Security and the class verifier

A look at the role played by the class verifier in the JVM's overall security model

This month's article continues the discussion of Java's security model begun in August's "Under the Hood." In that article, I gave a general overview of the security mechanisms built into the Java virtual machine (JVM). I also looked closely at one aspect of those security mechanisms: the JVM's built-in safety features. In September's "Under the Hood," I examined the class loader architecture, another aspect of the JVM's built-in security mechanisms. This month I'll focus on the third prong of the JVM's security strategy: the class verifier.

The class-file verifier

Every Java virtual machine has a class-file verifier, which ensures that loaded class files have a proper internal structure. If the class-file verifier discovers a problem with a class file, it throws an exception. Because a class file is just a sequence of binary data, a virtual machine can't know whether a particular class file was generated by a well-meaning Java compiler or by shady crackers bent on compromising the integrity of the virtual machine. As a consequence, all JVM implementations have a class-file verifier that can be invoked on untrusted classes, to make sure the classes are safe to use.

One of the security goals that the class-file verifier helps achieve is program robustness. If a buggy compiler or savvy cracker generated a class file that contained a method whose bytecodes included an instruction to jump beyond the end of the method, that method could, if it were invoked, cause the virtual machine to crash. Thus, for the sake of robustness, it is important that the virtual machine verify the integrity of the bytecodes it imports.

Although Java virtual machine designers are allowed to decide when their virtual machines will perform these checks, many implementations will do most checking just after a class is loaded. Such a virtual machine analyzes bytecodes (and verifies their integrity) once, before they are ever executed. As part of its verification of bytecodes, the Java virtual machine makes sure all jump instructions -- for example, goto (jump always), ifeq (jump if top of stack zero), etc. -- cause a jump to another valid instruction in the bytecode stream of the method. As a consequence, the virtual machine need not check for a valid target every time it encounters a jump instruction as it executes bytecodes. In most cases, checking all bytecodes once before they are executed is a more efficient way to guarantee robustness than checking each bytecode instruction every time it is executed.

A class-file verifier that performs its checking as early as possible most likely operates in two distinct phases. During phase one, which takes place just after a class is loaded, the class-file verifier checks the internal structure of the class file, including verifying the integrity of the bytecodes it contains. During phase two, which takes place as bytecodes are executed, the class-file verifier confirms the existence of symbolically referenced classes, fields, and methods.

Phase one: Internal checks

During phase one, the class-file verifier checks everything that's possible to check in a class file by looking at only the class file itself (without examining any other classes or interfaces). Phase one of the class-file verifier makes sure the imported class file is properly formed, internally consistent, adheres to the constraints of the Java programming language, and contains bytecodes that will be safe for the Java virtual machine to execute. If the class-file verifier finds that any of these are not true, it throws an error, and the class file is never used by the program.

Checking format and internal consistency

Besides verifying the integrity of the bytecodes, the verifier performs many checks for proper class file format and internal consistency during phase one. For example, every class file must start with the same four bytes, the magic number: 0xCAFEBABE. The purpose of magic numbers is to make it easy for file parsers to recognize a certain type of file. Thus, the first thing a class-file verifier likely checks is that the imported file does indeed begin with 0xCAFEBABE.

The class-file verifier also checks to make sure the class file is neither truncated nor enhanced with extra trailing bytes. Although different class files can be different lengths, each individual component contained inside a class file indicates its length as well as its type. The verifier can use the component types and lengths to determine the correct total length for each individual class file. In this way, it can verify that the imported file has a length consistent with its internal contents.

The verifier also looks at individual components to make sure they are well-formed instances of their type of component. For example, a method descriptor (the method's return type and the number and types of its parameters) is stored in the class file as a string that must adhere to a certain context-free grammar. One of the checks the verifier performs on individual components is to make sure each method descriptor is a well-formed string of the appropriate grammar.

In addition, the class-file verifier checks that the class itself adheres to certain constraints placed on it by the specification of the Java programming language. For example, the verifier enforces the rule that all classes, except class Object, must have a superclass. Thus, the class-file verifier checks at runtime some of the Java language rules that should have been enforced at compile-time. Because the verifier has no way of knowing if the class file was generated by a benevolent, bug-free compiler, it checks each class file to make sure the rules are followed.

The bytecode verifier

Once the class-file verifier has successfully completed the checks for proper format and internal consistency, it turns its attention to the bytecodes. During this part of phase one, which is commonly called the "bytecode verifier," the Java virtual machine performs a data-flow analysis on the streams of bytecodes that represent the methods of the class. To understand the bytecode verifier, you need to understand a bit about bytecodes and frames.

