Page History

There are two ways to synchronize access to shared mutable variables: method synchronization and block synchronization. Methods declared as synchronized and blocks that synchronize on the this reference both use the objectâ������������‚�š�š�š�š�š�������������‚�š�š�š�š�z�s object as a monitor (that is, its intrinsic lock). An attacker can manipulate the system to trigger contention and deadlock by obtaining and indefinitely holding the intrinsic lock of an accessible class, consequently causing a denial of service (DoS).

One technique for preventing this vulnerability is the private lock object idiom [Bloch 2001]. This idiom uses the intrinsic lock associated with the instance of a private final java.lang.Object declared within the class instead of the intrinsic lock of the object itself. This idiom requires the use of synchronized blocks within the classâ������������‚�š�š�š�š�š�������������‚�š�š�š�š�z�s class's methods rather than the use of synchronized methods. Lock contention between the classâ������������‚�š�š�š�š�š�������������‚�š�š�š�š�z�s class's methods and those of a hostile class becomes impossible because the hostile class cannot access the private final lock object.

...

The private lock object idiom is also suitable for classes that are designed for inheritance. When a superclass requests a lock on the objectâ������������‚�š�š�š�š�š�������������‚�š�š�š�š�z�s object's monitor, a subclass can interfere with its operation. For example, a subclass may use the superclass objectâ������������‚�š�š�š�š�š�������������‚�š�š�š�š�z�s object's intrinsic lock for performing unrelated operations, causing lock contention and deadlock. Separating the locking strategy of the superclass from that of the subclass ensures that they do not share a common lock and also permits fine-grained locking by supporting the use of multiple lock objects for unrelated operations. This increases the overall responsiveness of the application.

...

When any of these restrictions are violated, the objectâ������������‚�š�š�š�š�š�������������‚�š�š�š�š�z�s object's intrinsic lock cannot be trusted. But when these restrictions are obeyed, the private lock object idiom fails to add any additional security. Consequently, objects that comply with all of the restrictions are permitted to synchronize using their own intrinsic lock. However, block synchronization using the private lock object idiom is superior to method synchronization for methods that contain nonatomic operations that could either use a more fine-grained locking scheme involving multiple private final lock objects or that lack a requirement for synchronization. Nonatomic operations can be decoupled from those that require synchronization and can be executed outside the synchronized block. Both for this reason and for simplification of maintenance, block synchronization using the private lock object idiom is generally preferred over intrinsic synchronization.

...

This noncompliant code example exposes instances of the SomeObject class to untrusted code.

public class SomeObject {

  // Locks on the object's monitor
  public synchronized void changeValue() { 
    // ...
  }
 
  public static SomeObject lookup(String name) {
    // ...
  }
}

// Untrusted code
String name = // ...
SomeObject someObject = new SomeObject.lookup(name);
if (someObject == null) {
  // ... handle error
}
synchronized (someObject) {
  while (true) {
    // Indefinitely delaylock someObject
    Thread.sleep(Integer.MAX_VALUE); 
  }
}

The untrusted code attempts to acquire a lock on the objectâ������������‚�š�š�š�š�š�������������‚�š�š�š�š�z�s object's monitor and, upon succeeding, introduces an indefinite delay that prevents the synchronized changeValue() method from acquiring the same lock. Furthermore, the object locked is publicly available via the lookup() method.

Alternatively, an attacker could create a private SomeObject object and make it available to trusted code to use it before the attacker code grabs and holds the lock.

Note that in the untrusted code, the attacker intentionally violates rule LCK09-J. Do not perform operations that can block while holding a lock.

...

This noncompliant code example locks on a public nonfinal object in an attempt to use a lock other than {{SomeObject}}â������������‚�š�š�š�š�š�������������‚�š�š�š�š�z�s 's intrinsic lock.

public class SomeObject {
  public Object lock = new Object();

  public void changeValue() {
    synchronized (lock) {
      // ...
    }
  }
}

...

This noncompliant code example synchronizes on a publicly accessible but nonfinal field. The lock field is declared volatile so that changes are visible to other threads.

public class SomeObject {
  private volatile Object lock = new Object();

  public void changeValue() {
    synchronized (lock) {
      // ...
    }
  }

  public void setLock(Object lockValue) {
    lock = lockValue;
  }
}

Any thread can modify the fieldâ������������‚�š�š�š�š�š�������������‚�š�š�š�š�z�s field's value to refer to a different object in the presence of an accessor such as setLock(). That modification might cause two threads that intend to lock on the same object to lock on different objects, thereby permitting them to execute two critical sections in an unsafe manner. For example, if the lock were changed when one thread was in its critical section, a second thread would lock on the new object instead of the old one and would enter its critical section erroneously.

...

This noncompliant code example uses a public final lock object.

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

  public void changeValue() {
    synchronized (lock) {
      // ...
    }
  }
}

// Untrusted code
SomeObject someObject = new SomeObject();
someObject.lock.wait();

...

This noncompliant code example also violates rule OBJ01-J. Declare data members as private and provide accessible wrapper methodsLimit accessibility of fields.

Compliant Solution (Private Final Lock Object)

Thread-safe public classes that may interact with untrusted code must use a private final lock object. Existing classes that use intrinsic synchronization must be refactored to use block synchronization on such an object. In this compliant solution, calling changeValue() obtains a lock on a private final Object instance that is inaccessible to callers that are outside the class's scope.

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

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

...

This noncompliant code example exposes the class object of SomeObject to untrusted code.

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

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

The untrusted code attempts to acquire a lock on the class objectâ������������‚�š�š�š�š�š�������������‚�š�š�š�š�z�s object''s monitor and, upon succeeding, introduces an indefinite delay that prevents the synchronized changeValue() method from acquiring the same lock.

...

Thread-safe public classes that both use intrinsic synchronization over the class object and may interact with untrusted code must be refactored to use a static private final lock object and block synchronization.

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

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

In this compliant solution, changeValue() obtains a lock on a private static Object that is inaccessible to the caller.

Exceptions

LCK00-J-EX0: A class may violate this rule when all of the following conditions are met:

...

• Neither the client class nor any other class in the system passes objects of the violating class to untrusted code.
• The violating class cannot invoke methods, directly or indirectly, from untrusted classes that violate this rule.

LCK00-J-EX1: When a superclass of the class documents that it supports client-side locking and synchronizes on its class object, the class can support client-side locking in the same way and document this policy.

LCK00-J-EX2: Package-private classes that are never exposed to untrusted code are exempt from this rule because their accessibility protects against untrusted callers. However, use of this exemption should be documented explicitly to ensure that trusted code within the same package neither reuses the lock object nor changes the lock object inadvertently.

...

Exposing the lock object to untrusted code can result in DoS.

Automated Detection

Related Guidelines

Bibliography

08. Locking (LCK)      08. Locking (LCK)Image Added      Image Added