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The synchronized keyword is used to acquire a mutual-exclusion lock; that is, a lock that cannot be acquired by any other thread while it is held by the executing thread. There are two ways to synchronize access to shared mutable variables: method synchronization and block synchronization. Both methods Methods declared as synchronized and blocks that synchronize on the this reference both use the object’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).

Wiki MarkupOne technique for preventing this vulnerability is the “private lock object” idiom \[ private lock object idiom [Bloch 2001|AA. Bibliography#Bloch 01]\]. This idiom uses the intrinsic lock associated with the instance of a {{ private }} {{ final }} {{java.lang.Object}} declared within the class in place instead of the intrinsic lock of the object itself. This idiom requires the use of synchronized blocks within the class’s methods rather than the use of synchronized methods. Lock contention between the class’s methods and those of a hostile class is impossible, because the hostile class cannot access the {{private}} {{final}} lock the class's methods rather than the use of synchronized methods. Lock contention between the class's methods and those of a hostile class becomes impossible because the hostile class cannot access the private final lock object.

Static methods and state also share this vulnerability. When a static method is declared synchronized, it acquires the intrinsic lock of the class object before any statements in its body are executed, and it releases the intrinsic lock when the method completes. Untrusted code that has access to an object of the class, or of a subclass, can use the getClass() method to gain access to the class object , and consequently to its manipulate the class object's intrinsic lock. Protect static data by locking on a private static final Object. Reducing the accessibility of the class to package-private adds provides further protection against untrusted callers.

The private lock object idiom is also suitable for classes that are designed for inheritance. When a superclass thread requests a lock on the object’s object's monitor, a subclass thread can interfere with its operation. For example, a subclass may use the superclass object’s object's intrinsic lock for performing unrelated operations, caus-ing significant 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.

Objects that require synchronization must use the private lock object idiom rather than their own intrinsic lock in any case where untrusted code could:

• Subclass the class or its superclass. Note that trusted code is permitted to subclass the class.
• Create an object of the class , its superclass, or of a subclass.
• Access or acquire an object instance of the class , its superclass, or of a subclass.

Subclasses whose parents superclasses use the private lock object idiom must themselves use the idiom. However, when a class uses intrinsic synchronization over on the class object without documenting its locking policy, subclasses are forbidden to must not use intrinsic synchronization over on their own class object, unless they explicitly document their locking policy. When the superclass documents its policy by stating that client-side locking is supported, the subclasses have the option to choose between intrinsic locking over the class object or use of and using the private lock object idiom. Subclasses must document their locking policy regardless of which locking option is chosen. See rule TSM00-J. Do not override thread-safe methods with methods that are not thread-safe for related information.

When any of the above these restrictions is are violated, the object’s object's intrinsic lock cannot be trusted. When the 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 above 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 non-atomic nonatomic operations that could either can use a more fine-grained locking scheme involving multiple private final lock objects or that lack a requirement for synchronization. Non-atomic Nonatomic operations can be decoupled from those that require synchronization and can be executed outside the synchronized block. Both for this reason and also for simplification of maintenance, block synchronization using the private lock object idiom is generally recommendedpreferred over intrinsic synchronization.

Noncompliant Code Example (Method 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() { 
    // Locks on the object's monitor ...
  }
 
  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 lock someObject
    Thread.sleep(Integer.MAX_VALUE); // Indefinitely delay someObject
  }
}

The untrusted code attempts to acquire a lock on the object’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.

Noncompliant Code Example (Public Non-

...

final Lock Object)

This noncompliant code example locks on a public non-final nonfinal object in an attempt to use a lock other than {{SomeObject}}’s 's intrinsic lock.

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

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

However, it is possible for untrusted code to change This change fails to protect against malicious code. For example, untrusted or malicious code could disrupt proper synchronization by changing the value of the lock object and disrupt proper synchronization.

Noncompliant Code Example (Publicly Accessible Non-

...

final Lock Object)

This noncompliant code example synchronizes on a private but non-final fieldpublicly 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 field’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 enabling permitting them to execute the two critical sections in an unsafe manner. For example, if the lock were changed when one thread is was in its critical section and the lock is changed, a second thread will would lock on the new object instead of the old one and would enter its critical section erroneously.

A class that lacks accessible methods to change the lock is secure against untrusted manipulation. However, it remains susceptible to inadvertent modification by the programmer. For maintainability reasons, eliminating the accessor method (which is presumably needed for other reasons) is not the preferred solution.

Noncompliant Code Example (Public Final Lock Object)

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. Limit accessibility of fields.Untrusted code that has the ability to create an instance of the class or has access to an already created instance can invoke the wait() method on the publicly accessible lock, causing the lock in the changeValue() method to be released immediately. Furthermore, if the method invokes lock.wait() from its body and does not test a condition predicate, it will be vulnerable to malicious notifications. (See rule THI03-J. Always invoke wait() and await() methods inside a loop for more information.)

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 from 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
      // ...
    }
  }
}

A private final lock object can only be used only with block synchronization. Block synchronization is preferred over method synchronization , because operations without a requirement for synchronization can be moved outside the synchronized region, reducing lock contention and blocking. Note that it is unnecessary to declare the lock field volatile because of the strong visibility semantics of final fields. When granularity issues require the use of multiple locks, declare and use multiple private final lock objects to satisfy the granularity requirements rather than using a mutable reference to a lock object along with a setter method.

...

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 object’s object''s monitor and, upon succeeding, introduces an indefinite delay that prevents the synchronized changeValue() method from acquiring the same lock.

A compliant solution must also comply with rule LCK05-J. Synchronize access to static fields that can be modified by untrusted code. However, in
In the untrusted code, the attacker intentionally violates rule LCK09-J. Do not perform operations that can block while holding a lock.

...

Thread-safe public classes that both use intrinsic synchronization over the class object and also 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(); // private final lock 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-EX1EX0: A class may violate this rule when all of the following conditions are met:

• It sufficiently documents that callers are forbidden to must not pass objects of this class to untrusted code.
• The class cannot invoke methods, directly or indirectly, on objects of any untrusted classes that violate this guide-line, whether directly or indirectlyrule.
• The synchronization policy of the class is documented properly.

...

• 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, whether directly or indirectly.

LCK00-J-EX2EX1: 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-EX3J-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 denial of serviceDoS.

Bibliography

\[[Bloch 2001|AA. Bibliography#Bloch 01]\] Item 52: "Document Thread Safety"

Automated Detection

Related Guidelines

Bibliography

Image Added      Image Added08. Locking (LCK)      08. Locking (LCK)