NFSD: Prevent post-shutdown use-after-free in NFSD_CMD_UNLOCK_FILESYSTEM

NFSD: Prevent post-shutdown use-after-free in NFSD_CMD_UNLOCK_FILESYSTEM

The NFSD_CMD_UNLOCK_FILESYSTEM netlink command runs
nfsd4_cancel_copy_by_sb() before nfsd_mutex is held and before
nn->nfsd_serv is confirmed set, the same pre-mutex ordering the procfs
unlock_filesystem path carried. Once nfsd has shut down,
nfs4_state_destroy_net() has freed nn->conf_id_hashtbl but left the
pointer intact, so the cancel helper iterates freed slab memory as an
array of struct list_head and then dereferences a bogus nfs4_client
when it takes clp->async_lock. A local administrator holding
CAP_SYS_ADMIN can reach this use-after-free by stopping the server and
then issuing the command.

Move the async COPY cancel into the nfsd_mutex section, after
nn->nfsd_serv is confirmed, so every NFSv4 state-table walker on this
path observes a running server. Async copies exist only while the
server runs, so gating the cancel on nn->nfsd_serv loses nothing.

Fixes: 327c5168eff2 ("NFSD: Add NFSD_CMD_UNLOCK_FILESYSTEM netlink command")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260613-unlock-filesystem-uaf-v1-2-462b9bec8c84@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>