The recommendations suggested in the guideline CON13-J. Ensure that threads are stopped cleanly are insufficient to terminate a thread that is blocked on a blocking operation such as network or file input-output (IO). Consequently, threads and tasks should provide callers with an explicit termination mechanism to prevent denial of service vulnerabilities.
Noncompliant Code Example (blocking IO, volatile flag)
This noncompliant code example uses a volatile done flag to indicate that it is safe to shut down the thread, as suggested in CON13-J. Ensure that threads are stopped cleanly. However, this does not help in terminating the thread because the thread is blocked on network IO as a consequence of using the readLine() method.
public final class SocketReader implements Runnable { // Thread-safe class
private final Socket socket;
private final BufferedReader in;
private volatile boolean done = false;
private final Object lock = new Object();
public SocketReader(String host, int port) throws IOException {
this.socket = new Socket(host, port);
this.in = new BufferedReader(new InputStreamReader(this.socket.getInputStream()));
}
// Only one thread can use the socket at a particular time
public void run() {
try {
synchronized (lock) {
readData();
}
} catch (IOException ie) {
// Forward to handler
}
}
public void readData() throws IOException {
String string;
while (!done && (string = in.readLine()) != null) {
// Blocks until end of stream (null)
}
}
public void shutdown() {
done = true;
}
public static void main(String[] args) throws IOException, InterruptedException {
SocketReader reader = new SocketReader("somehost", 25);
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
reader.shutdown();
}
}
Noncompliant Code Example (blocking IO, interruptible)
This noncompliant code example is similar to the preceding one but uses thread interruption to indicate that it is safe to shut down the thread. Network IO is not responsive to thread interruption when a java.net.Socket is being used.
public final class SocketReader implements Runnable { // Thread-safe class
// ...
public void readData() throws IOException {
String string;
while (!Thread.interrupted() && (string = in.readLine()) != null) {
// Blocks until end of stream (null)
}
}
public static void main(String[] args) throws IOException, InterruptedException {
SocketReader reader = new SocketReader("somehost", 25);
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
thread.interrupt();
}
}
Compliant Solution (close socket connection)
This compliant solution resumes the thread, by having the shutdown() method close the socket. As a result, the thread is bound to stop because the readLine() method will throw a SocketException when the socet is closed. Note that there is no way to keep the connection alive if the thread is to be cleanly halted immediately.
public final class SocketReader implements Runnable {
// ...
public void readData() throws IOException {
String string;
try {
while ((string = in.readLine()) != null) {
// Blocks until end of stream (null)
}
} finally {
shutdown();
}
}
public void shutdown() throws IOException {
socket.close();
}
public static void main(String[] args) throws IOException, InterruptedException {
SocketReader reader = new SocketReader();
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
reader.shutdown();
}
}
The finally block executes after shutdown() is called. Because the finally block also calls the shutdown() method, it is possible that the socket will be closed a second time, which has no effect.
When performing asynchronous IO, a java.nio.channels.Selector may also be brought out of the blocked state by either invoking its close() or wakeup() method.
A boolean flag can be used (as shown earlier) if additional operations need to be performed. When supplementing the code with a flag, the shutdown() method should also set the flag to false so that the thread can cleanly exit from the while loop.
Compliant Solution (interruptible channel)
This compliant solution uses an interruptible channel, SocketChannel, instead of a Socket connection. If the thread performing the network IO is interrupted using the Thread.interrupt() method while reading the data, the thread receives a ClosedByInterruptException and the channel is closed immediately. The thread's interrupt status is also set.
public final class SocketReader implements Runnable {
private final SocketChannel sc;
private final Object lock = new Object();
public SocketReader(String host, int port) throws IOException {
sc = SocketChannel.open(new InetSocketAddress(host, port));
}
public void run() {
ByteBuffer buf = ByteBuffer.allocate(1024);
try {
synchronized (lock) {
while (!Thread.interrupted()) {
sc.read(buf);
// ...
