CON17-J. Avoid using ThreadGroup APIs

A ThreadGroup is a group of threads as defined by the class java.lang.ThreadGroup. A group is assigned to a thread upon its creation. If the group name is not specified explicitly, the default group called main is assigned by the Java Virtual Machine (JVM). The convenience methods of the ThreadGroup class can be used to operate on all threads at once, such as, by using the interrupt() method. Another use is to reinforce layered security by confining threads into groups so that they do not interfere with each other. [[JavaThreads 04]]

While there may be a few benefits, the associated dangers demand deeper insight into the real utility of the ThreadGroup class. Several of the ThreadGroup APIs (allowThreadSuspension, resume, stop, suspend) have been deprecated and the remainder are seldom used because they offer little desirable functionality. Ironically, a few APIs are not even thread-safe. [[Bloch 01]]

A programmer may sometimes wish to enumerate all the threads in a group as a precursor to other operations. This is accomplished by using the activeCount() method that "Returns an estimate of the number of active threads in this thread group. " [[API 06]]. Notice that there is no absolute word on whether it returns the exact count or not; the definition of active also has a different connotation here. A thread that is constructed and not started is still counted by the activeCount() method as active.

According to the Java API [[API 06]], class ThreadGroup documentation:

[The enumerate method] Copies into the specified array every active thread in this thread group and its subgroups. An application should use the activeCount method to get an estimate of how big the array should be. If the array is too short to hold all the threads, the extra threads are silently ignored.

Threads are removed from the thread array either when they are stopped or when their run() method has finished executing. As a result, if a thread is not started, it continues to reside in the array despite the loss of the original reference. [[JavaThreads 99]]

Using the ThreadGroup APIs to facilitate thread shutdown also has pitfalls. "One way is for the thread initiating the termination to join the other threads and so know when those threads have terminated. However, an application may have to maintain its own list of the threads it creates because simply inspecting the ThreadGroup may return library threads that do not terminate and for which join will not return." [[JPL 06]].

Noncompliant Code Example

This noncompliant code example shows a NetworkHandler class that maintains a controller thread t. This thread is responsible for spawning a new thread every time a new network connection request is received. For the sake of brevity, it is assumed that the controller thread t starts three nonterminating threads in succession and waits for a few seconds. All threads are defined to belong to the same group, Chief.

class NetworkHandler implements Runnable {
  private static ThreadGroup tg = new ThreadGroup("Chief");

  public void run() {
    try {
      new Thread(tg, new HandleRequest(), "t1").start(); // Start t1
      new Thread(tg, new HandleRequest(), "t2").start(); // Start t2	
      new Thread(tg, new HandleRequest(), "t3").start(); // Start t3
      Thread.sleep(2000);
    } catch(InterruptedException e) {
      // Forward to handler
    }
  }
	
  public static void printThreads(Thread t, int point) {
    System.out.println("Active Threads in Thread Group " + t.getThreadGroup().getName() + 
      " at point(" + point + "):" + " " + t.activeCount());
  }

  public static void printEnumeratedThreads(Thread ta[], int len) {
    System.out.println("Enumerating all threads...");
    for(int i = 0; i < len; i++) {
      System.out.println("Thread " + i + " = " + ta[i].getName());
    }   
  }

  public static void main(String[] args) throws InterruptedException {
    Thread t = new Thread(tg, new NetworkHandler(), "t");
    t.start();
	     
    Thread ta[] = new Thread[t.activeCount()];		

    printThreads(t, 1);      // At point 1 
    Thread.sleep(1000);      // Delay to demonstrate TOCTOU condition (race window)   
    printThreads(t, 2);      // At point 2, the thread count changes as new threads are initiated
    int n = t.enumerate(ta); // Incorrectly uses the (now stale) thread count obtained at point 1	
    printEnumeratedThreads(ta, n); // Silently ignores printing newly initiated threads
  }
}

class HandleRequest implements Runnable {
  public void run() {
    System.out.println("Active Threads in Thread Group " +   
      Thread.currentThread().getThreadGroup().getName() + 
      " (Handler thread invoked this): " + " " + Thread.activeCount());	

    while(true) {} // Infinite loop to keep threads active for demonstration	
  }
}

There is a Time of Check-Time of Use (TOCTOU) vulnerability in this implementation because obtaining the count and enumerating the list do not constitute an atomic operation. If new requests come in during the time interval between calling activeCount() and enumerate() in the main() method, the total number of threads in the group will increase but the enumerated list ta will contain only theinitial number, that is two thread references, each corresponding to the thread group main and TG, respectively. Consequently, the program operates on stale data instead of taking into account the newly started threads in the thread group Chief.

Different runs of the program produce different values of the number of threads in the thread group Chief because of the race conditions and demonstrate how a few incoming requests can get completely ignored. Any subsequent use of the ta array may lead to undesirable object retention.

Compliant Solution

To be compliant, avoid using the ThreadGroup class. Before Java 5.0, this class had to be extended as there was no other way to control the UncaughtExceptionHandler. The application provided handler UncaughtExceptionHandler comes into the picture when a thread exits because of an uncaught exception. In recent versions, the UncaughtExceptionHandler is maintained on a per-thread basis using an interface enclosed by the Thread class, leaving little to no functionality for the ThreadGroup class. [[Goetz 06]]

Risk Assessment

Using the ThreadGroup APIs may result in race conditions, memory leaks and inconsistent object state.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[API 06]] Methods activeCount and enumerate, Classes ThreadGroup and Thread
[[JavaThreads 04]] 13.1 ThreadGroups
[[Bloch 01]] Item 53: Avoid thread groups
[[Goetz 06]] 7.3.1. Uncaught Exception Handlers
[[JPL 06]] 23.3.3. Shutdown Strategies
[[SDN 06]] Bug ID: 4089701 and 4229558

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