signed-off-by: Yang Erkun <yangerkun@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>