CON04-J. Use the private lock object idiom instead of the Class object's intrinsic locking mechanism

The synchronized keyword is used to acquire a mutual-exclusion lock so that no other thread can acquire the lock, while it is being held by the executing thread. The synchronized keyword always uses the object's intrinsic 'monitor' lock. This lock is available to any code that the object itself is available to; consequently any code can lock on the object, and potentially cause a denial of service.

Recall that there are two ways to synchronize access to shared mutable variables, method synchronization and block synchronization. Excessive synchronization can induce a denial of service (DoS) vulnerability because another class whose member locks on the class object, can fail to release the lock promptly. However, this requires the victim class to be accessible from the hostile class.

The private lock object idiom can be used to prevent the DoS vulnerability. The idiom consists of a private object declared as an instance field. The private object must be explicitly used for locking purposes in synchronized blocks, within the class's methods. This lock object belongs to an instance of the object and is not associated with the class object itself. Consequently, there is no lock contention between a class method and a method of a hostile class when both try to lock on the class object. [[Bloch 01]]

This idiom can also be suitably used by classes designed for inheritance. If a superclass thread requests a lock on the class object's monitor, a subclass thread can interfere with its operation. Refer to the guideline CON02-J. Always synchronize on the appropriate object for more details.

An object should use a private internal lock object rather than its own intrinsic lock unless the class can guarantee that untrusted code may not:

• Subclass the class (However, trusted code may subclass the class.)
• Create an object of the class (or a subclass)
• Access an object of the class (or a subclass)
  Furthermore, the class must also ensure that no superclasses it inherits from use synchronization at all.

If these restrictions are not met, the object's intrinsic lock is not trustworthy. If all conditions are satisfied, then the object gains no significant security from using a private internal lock object, and may synchronize using its own intrinsic lock.

Noncompliant Code Example

This noncompliant code example exposes the class object someObject to untrusted code. The untrusted code attempts to acquire a lock on the class object's monitor and upon succeeding, introduces an indefinite delay which holds up the synchronized changeValue() method from acquiring the same lock. Note that the untrusted code also violates CON06-J. Do not defer a thread that is holding a lock.

public class SomeObject {
  public synchronized void changeValue() { // Locks on the class object's monitor
    // ...   
  }
}

// Untrusted code
synchronized (someObject) {
  while (true) {
    Thread.sleep(Integer.MAX_VALUE); // Indefinitely delay someObject
  }
}

Compliant Solution

Thread-safe classes that use intrinsic synchronization of the class object may be protected by using the private lock object idiom and adapting them to use block synchronization. In this compliant solution, if the method changeValue() is called, the lock is obtained on a private Object that is inaccessible from the caller.

public class SomeObject {
  private final Object lock = new Object(); // private lock object

  public void changeValue() {
    synchronized (lock) { // Locks on the private Object
      // ...
    }
  }
}

Using a private lock may only be achieved with block synchronization, as method synchronization always uses the object's intrinsic lock. However, block synchronization is also preferred over method synchronization, because it is easy to move operations out of the synchronized block when they might take a long time and they are not truly a critical section.

Risk Assessment

Exposing the class object to untrusted code can result in denial-of-service.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[Bloch 01]] Item 52: "Document Thread Safety"

CON03-J. Do not use background threads during class initialization      11. Concurrency (CON)      CON05-J. Ensure that threads do not fail during activation