ERR03-J. Restore prior object state on method failure

Objects in general should — and security-critical objects must — be maintained in a consistent state even when exceptional conditions arise. Common techniques for maintaining object consistency include

• Input validation (on method arguments, for example)
• Reordering logic so that code that can result in the exceptional condition executes before the object is modified
• Using rollbacks in the event of failure
• Performing required operations on a temporary copy of the object and committing changes to the original object only after their successful completion
• Avoiding the need to modify the object at all

Noncompliant Code Example

This noncompliant code example shows a Dimensions class that contains three internal attributes, the length, width, and height of a rectangular box. The getVolumePackage() method is designed to return the total volume required to hold the box after accounting for packaging material, which adds 2 units to the dimensions of each side. Nonpositive values of the dimensions of the box (exclusive of packaging material) are rejected during input validation. No dimension can be larger than 10. Also, the weight of the object is passed in as an argument and cannot be more than 20 units.

Consider the case where the weight is more than 20 units. This causes an IllegalArgumentException, which is intercepted by the custom error reporter. While the logic restores the object's original state in the absence of this exception, the rollback code fails to execute in the event of an exception. Consequently, subsequent invocations of getVolumePackage() produce incorrect results.

class Dimensions {
  private int length;
  private int width;
  private int height;
  static public final int PADDING = 2;
  static public final int MAX_DIMENSION = 10;

  public Dimensions(int length, int width, int height) {
    this.length = length;
    this.width = width;
    this.height = height;
  }

  protected int getVolumePackage(int weight) {
    length += PADDING;
    width  += PADDING;
    height += PADDING;
    try {
      if (length <= PADDING || width <= PADDING || height <= PADDING ||
        length > MAX_DIMENSION + PADDING || width > MAX_DIMENSION + PADDING ||
        height > MAX_DIMENSION + PADDING || weight <= 0 || weight > 20) {
        throw new IllegalArgumentException();
      }

      int volume = length * width * height; // 12 * 12 * 12 = 1728
      length -= PADDING; width -= PADDING; height -= PADDING; // Revert
      return volume;
    } catch (Throwable t) {
      MyExceptionReporter mer = new MyExceptionReporter();
      mer.report(t); // Sanitize
      return -1; // Non-positive error code
    }
  }

  public static void main(String[] args) {
    Dimensions d = new Dimensions(10, 10, 10);
    System.out.println(d.getVolumePackage(21)); // Prints -1 (error)
    System.out.println(d.getVolumePackage(19)); // Prints 2744 instead of 1728
  }
}

The catch clause is permitted by exception ERR08-EX0 ERR08-J. Do not catch NullPointerException or any of its ancestors because it serves as a general filter passing exceptions to the MyExceptionReporter class, which is dedicated to safely reporting exceptions as recommended by rule ERR00-J. Do not suppress or ignore checked exceptions. While this code only throws IllegalArgumentException, the catch clause is general enough to handle any exception in case the try block should be modified to throw other exceptions.

Compliant Solution (Rollback)

This compliant solution replaces the catch block in the getVolumePackage() method with code that restores prior object state in the event of an exception.

  // ...

  } catch (Throwable t) {
    MyExceptionReporter mer = new MyExceptionReporter();
    mer.report(t); // Sanitize
    length -= PADDING; width -= PADDING; height -= PADDING; // Revert
    return -1;
  }

Compliant Solution (finally Clause)

This compliant solution uses a finally clause to perform rollback, guaranteeing that rollback occurs whether or not an error occurs.

protected int getVolumePackage(int weight) {
  length += PADDING;
  width  += PADDING;
  height += PADDING;
  try {
    if (length <= PADDING || width <= PADDING || height <= PADDING ||
      length > MAX_DIMENSION + PADDING || 
      width > MAX_DIMENSION + PADDING || 
      height > MAX_DIMENSION + PADDING || 
      weight <= 0 || weight > 20) {
      throw new IllegalArgumentException();
    }

    int volume = length * width * height; // 12 * 12 * 12 = 1728
    return volume;
  } catch (Throwable t) {
    MyExceptionReporter mer = new MyExceptionReporter();
    mer.report(t); // Sanitize
    return -1; // Non-positive error code
  } finally {
    // Revert
    length -= PADDING; width -= PADDING; height -= PADDING; 
  }
}

Compliant Solution (Input Validation)

This compliant solution improves on the previous solution by performing input validation before modifying the state of the object. Note that the try block contains only those statements that could throw the exception; all others have been moved outside the try block.

protected int getVolumePackage(int weight) {
  try {
    if (length <= 0 || width <= 0 || height <= 0 ||
        length > MAX_DIMENSION || width > MAX_DIMENSION || height > MAX_DIMENSION ||
        weight <= 0 || weight > 20) {
      throw new IllegalArgumentException(); // Validate first
    }
  } catch (Throwable t) { MyExceptionReporter mer = new MyExceptionReporter();
    mer.report(t); // Sanitize
    return -1;
  }

  length += PADDING;
  width  += PADDING;
  height += PADDING;

  int volume = length * width * height;
  length -= PADDING; width -= PADDING; height -= PADDING;
  return volume;
}

Compliant Solution (Unmodified Object)

This compliant solution avoids the need to modify the object. The object's state cannot be made inconsistent, and rollback is consequently unnecessary. This approach is preferred to solutions that modify the object but may be infeasible for complex code.

protected int getVolumePackage(int weight) {
  try {
    if (length <= 0 || width <= 0 || height <= 0 ||
        length > MAX_DIMENSION || width > MAX_DIMENSION ||
        height > MAX_DIMENSION || weight <= 0 || weight > 20) {
      throw new IllegalArgumentException(); // Validate first
    }
  } catch (Throwable t) { 
    MyExceptionReporter mer = new MyExceptionReporter();
    mer.report(t); // Sanitize
    return -1;
  }

  int volume = (length + PADDING) * (width + PADDING) *
               (height + PADDING);
  return volume;
}

Risk Assessment

Failure to restore prior object state on method failure can leave the object in an inconsistent state and can violate required state invariants.

Related Vulnerabilities

CVE-2008-0002 describes a vulnerability in several versions of Apache Tomcat. If an exception occurs during parameter processing, the program can be left in the context of the wrong request, which might allow remote attackers to obtain sensitive information. An exception can be triggered by disconnecting from Tomcat during this processing.

Related Guidelines

Bibliography

      06. Exceptional Behavior (ERR)      ERR04-J. Do not exit abruptly from a finally block