SUNRPC: fix races on PipeFS MOUNT notifications

Below are races, when RPC client can be created without PiepFS dentries

CPU#0					CPU#1
-----------------------------		-----------------------------
rpc_new_client				rpc_fill_super
rpc_setup_pipedir
mutex_lock(&sn->pipefs_sb_lock)
rpc_get_sb_net == NULL
(no per-net PipeFS superblock)
					sn->pipefs_sb = sb;
					notifier_call_chain(MOUNT)
					(client is not in the list)
rpc_register_client
(client without pipes dentries)

To fix this patch:
1) makes PipeFS mount notification call with pipefs_sb_lock being held.
2) releases pipefs_sb_lock on new SUNRPC client creation only after
registration.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/clnt.c

@@ -157,20 +157,15 @@ static struct dentry *rpc_setup_pipedir_sb(struct super_block *sb,
 }
 
 static int
-rpc_setup_pipedir(struct rpc_clnt *clnt, const char *dir_name)
+rpc_setup_pipedir(struct rpc_clnt *clnt, const char *dir_name,
+		  struct super_block *pipefs_sb)
 {
-	struct net *net = rpc_net_ns(clnt);
-	struct super_block *pipefs_sb;
 	struct dentry *dentry;
 
 	clnt->cl_dentry = NULL;
 	if (dir_name == NULL)
 		return 0;
-	pipefs_sb = rpc_get_sb_net(net);
-	if (!pipefs_sb)
-		return 0;
 	dentry = rpc_setup_pipedir_sb(pipefs_sb, clnt, dir_name);
-	rpc_put_sb_net(net);
 	if (IS_ERR(dentry))
 		return PTR_ERR(dentry);
 	clnt->cl_dentry = dentry;
@@ -296,6 +291,7 @@ static struct rpc_clnt * rpc_new_client(const struct rpc_create_args *args, stru
 	struct rpc_clnt		*clnt = NULL;
 	struct rpc_auth		*auth;
 	int err;
+	struct super_block *pipefs_sb;
 
 	/* sanity check the name before trying to print it */
 	dprintk("RPC:       creating %s client for %s (xprt %p)\n",
@@ -354,9 +350,12 @@ static struct rpc_clnt * rpc_new_client(const struct rpc_create_args *args, stru
 
 	atomic_set(&clnt->cl_count, 1);
 
-	err = rpc_setup_pipedir(clnt, program->pipe_dir_name);
-	if (err < 0)
-		goto out_no_path;
+	pipefs_sb = rpc_get_sb_net(rpc_net_ns(clnt));
+	if (pipefs_sb) {
+		err = rpc_setup_pipedir(clnt, program->pipe_dir_name, pipefs_sb);
+		if (err)
+			goto out_no_path;
+	}
 
 	auth = rpcauth_create(args->authflavor, clnt);
 	if (IS_ERR(auth)) {
@@ -369,11 +368,16 @@ static struct rpc_clnt * rpc_new_client(const struct rpc_create_args *args, stru
 	/* save the nodename */
 	rpc_clnt_set_nodename(clnt, utsname()->nodename);
 	rpc_register_client(clnt);
+	if (pipefs_sb)
+		rpc_put_sb_net(rpc_net_ns(clnt));
 	return clnt;
 
 out_no_auth:
-	rpc_clnt_remove_pipedir(clnt);
+	if (pipefs_sb)
+		__rpc_clnt_remove_pipedir(clnt);
 out_no_path:
+	if (pipefs_sb)
+		rpc_put_sb_net(rpc_net_ns(clnt));
 	kfree(clnt->cl_principal);
 out_no_principal:
 	rpc_free_iostats(clnt->cl_metrics);

net/sunrpc/rpc_pipe.c

@@ -1127,6 +1127,7 @@ rpc_fill_super(struct super_block *sb, void *data, int silent)
 		return -ENOMEM;
 	dprintk("RPC:       sending pipefs MOUNT notification for net %p%s\n",
 		net, NET_NAME(net));
+	mutex_lock(&sn->pipefs_sb_lock);
 	sn->pipefs_sb = sb;
 	err = blocking_notifier_call_chain(&rpc_pipefs_notifier_list,
 					   RPC_PIPEFS_MOUNT,
@@ -1134,6 +1135,7 @@ rpc_fill_super(struct super_block *sb, void *data, int silent)
 	if (err)
 		goto err_depopulate;
 	sb->s_fs_info = get_net(net);
+	mutex_unlock(&sn->pipefs_sb_lock);
 	return 0;
 
 err_depopulate:
@@ -1142,6 +1144,7 @@ err_depopulate:
 					   sb);
 	sn->pipefs_sb = NULL;
 	__rpc_depopulate(root, files, RPCAUTH_lockd, RPCAUTH_RootEOF);
+	mutex_unlock(&sn->pipefs_sb_lock);
 	return err;
 }