The bytecode streams that represent Java methods are a series of one-byte instructions, called opcodes, each of which may be followed by one or more operands. The operands supply extra data needed by the Java virtual machine to execute the opcode instruction. The activity of executing bytecodes, one opcode after another, constitutes a thread of execution inside the Java virtual machine. Each thread is awarded its own Java stack, which is made up of discrete frames. Each method invocation gets its own frame, a section of memory where it stores, among other things, local variables and intermediate results of computation. The part of the frame in which a method stores intermediate results is called the method's operand stack. An opcode and its (optional) operands may refer to the data stored on the operand stack or in the local variables of the method's frame. Thus, the virtual machine may use data on the operand stack, in the local variables, or both, in addition to any data stored as operands following an opcode when it executes the opcode.

The bytecode verifier does a great deal of checking. It checks to make sure that no matter what path of execution is taken to get to a certain opcode in the bytecode stream, the operand stack always contains the same number and types of items. It checks to make sure no local variable is accessed before it is known to contain a proper value. It checks that fields of the class are always assigned values of the proper type, and that methods of the class are always invoked with the correct number and types of arguments. The bytecode verifier also checks to make sure that each opcode is valid, that each opcode has valid operands, and that for each opcode, values of the proper type are in the local variables and on the operand stack. These are just a few of the many checks performed by the bytecode verifier, which is able, through all its checking, to verify that a stream of bytecodes is safe for the JVM to execute.

Phase two: Verification of symbolic references

Although phase one of verification likely takes place soon after the JVM loads a class file, phase two is delayed until the bytecodes contained in the class file actually are executed. Phase two verifies symbolic references. A symbolic reference is a character string that gives the name and possibly other information about the referenced item -- enough information to uniquely identify a class, field, or method. Thus, symbolic references to other classes give the full name of the class. Symbolic references to the fields of other classes give the class name, field name, and field descriptor. Symbolic references to the methods of other classes give the class name, method name, and method descriptor.

During phase two, the JVM follows the references from the class file being verified to the referenced class files, to make sure the references are correct. Because phase two has to look at other classes external to the class file being checked, phase two may require that new classes be loaded. Most JVM implementations will likely delay loading classes until they actually are used by the program.

If an implementation does load classes earlier -- perhaps in an attempt to speed up the loading process -- then it must still give the impression that it is loading classes as late as possible. If, for example, a Java virtual machine discovers during early loading that it can't find a certain referenced class, it doesn't throw a "class definition not found" error until (and unless) the referenced class is used for the first time by the running program.

Phase two and dynamic linking

Phase two of class-file verification is really just part of the process of dynamic linking. When a class file is loaded, it contains symbolic references to other classes and their fields and methods. Dynamic linking is the process of resolving symbolic references into direct references. As the JVM executes bytecodes and encounters an opcode that, for the first time, uses a symbolic reference to another class, the virtual machine must resolve the symbolic reference. The virtual machine performs two basic tasks during resolution:

  • It finds the class being referenced (loading it if necessary).
  • It replaces the symbolic reference with a direct reference, such as a pointer or offset, to the class, field, or method.

The virtual machine remembers the direct reference so that if it encounters the same reference again later, it can immediately use the direct reference without needing to spend time resolving the symbolic reference again.

When the Java virtual machine resolves a symbolic reference, phase two of the class-file verifier makes sure the reference is valid. If the reference is not valid -- for instance, if the class cannot be loaded or if the class exists but doesn't contain the referenced field or method -- the class-file verifier throws an error.

As an example, consider a class named Volcano. If a method of class Volcano invokes a method in a class named Lava, the name and descriptor of the method in Lava are included as part of the binary data in the class file for Volcano. So, during the course of execution when Volcano's method first invokes Lava's method, the JVM makes sure a method exists in class Lava that has a name and descriptor that matches those expected by class Volcano. If the symbolic reference (class name, method name, and descriptor) is correct, the virtual machine replaces it with a direct reference, such as a pointer, which it will use from then on. But if the symbolic reference from class Volcano doesn't match any method in class Lava, phase two verification fails, and the JVM throws a "no such method" error.

Binary compatibility

The reason phase two of the class-file verifier must look at classes that refer to one another to make sure they are compatible is because Java programs are dynamically linked. Java compilers often will recompile classes that depend on a class you have changed, and in so doing, detect any incompatible changes at compile-time. But there may be times when your compiler doesn't recompile a dependent class. For example, if you are developing a large system, you will likely partition the various parts of the system into packages. If you compile each package separately, then a change to one class in a package would likely cause a recompilation of affected classes within that same package but not necessarily in any other package. Moreover, if you are using someone else's packages, especially if your program downloads class files from someone else's package across a network as it runs, it may be impossible for you to check for compatibility at compile-time. That's why phase two of the class-file verifier must check for compatibility at runtime.

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