SEC08-J. Define wrappers around native methods

Native methods are defined in Java and written in traditional languages such as C and C++ [[JNI 2006]]. The added extensibility comes at the cost of flexibility and portability as the code no longer conforms to the policies enforced by Java. In the past, native methods were used for performing platform specific operations, interfacing with legacy library code and improving program performance [[Bloch 2008]]. Although this is no longer completely true — because of poor portability, security, and (ironically) performance issues — native code is still used to interface with legacy code.

Defining a wrapper method facilitates installing appropriate security manager checks, performing input validation before passing the arguments to the native code or when obtaining return values, defensively copying mutable inputs, and sanitizing untrusted data. As a result every native method must be private, and must be invoked only by a wrapper method.

Noncompliant Code Example

In this noncompliant code example, the nativeOperation is both native and public; for that reason, untrusted callers may invoke it. Native method invocations bypass security manager checks.

This example includes the doOperation() wrapper method which invokes the nativeOperation() native method but fails to provide input validation or security checks.

public final class NativeMethod {

  // public native method
  public native void nativeOperation(byte[] data, int offset, int len);

  // wrapper method that lacks security checks and input validation
  public void doOperation(byte[] data, int offset, int len) {
    nativeOperation(data, offset, len);
  }
  
  static {
    // load native library in static initializer of class
    System.loadLibrary("NativeMethodLib"); 
  }
}

Compliant Solution

This compliant solution declares the native method private. The doOperation() wrapper method performs routine permission checking to determine whether the succeeding operations are permitted to continue. The method also creates a defensive copy of the mutable input array data as well as performs range checking of the arguments. The nativeOperation() method is consequently called with safe inputs. Note that the validation checks must produce outputs that conform to the input requirements of the native methods.

public final class NativeMethodWrapper {

  // private native method
  private native void nativeOperation(byte[] data, int offset, int len);

  // wrapper method performs SecurityManager and input validation checks
  public void doOperation(byte[] data, int offset, int len) {
    // permission needed to invoke native method
    securityManagerCheck();

    if (data == null) {
      throw new NullPointerException();
    }

    // copy mutable input
    data = data.clone();

    // validate input
    if ((offset < 0) || (len < 0) || (offset > (data.length - len))) {
      throw new IllegalArgumentException();
    }

    nativeOperation(data, offset, len);
  }

  static {
    // load native library in static initializer of class
    System.loadLibrary("NativeMethodLib"); 
  }
}

Exceptions

SEC08-EX00: Native methods that do not require security manager checks, validation of arguments or return values, defensively copying of mutable inputs do not need to be wrapped — for example, the standard C function int rand(void).

Risk Assessment

Failure to define wrappers around native methods can allow unprivileged callers to invoke them and consequently exploit inherent vulnerabilities such as those resulting from invalid inputs.

Automated Detection

Automated detection is not feasible in the fully general case. However, an approach similar to Design Fragments [[Fairbanks 07]] could assist both programmers and static analysis tools.

Related Guidelines

Bibliography

SEC07-J. Call the superclass's getPermissions method when writing a custom class loader      14. Platform Security (SEC)