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Declaring a variable volatile guarantees the happens-before relationship so that writes are always visible to subsequent reads from any thread. It also ensures sequential consistency, in that, volatile read and write operations cannot be reordered with respect to each other and in addition, as required by the modern JMM, volatile read and write operations are also not reordered with respect to operations on non-volatile variables.

Noncompliant Code Example

This noncompliant code example uses a shutdown() method to set a non-volatile done flag that is checked in the run() method. If some thread invokes the shutdown() method to set the flag, it is possible that another thread might not observe this change. Consequently, it may be forced to sleep even though the condition variable (or flag) disallows this.

final class ControlledStop implements Runnable {
  private boolean done = false;
 
  public void run() {
    while(!done) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        // handle
      } 
    } 	 
  }

  protected void shutdown(){
    done = true;
  }
}

Compliant Solution

This compliant solution declares the flag volatile so that updates by one thread are immediately visible to another thread.

final class ControlledStop implements Runnable {
  private volatile boolean done = false;
  // ...
}

Noncompliant Code Example

This noncompliant code example declares a non-volatile int variable that is initialized in the constructor depending on a security check. In a multi-threading scenario, it is possible that the statements will be reordered so that the boolean flag initialized is set to true before the initialization has concluded. If it is possible to obtain a partially initialized instance of the class in a subclass using a finalizer attack (OBJ04-J. Do not allow partially initialized objects to be accessed), a race condition can be exploited by invoking the getBalance() method to obtain the balance even though initialization is still underway.

class BankOperation {
  private int balance = 0;
  private boolean initialized = false;
 
  public BankOperation() {
    if (!performAccountVerification()) {
      throw new SecurityException("Invalid Account"); 
    }
    balance = 1000;   
    initialized = true; 
  }
  
  private int getBalance() {
    if(initialized == true)
      return balance;
    else
      return -1;
  }
}

Compliant Solution

This compliant solution declares the initialized flag as volatile to ensure that the initialization statements are not reordered.

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The use of the volatile keyword is inappropriate for composite operations on shared variables (CON01-J. Ensure atomicity of composite operations and visibility of results).

Risk Assessment

Failing to use volatile to guarantee visibility of shared values across multiple thread and prevent reordering of statements can result in unpredictable control flow.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[JLS 05|AA. Java References#JLS 05]\] [Chapter 17, Threads and Locks|http://java.sun.com/docs/books/jls/third_edition/html/memory.html], section 17.4.5 Happens-before Order, section 17.4.3 Programs and Program Order, section 17.4.8 Executions and Causality Requirements
\[[Tutorials 08|AA. Java References#Tutorials 08]\] [Java Concurrency Tutorial|http://java.sun.com/docs/books/tutorial/essential/concurrency/index.html]
\[[Lea 00|AA. Java References#Lea 00]\] Sections, 2.2.7 The Java Memory Model, 2.2.5 Deadlock, 2.1.1.1 Objects and locks
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 66: Synchronize access to shared mutable data
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 667|http://cwe.mitre.org/data/definitions/667.html] "Insufficient Locking", [CWE ID 413|http://cwe.mitre.org/data/definitions/413.html] "Insufficient Resource Locking", [CWE ID 366|http://cwe.mitre.org/data/definitions/366.html]  "Race Condition within a Thread", [CWE ID 567|http://cwe.mitre.org/data/definitions/567.html]  "Unsynchronized Access to Shared Data"

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