}
}
} catch (IOException ie) {
// Forward to handler
}
}
public static void main(String[] args) throws IOException, InterruptedException {
SocketReader reader = new SocketReader("somehost", 25);
Thread thread = new Thread(reader);
thread.start();
Thread.sleep(1000);
thread.interrupt();
}
}
This technique interrupts the current thread, however, it only stops the thread because the code polls the interrupted flag using the method Thread.interrupted(), and shuts down the thread when it is interrupted. Using a SocketChannel ensures that the condition in the while loop is tested as soon as an interruption is received, despite the read operation being a blocking operation. Invoking the interrupt() method of a thread that is blocked because of a java.nio.channels.Selector also causes the thread to awaken.
Noncompliant Code Example (database connection)
This noncompliant code example shows a thread-safe class DBConnector that creates one JDBC connection per thread. Each connection belongs to one thread, and is not shared by other threads. This is a common use-case because JDBC connections are not meant to be shared by multiple-threads.
public final class DBConnector implements Runnable {
final String query;
DBConnector(String query) {
this.query = query;
}
public void run() {
Connection connection;
try {
// Username and password are hard coded for brevity
connection = DriverManager.getConnection("jdbc:driver:name", "username", "password");
Statement stmt = connection.createStatement();
ResultSet rs = stmt.executeQuery(query);
} catch (SQLException e) {
// Forward to handler
}
// ...
}
public static void main(String[] args) throws InterruptedException {
DBConnector connector = new DBConnector("suitable query");
Thread thread = new Thread(connector);
thread.start();
Thread.sleep(5000);
thread.interrupt();
}
}
Unfortunately database connections, like sockets, are not inherently interruptible. So this design does not permit a client to cancel a task by closing it if the corresponding thread is blocked on a long running activity such as a join query. Furthermore, it is important to provide a mechanism to close connections to prevent thread starvation, because there is a limited number of database connections available in the pool. Similar task cancellation mechanisms are required when using objects local to a method, such as sockets.
Compliant Solution (ThreadLocal)
This compliant solution uses a ThreadLocal wrapper around the connection so that a thread that calls initialValue() obtains a unique connection instance. The advantage of this approach is that a shutdownConnection() method can be provided so that clients external to the class can also close the connection when the corresponding thread is blocked, or is performing some time consuming activity.
public final class DBConnector implements Runnable {
final String query;
DBConnector(String query) {
this.query = query;
}
private static ThreadLocal<Connection> connectionHolder = new ThreadLocal<Connection>() {
Connection connection = null;
@Override public Connection initialValue() {
try {
// Username and password are hard coded for brevity
connection = DriverManager.getConnection
("jdbc:driver:name", "username", "password");
} catch (SQLException e) {
// Forward to handler
}
return connection;
}
@Override public void set(Connection con) {
if(connection == null) { // Shuts down connection when null value is passed
try {
connection.close();
} catch (SQLException e) {
// Forward to handler
}
} else {
connection = con;
}
}
};
public static Connection getConnection() {
return connectionHolder.get();
}
public static void shutdownConnection() { // Allows client to close connection anytime
connectionHolder.set(null);
}
public void run() {
Connection dbConnection = getConnection();
Statement stmt;
try {
stmt = dbConnection.createStatement();
ResultSet rs = stmt.executeQuery(query);
} catch (SQLException e) {
// Forward to handler
}
// ...
}
public static void main(String[] args) throws InterruptedException {
DBConnector connector = new DBConnector(/* suitable query */);
Thread thread = new Thread(connector);
thread.start();
Thread.sleep(5000);
connector.shutdown();
}
}
Risk Assessment
Failing to provide facilities for thread shutdown can cause non-responsiveness and denial of service.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
CON24- J |
low |
probable |
medium |
P4 |
L3 |
Automated Detection
TODO
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
[[API 06]] Class Thread, method stop, interface ExecutorService
[[Darwin 04]] 24.3 Stopping a Thread
[[JDK7 08]] Concurrency Utilities, More information: Java Thread Primitive Deprecation
[[JPL 06]] 14.12.1. Don't stop and 23.3.3. Shutdown Strategies
[[JavaThreads 04]] 2.4 Two Approaches to Stopping a Thread
[[Goetz 06]] Chapter 7: Cancellation and shutdown
CON12-J. Avoid deadlock by requesting and releasing locks in the same order 11. Concurrency (CON) VOID CON14-J. Ensure atomicity of 64-bit operations