SUNRPC: Fix a race in rpc_info_open

There is a race between rpc_info_open and rpc_release_client()
in that nothing stops a process from opening the file after
the clnt->cl_kref goes to zero.

Fix this by using atomic_inc_unless_zero()...
Reported-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@kernel.org

include/linux/sunrpc/clnt.h

@@ -30,7 +30,7 @@ struct rpc_inode;
  * The high-level client handle
  */
 struct rpc_clnt {
-	struct kref		cl_kref;	/* Number of references */
+	atomic_t		cl_count;	/* Number of references */
 	struct list_head	cl_clients;	/* Global list of clients */
 	struct list_head	cl_tasks;	/* List of tasks */
 	spinlock_t		cl_lock;	/* spinlock */

net/sunrpc/clnt.c

@@ -226,7 +226,7 @@ static struct rpc_clnt * rpc_new_client(const struct rpc_create_args *args, stru
 			goto out_no_principal;
 	}
 
-	kref_init(&clnt->cl_kref);
+	atomic_set(&clnt->cl_count, 1);
 
 	err = rpc_setup_pipedir(clnt, program->pipe_dir_name);
 	if (err < 0)
@@ -390,14 +390,14 @@ rpc_clone_client(struct rpc_clnt *clnt)
 		if (new->cl_principal == NULL)
 			goto out_no_principal;
 	}
-	kref_init(&new->cl_kref);
+	atomic_set(&new->cl_count, 1);
 	err = rpc_setup_pipedir(new, clnt->cl_program->pipe_dir_name);
 	if (err != 0)
 		goto out_no_path;
 	if (new->cl_auth)
 		atomic_inc(&new->cl_auth->au_count);
 	xprt_get(clnt->cl_xprt);
-	kref_get(&clnt->cl_kref);
+	atomic_inc(&clnt->cl_count);
 	rpc_register_client(new);
 	rpciod_up();
 	return new;
@@ -465,10 +465,8 @@ EXPORT_SYMBOL_GPL(rpc_shutdown_client);
  * Free an RPC client
  */
 static void
-rpc_free_client(struct kref *kref)
+rpc_free_client(struct rpc_clnt *clnt)
 {
-	struct rpc_clnt *clnt = container_of(kref, struct rpc_clnt, cl_kref);
-
 	dprintk("RPC:       destroying %s client for %s\n",
 			clnt->cl_protname, clnt->cl_server);
 	if (!IS_ERR(clnt->cl_path.dentry)) {
@@ -495,12 +493,10 @@ rpc_free_client(struct kref *kref)
  * Free an RPC client
  */
 static void
-rpc_free_auth(struct kref *kref)
+rpc_free_auth(struct rpc_clnt *clnt)
 {
-	struct rpc_clnt *clnt = container_of(kref, struct rpc_clnt, cl_kref);
-
 	if (clnt->cl_auth == NULL) {
-		rpc_free_client(kref);
+		rpc_free_client(clnt);
 		return;
 	}
 
@@ -509,10 +505,11 @@ rpc_free_auth(struct kref *kref)
 	 *       release remaining GSS contexts. This mechanism ensures
 	 *       that it can do so safely.
 	 */
-	kref_init(kref);
+	atomic_inc(&clnt->cl_count);
 	rpcauth_release(clnt->cl_auth);
 	clnt->cl_auth = NULL;
-	kref_put(kref, rpc_free_client);
+	if (atomic_dec_and_test(&clnt->cl_count))
+		rpc_free_client(clnt);
 }
 
 /*
@@ -525,7 +522,8 @@ rpc_release_client(struct rpc_clnt *clnt)
 
 	if (list_empty(&clnt->cl_tasks))
 		wake_up(&destroy_wait);
-	kref_put(&clnt->cl_kref, rpc_free_auth);
+	if (atomic_dec_and_test(&clnt->cl_count))
+		rpc_free_auth(clnt);
 }
 
 /**
@@ -588,7 +586,7 @@ void rpc_task_set_client(struct rpc_task *task, struct rpc_clnt *clnt)
 	if (clnt != NULL) {
 		rpc_task_release_client(task);
 		task->tk_client = clnt;
-		kref_get(&clnt->cl_kref);
+		atomic_inc(&clnt->cl_count);
 		if (clnt->cl_softrtry)
 			task->tk_flags |= RPC_TASK_SOFT;
 		/* Add to the client's list of all tasks */

net/sunrpc/rpc_pipe.c

@@ -371,21 +371,23 @@ rpc_show_info(struct seq_file *m, void *v)
 static int
 rpc_info_open(struct inode *inode, struct file *file)
 {
-	struct rpc_clnt *clnt;
+	struct rpc_clnt *clnt = NULL;
 	int ret = single_open(file, rpc_show_info, NULL);
 
 	if (!ret) {
 		struct seq_file *m = file->private_data;
-		mutex_lock(&inode->i_mutex);
+
+		spin_lock(&file->f_path.dentry->d_lock);
+		if (!d_unhashed(file->f_path.dentry))
 			clnt = RPC_I(inode)->private;
-		if (clnt) {
-			kref_get(&clnt->cl_kref);
+		if (clnt != NULL && atomic_inc_not_zero(&clnt->cl_count)) {
+			spin_unlock(&file->f_path.dentry->d_lock);
 			m->private = clnt;
 		} else {
+			spin_unlock(&file->f_path.dentry->d_lock);
 			single_release(inode, file);
 			ret = -EINVAL;
 		}
-		mutex_unlock(&inode->i_mutex);
 	}
 	return ret;
 }