Page History

Reading a shared primitive variable in one thread may not yield the value of the most recent write to the variable from another thread. Consequently, the thread may observe a stale value of the shared variable. To ensure the visibility of the most recent update, either the variable must be declared volatile or the reads and writes must be synchronized.

Declaring a shared variable volatile guarantees visibility in a thread-safe manner only when both of the following conditions are met:

• A write to a variable is independent from its current value.
• A write to a variable is independent from the result of any nonatomic compound operations involving reads and writes of other variables (see VNA02-J. Ensure that compound operations on shared variables are atomic for more information).

The first condition can be relaxed when you can be sure that only one thread will ever update the value of the variable [Goetz 2006]. However, code that relies on a single-thread confinement is error prone and difficult to maintain. This design approach is permitted under this rule but is discouraged.

Synchronizing the code makes it easier to reason about its behavior and is frequently more secure than simply using the volatile keyword. However, synchronization has somewhat higher performance overhead and can result in thread contention and deadlocks when used excessively.

Declaring a variable volatile or correctly synchronizing the code guarantees that 64-bit primitive long and double variables are accessed atomically. For more information on sharing those variables among multiple threads, see VNA05-J. Ensure atomicity when reading and writing 64-bit values.

Noncompliant Code Example (Non-volatile Flag)

This noncompliant code example uses a shutdown() method to set the nonvolatile done flag that is checked in the run() method:

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

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

If one thread invokes the shutdown() method to set the flag, a second thread might not observe that change. Consequently, the second thread might observe that done is still false and incorrectly invoke the sleep() method. Compilers and just-in-time compilers (JITs) are allowed to optimize the code when they determine that the value of done is never modified by the same thread, resulting in an infinite loop.

Compliant Solution (Volatile)

In this compliant solution, the done flag is declared volatile to ensure that writes are visible to other threads:

Memory that can be shared between threads is called _shared memory_ or _heap memory_. The term _variable_ is used in the context of this guideline, to refer to both fields and array elements \[[JLS 05|AA. Java References#JLS 05]\].

All instance fields, static fields, and array elements are stored in heap memory. Local variables, formal method parameters, or exception handler parameters are never shared between threads and are not affected by the memory model.

The Java Language Specification defines the Java Memory Model (JMM) which describes possible behaviors of a multi-threaded Java program. Concurrent executions are typically interleaved but the situation is complicated by statements that may be reordered by the compiler or runtime system. This results in execution orders that are not immediately obvious from an examination of the source-code.

There are two requirements for implementing synchronization correctly:

1. Happens-before consistency: If two accesses follow the happens-before relationship, data races cannot occur. However, this is necessary but not sufficient for acceptable program behavior. In addition, often the particular execution order of a program must be sequential consistent.

Consider the following example in which a and b are (shared) global variables or instance fields but r1 and r2 are local variables not accessible by other threads.

Initially, let a = 0 and b = 0.

Because, in Thread 1, the two assignments a = 10; and r1 = b; are not related, the compiler or runtime system is free to reorder them. Similarly in Thread 2, the statements may be freely reordered. Although it may seem counter-intuitive, the Java memory model allows a read to see a write that occurs later in the execution order.

Two possible execution orders and actual assignments are:

In this ordering, r1 and r2 read the original values of the variables a and b even though they are expected to see the updated values.

In this ordering, r1 and r2 read the values of a and b written from step 3 and 4, before the steps are executed.

Even if statements execute in the expected order, caching can prevent the latest values from being reflected in the main memory. Such counter-intuitive behavior necessitates the sequential consistency property.

2. [Sequential consistency|BB. Definitions#sequential consistency]: "The fact that we allow a read to see a write that comes later in the execution order can sometimes thus result in unacceptable behaviors." \[[JLS 05|AA. Java References#JLS 05]\]. In such cases, sequential consistency is required. This condition ensures that the compiler does not optimize away or reorder any statements. It also ensures that each operation is atomic and immediately visible to other threads. This makes it easy for a programmer to follow the logic, however, introduces a performance penalty. Synchronization guarantees sequential consistency and so does use of the {{volatile}} keyword.

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 volatile boolean done = false;
 
  @Override public void run() {
    while (!done) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        Thread.currentThread().interrupt(); // Reset interrupted handlestatus
      } 
    } 	 
  }

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

Compliant Solution (AtomicBoolean)

This In this compliant solution declares , the flag volatile so that updates by one thread are immediately visible to another thread done flag is declared to be of type java.util.concurrent.atomic.AtomicBoolean. Atomic types also guarantee that writes are visible to other threads.

final class ControlledStop implements Runnable {
  private volatilefinal booleanAtomicBoolean done = new AtomicBoolean(false);
 
  @Override public void run() {
    while (!done.get()) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        Thread.currentThread().interrupt(); // Reset interrupted status
      } 
    } 	 
  }

  public void shutdown() {
    done.set(true);
  }
}

Noncompliant Code Example

Compliant Solution (synchronized)

This compliant solution uses the intrinsic lock of the Class object to ensure that updates are visible to other threads: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.

final class BankOperationControlledStop {
implements  private int balance = 0;Runnable {
  private boolean initializeddone = false;
 
  @Override public void BankOperationrun() {
    ifwhile (!performAccountVerificationisDone()) {
      throwtry {
  new SecurityException("Invalid Account"); 
   // }...
     balance  = 1000 Thread.currentThread().sleep(1000); // Do something
      } initialized = true;catch(InterruptedException ie) { 
  }
  
  private int getBalance() { Thread.currentThread().interrupt(); // Reset interrupted status
    if(initialized == true)  } 
    } 	 
 return balance;}

  public synchronized else
boolean isDone() {
    return -1done;
  }
}

Compliant Solution

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

class BankOperation {
  privatepublic intsynchronized balancevoid = 0;shutdown() {
  private volatile booleandone initialized = falsetrue;
 // Declared volatile
  // ...
}
 }
}

Although this compliant solution is acceptable, intrinsic locks cause threads to block and may introduce contention. On the other hand, volatile-qualified shared variables do not block. Excessive synchronization can also make the program prone to deadlock.

Synchronization is a more secure alternative in situations where the volatile keyword or a java.util.concurrent.atomic.Atomic* field is inappropriate, such as when a variable's new value depends on its current value (see VNA02-J. Ensure that compound operations on shared variables are atomic for more information).

Compliance with LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code can reduce the likelihood of misuse by ensuring that untrusted callers cannot access the lock object.

Exceptions

VNA00-J-EX0: Class objects are created by the virtual machine; their initialization always precedes any subsequent use. Consequently, cross-thread visibility of Class objects is already assured by defaultThe use of the volatile keyword is inappropriate for composite operations on shared variables (CON01-J. Ensure visibility of shared variables and atomicity of composite operations).

Risk Assessment

Failing to use volatile to guarantee ensure the visibility of shared values across multiple thread and prevent reordering of statements can result in unpredictable control flowa shared primitive variable may result in a thread observing a stale value of the variable.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation Some static analysis tools are capable of detecting violations 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"

.

Related Guidelines

Bibliography

...

Image Added Image Added Image Added11. Concurrency (CON)      11. Concurrency (CON)      CON02-J. Always synchronize on the appropriate object