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Concurrency related vulnerabilities can manifest themselves when assumptions are made about the multithreaded behavior of derived classes. An Overriding thread-safe methods with methods that are unsafe for concurrent use can result in improper synchronization when a client that depends on the thread-safety promised by the parent inadvertently operates on an instance of the subclass. For example, an overridden synchronized method's contract may can be violated if when a subclass defines provides an implementation that is unsafe for concurrent use. Such overriding can easily result in errors that are difficult to diagnose. Consequently, programs must not override thread-safe methods with methods that are unsafe for concurrent use.

The locking strategy of classes designed for inheritance should always be documented. This information can subsequently be used to determine an appropriate locking strategy for subclasses (see LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code and LCK11-J. Avoid client-side locking when using classes that do not commit to their locking strategy).

Noncompliant Code Example (Synchronized Method)

This noncompliant code example defines a overrides the synchronized doSomething() method in a class called SynchronizedBase. The method can be safely used by many threads. However, if a subclass UnsynchronizedSubclass overrides the method but leaves it unsynchronized, its instance cannot be safely passed to multiple threads. the Base class with an unsynchronized method in the Derived class:

public class SynchronizedBaseBase {
  public synchronized void doSomething() {
    // ...	
  }
}

public class UnsynchronizedSubclassDerived extends SynchronizedBaseBase {
  @Override public void doSomething() {
    // ...		
  }
}

The doSomething() method of the Base class can be safely used by multiple threads, but instances of the Derived subclass cannot.

This programming error can be difficult to diagnose This problem is hard to notice because threads that accept instances of SynchronizedBase Base can also accept instances of its subclasses. Consequently, the threads may incorrectly assume that the subclasses are clients could be unaware that they are operating on a thread-unsafe instance of a subclass of a thread-safe class.

Compliant Solution (Synchronized Method)

This compliant solution synchronizes the doSomething() method of the subclass. :

public class SynchronizedBaseBase {
  public synchronized void doSomething() {
    // ...	
  }
}

public class SynchronizedSubclassDerived extends SynchronizedBaseBase {
  @Override public synchronized void doSomething() {
    // ...		
  }
}

This solution also complies with LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code because the accessibility of the class is package-private. Package-private accessibility is permitted when untrusted code cannot infiltrate the package.

Compliant Solution

...

An alternative is to use block synchronization by defining an internal private final lock object in the extending class. This is the preferred choice when untrusted code can obtain the intrinsic lock of the class object (CON04-J. Use the private lock object idiom instead of intrinsic synchronization).

(Private Final Lock Object)

This compliant solution ensures that the Derived class is thread-safe by overriding the synchronized doSomething() method of the Base class with a method that synchronizes on a private final lock object.

class Base {

  public synchronized void doSomething() {
    // ...
  }
}

class Derived extends Base {
  private final Object lock = new Object();

  @Override public void doSomething() {
    synchronized (lock) {
      // ...
    }
  }
}

This is an acceptable solution, provided the locking policy of the Derived class is consistent with that of the Base class.

Noncompliant Code Example (Private Lock)

This noncompliant code example defines a doSomething() method in the Base class that uses a private final lock in accordance with LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code.

class Base {
  private final Object lock = new Object();

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

class Derived extends Base {
  Logger logger = // Initialize

  @Override public void doSomething() {
    try {
      super.doSomething();
    } finally {
      logger.log(Level.FINE, "Did something");
    }
  }
}

It is possible for multiple threads to cause the entries to be logged in an order that differs from the order in which the tasks are performed. Consequently, the doSomething() method of the Derived class cannot be used safely by multiple threads because it is not thread-safe.

Compliant Solution (Private Lock)

This compliant solution synchronizes the doSomething() method of the subclass using its own private final lock object:

class Base {
  private final Object lock = new Object();

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

class Derived extends Base {
  Logger logger = // Initialize

  

// ...
public class SynchronizedSubclass extends SynchronizedBase {
  private final Object lock = new Object(); 

  @Override public void doSomething() {
    synchronized (lock) {
      try {
       // super.doSomething();
      } finally {
        logger..	log(Level.FINE, "Did something");
      }
    }	
  }
}

Exceptions

EX1: If the subclass implements block synchronization instead of method synchronization, it is likely that thread-safety has been accounted for. This condition should be sufficiently documented.

Risk Assessment

Note that the Base and Derived objects maintain distinct locks that are inaccessible from each other's classes. Consequently, Derived can provide thread-safety guarantees independent of Base.

Risk Assessment

Overriding thread-safe methods with methods that are unsafe for concurrent access Failure to provide synchronization in overriding methods can result in unexpected behavior.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[API 06|AA. Java References#API 06]\]
\[[SDN 08|AA. Java References#SDN 08]\] Sun bug database, [Bug ID 4294756|http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4294756]

Sound automated detection is infeasible; heuristic checks could be useful.

Bibliography

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Image Added Image Added Image AddedCON09-J. Do not invoke alien methods that rely on invariants protected by the same lock object      11. Concurrency (CON)      CON11-J. Do not assume that declaring an object volatile guarantees visibility of its members