ksmbd: fix use-after-free in ksmbd_tree_connect_put under concurrency

Under high concurrency, A tree-connection object (tcon) is freed on
a disconnect path while another path still holds a reference and later
executes *_put()/write on it.

Reported-by: Qianchang Zhao <pioooooooooip@gmail.com>
Reported-by: Zhitong Liu <liuzhitong1993@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: namjaejeon

fs/smb/server/mgmt/tree_connect.c

@@ -78,7 +78,6 @@ ksmbd_tree_conn_connect(struct ksmbd_work *work, const char *share_name)
 	tree_conn->t_state = TREE_NEW;
 	status.tree_conn = tree_conn;
 	atomic_set(&tree_conn->refcount, 1);
-	init_waitqueue_head(&tree_conn->refcount_q);
 
 	ret = xa_err(xa_store(&sess->tree_conns, tree_conn->id, tree_conn,
 			      KSMBD_DEFAULT_GFP));
@@ -100,14 +99,8 @@ ksmbd_tree_conn_connect(struct ksmbd_work *work, const char *share_name)
 
 void ksmbd_tree_connect_put(struct ksmbd_tree_connect *tcon)
 {
-	/*
-	 * Checking waitqueue to releasing tree connect on
-	 * tree disconnect. waitqueue_active is safe because it
-	 * uses atomic operation for condition.
-	 */
-	if (!atomic_dec_return(&tcon->refcount) &&
-	    waitqueue_active(&tcon->refcount_q))
-		wake_up(&tcon->refcount_q);
+	if (atomic_dec_and_test(&tcon->refcount))
+		kfree(tcon);
 }
 
 int ksmbd_tree_conn_disconnect(struct ksmbd_session *sess,
@@ -119,14 +112,11 @@ int ksmbd_tree_conn_disconnect(struct ksmbd_session *sess,
 	xa_erase(&sess->tree_conns, tree_conn->id);
 	write_unlock(&sess->tree_conns_lock);
 
-	if (!atomic_dec_and_test(&tree_conn->refcount))
-		wait_event(tree_conn->refcount_q,
-			   atomic_read(&tree_conn->refcount) == 0);
-
 	ret = ksmbd_ipc_tree_disconnect_request(sess->id, tree_conn->id);
 	ksmbd_release_tree_conn_id(sess, tree_conn->id);
 	ksmbd_share_config_put(tree_conn->share_conf);
-	kfree(tree_conn);
+	if (atomic_dec_and_test(&tree_conn->refcount))
+		kfree(tree_conn);
 	return ret;
 }
 

fs/smb/server/mgmt/tree_connect.h

@@ -33,7 +33,6 @@ struct ksmbd_tree_connect {
 	int				maximal_access;
 	bool				posix_extensions;
 	atomic_t			refcount;
-	wait_queue_head_t		refcount_q;
 	unsigned int			t_state;
 };
 

fs/smb/server/smb2pdu.c

@@ -2190,7 +2190,6 @@ int smb2_tree_disconnect(struct ksmbd_work *work)
 		goto err_out;
 	}
 
-	WARN_ON_ONCE(atomic_dec_and_test(&tcon->refcount));
 	tcon->t_state = TREE_DISCONNECTED;
 	write_unlock(&sess->tree_conns_lock);
 
@@ -2200,8 +2199,6 @@ int smb2_tree_disconnect(struct ksmbd_work *work)
 		goto err_out;
 	}
 
-	work->tcon = NULL;
-
 	rsp->StructureSize = cpu_to_le16(4);
 	err = ksmbd_iov_pin_rsp(work, rsp,
 				sizeof(struct smb2_tree_disconnect_rsp));