Setting Privileges for Extensions

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Trail: The Extension Mechanism
Lesson: Making Extensions Secure

Setting Privileges for Extensions

If a security manager is in force, the following conditions must be met to enable any software, including extension software, to perform security-sensitive operations:

The security-sensitive code in the extension must be wrapped in a PrivilegedAction object.
The security policy implemented by the security manager must grant the appropriate permission to the extension. By default, installed extensions are granted all security permissions as if they were part of the core platform API. The permissions granted by the security policy apply only to code wrapped in the PrivilegedAction instance.

Let's look at each of these conditions in a little more detail, with some examples.

Using the PrivilegedAction Class

Suppose that you want to modify the RectangleArea class in the extension example of the previous lesson to write rectangle areas to a file rather than to stdout. Writing to a file, however, is a security-sensitive operation, so if your software is going to be running under a security manager, you'll need to mark your code as being privileged. There are two steps you need to take to do so:

You need to place code that performs security-sensitive operations within the run method of an object of type java.security.PrivilegedAction.
You must use that PrivilegedAction object as the argument in a call to the doPrivileged method of java.security.AccessController.

If we apply those guidelines to the RectangleArea class, our class definition would look something like this:
import java.io.*;
import java.security.*;

public final class RectangleArea {
    public static void writeArea(final java.awt.Rectangle r) {
        AccessController.doPrivileged(new PrivilegedAction() {
	    public Object run() {
	        try { 
		    int area = r.width * r.height;
		    FileWriter fw = new FileWriter("/tmp/AreaOutput");
		    fw.write("The rectangle's area is " + area);
		    fw.flush();
		    fw.close();
	        } catch(IOException ioe) {
		    System.err.println(ioe);
	        }
	        return null;
	    }
        });
    }
}
The single method in this class, writeArea, computes the area of a rectangle, and writes the area to a file called AreaOutput in the /tmp directory.
The security-sensitive statements dealing with the output file are placed within the run method of a new instance of PrivilegedAction. (Note that run requires that an Object instance be returned. The returned object can be null.) The new PrivilegedAction instance is then passed as an argument in a call to AccessController.doPrivileged.
For more information about using doPrivileged, see New API for Privileged Blocks in the JDK^TM documentation.
Wrapping security-sensitive code in a PrivilegedAction object in this manner is the first requirement for enabling an extension to perform security-sensitive operations. The second requirement is: getting the security manager to grant the privileged code the appropriate permissions.

Specifying Permissions with the Security Policy

The security policy in force at runtime is specified by a policy file. The default policy is set by the file lib/security/java.policy in the JRE software.
The policy file assigns security privileges to software by using grant entries. The policy file can contain any number of grant entries. The default policy file has this grant entry for installed extensions:
grant codeBase "file:${java.home}/lib/ext/" {
    permission java.security.AllPermission;
};
This entry specifies that files at the location file:${java.home}/lib/ext/ are to be granted the permission called java.security.AllPermission. (Note that in the Java 1.2 Java platform, java.home refers to the jre directory.) It's not too hard to guess that java.security.AllPermission grants installed extensions all the security privileges that it's possible to grant.
By default, then, installed extensions have no security restrictions. Extension software can perform security-sensitive operations as if there were no security manager installed, provided that security-sensitive code is contained in an instance of PrivilegedAction passed as an argument in a doPrivileged call.
To limit the privileges granted to extensions, you need to modify the policy file. To deny all privileges to all extensions, you could simply remove the above grant entry.
Not all permissions are as comprehensive as the java.security.AllPermissions granted by default. After deleting the default grant entry, you can enter a new grant entry for one or more of the following limited permissions:

java.awt.AWTPermission
java.io.FilePermission
java.net.NetPermission
java.util.PropertyPermission
java.lang.reflect.ReflectPermission
java.lang.RuntimePermission
java.security.SecurityPermission
java.io.SerializablePermission
java.net.SocketPermission

The Policy Permissions file in the JDK documentation provides details about each of these permissions. Let's look at just one, java.io.FilePermission, as an example.
The only permission that the RectangleArea.writeArea method really needs is the permission to write to a file. Assuming that the RectangleArea class is bundled in the file area.jar, you could grant write privileges by adding this entry to the policy file:
grant codeBase "file:${java.home}/lib/ext/area.jar" {
    permission java.io.FilePermission "/tmp/*", "write";  
};
The codeBase "file:${java.home}/lib/ext/area.jar" part of this entry guarantees that any permissions specified by this entry will apply only to your JAR file. The java.io.FilePermission permits access to files. The first string argument, "/tmp/*", indicates that area.jar is being granted permission to access all files in the /tmp directory. The second argument indicates that the file access being granted is only for writing. (Other choices for the second argument are "read", "delete", and "execute".)

Signing Extensions

You can use the policy file to place additional restrictions on the permissions granted to extensions by requiring them to be signed by a trusted entity. (For a review of signing and verifying JAR files, see the JAR File Format trail in this tutorial.)
To allow signature verification of extensions or other software in conjunction with granting permissions, the policy file must contain a keystore entry. The keystore entry specifies which keystore is to be used in the verification. Keystore entries have the form
keystore "keystore_url";
The URL keystore_url is either an absolute or relative. If it's relative, the URL is relative to the location of the policy file.
To indicate that an extension must be signed in order to be granted security privileges, you use the signedBy field. For example, the following entry indicates that the extension area.jar is to be granted write privileges only if it is signed by the users identified in the keystore by the aliases Robert and Rita:
grant signedBy "Robert,Rita",
    codeBase "file:${java.home}/lib/ext/area.jar" {
        permission java.io.FilePermission "*", "write";  
};
If the codeBase field is omitted, as in the following "grant", the permissions are granted to any software, including installed or download extensions, that are signed by "Robert" or "Rita":
grant signedBy "Robert,Rita" {
    permission java.io.FilePermission "*", "write";  
};
For further details about the policy file format, see section 3.3.1 of the Security Architecture Specification in the JDK documentation.

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