nfsd: Fix nsfd startup race (again)

Commit bd5ae928 ("nfsd: register pernet ops last, unregister first")
has re-opened rpc_pipefs_event() race against nfsd_net_id registration
(register_pernet_subsys()) which has been fixed by commit bb7ffbf2
("nfsd: fix nsfd startup race triggering BUG_ON").

Restore the order of register_pernet_subsys() vs register_cld_notifier().
Add WARN_ON() to prevent a future regression.

Crash info:
Unable to handle kernel NULL pointer dereference at virtual address 0000000000000012
CPU: 8 PID: 345 Comm: mount Not tainted 5.4.144-... #1
pc : rpc_pipefs_event+0x54/0x120 [nfsd]
lr : rpc_pipefs_event+0x48/0x120 [nfsd]
Call trace:
 rpc_pipefs_event+0x54/0x120 [nfsd]
 blocking_notifier_call_chain
 rpc_fill_super
 get_tree_keyed
 rpc_fs_get_tree
 vfs_get_tree
 do_mount
 ksys_mount
 __arm64_sys_mount
 el0_svc_handler
 el0_svc

Fixes: bd5ae928 ("nfsd: register pernet ops last, unregister first")
Cc: stable@vger.kernel.org
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4recover.c

@@ -2156,6 +2156,7 @@ static struct notifier_block nfsd4_cld_block = {
 int
 register_cld_notifier(void)
 {
+	WARN_ON(!nfsd_net_id);
 	return rpc_pipefs_notifier_register(&nfsd4_cld_block);
 }
 

fs/nfsd/nfsctl.c

@@ -1521,12 +1521,9 @@ static int __init init_nfsd(void)
 	int retval;
 	printk(KERN_INFO "Installing knfsd (copyright (C) 1996 okir@monad.swb.de).\n");
 
-	retval = register_cld_notifier();
-	if (retval)
-		return retval;
 	retval = nfsd4_init_slabs();
 	if (retval)
-		goto out_unregister_notifier;
+		return retval;
 	retval = nfsd4_init_pnfs();
 	if (retval)
 		goto out_free_slabs;
@@ -1545,9 +1542,14 @@ static int __init init_nfsd(void)
 		goto out_free_exports;
 	retval = register_pernet_subsys(&nfsd_net_ops);
 	if (retval < 0)
+		goto out_free_filesystem;
+	retval = register_cld_notifier();
+	if (retval)
 		goto out_free_all;
 	return 0;
 out_free_all:
+	unregister_pernet_subsys(&nfsd_net_ops);
+out_free_filesystem:
 	unregister_filesystem(&nfsd_fs_type);
 out_free_exports:
 	remove_proc_entry("fs/nfs/exports", NULL);
@@ -1561,13 +1563,12 @@ static int __init init_nfsd(void)
 	nfsd4_exit_pnfs();
 out_free_slabs:
 	nfsd4_free_slabs();
-out_unregister_notifier:
-	unregister_cld_notifier();
 	return retval;
 }
 
 static void __exit exit_nfsd(void)
 {
+	unregister_cld_notifier();
 	unregister_pernet_subsys(&nfsd_net_ops);
 	nfsd_drc_slab_free();
 	remove_proc_entry("fs/nfs/exports", NULL);
@@ -1577,7 +1578,6 @@ static void __exit exit_nfsd(void)
 	nfsd4_free_slabs();
 	nfsd4_exit_pnfs();
 	unregister_filesystem(&nfsd_fs_type);
-	unregister_cld_notifier();
 }
 
 MODULE_AUTHOR("Olaf Kirch <okir@monad.swb.de>");