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... Preface -- 1 -- 2 -- 3 -- 4 -- 5 -- 6 -- 7 -- 8 -- 9 -- A -- B -- C -- Refs Front -- Contents -- Help
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Copyright 1999 Gary McGraw and Edward Felten. All rights reserved. Published by John Wiley & Sons, Inc.
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The Base Java Security Model: The Original Applet Sandbox
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CHAPTER SECTIONS: 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13
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Section 6 -- The Verifier
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Recall that when a Java program is compiled, it compiles down to platform-independent Java byte code. As Figure 2.3 shows, Java byte code is verified before it can run. This verification scheme is meant to ensure that the byte code, which may or may not have been created by a Java compiler, plays by the rules. After all, byte code could well have been created by a "hostile compiler'' that assembled byte code meant to violate the rules of the Java VM; created directly with an editor like emacs; created almost directly with a Java byte code assembler like Jasmin; or compiled from another source language like C++, Scheme, or Ada into Java byte code. The important thing is not what the source code looked like or even what language it was written in, but what the byte code (the code that actually ends up running) does. In this sense, the Verifier makes mystery code a bit less mysterious.
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Figure 2.3 The Verifier scrutinizes byte code before it is allowed to be run on a local VM. The Verifier plays an essential role in Java's language-based approach to security, which is built on the foundation of type safety.
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Verifying class files containing byte code is one way in which Java automatically checks untrusted code before it is allowed to run. Once Java code has been verified, it can execute in uninterrupted fashion on a VM (with much less need to make security-critical checks while the code runs). This strategy leads to improvements in the efficiency of Java execution, which offset the speed concerns raised by Java's
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security checking. The Verifier is built in to the VM and cannot be accessed by Java programmers or Java users. In most Java implementations, when Java code arrives at the VM and is formed into a Class by the Class Loader, the Verifier automatically examines it. The Verifier checks byte code at a number of different levels. The simplest test makes sure that the format of a code fragment is correct. On a less-basic level, a built-in theorem prover is applied to each code fragment. The theorem prover helps to make sure that byte code does not forge pointers, violate access restrictions, or access objects using incorrect type information. If the Verifier discovers a problem with a class file, it throws an exception, loading ceases, and the class file never executes. The verification process, in concert with the security features built into the language and checked at runtime, helps to establish a base set of security guarantees.3 The Verifier also ensures that class files that refer to each other preserve binary compatibility. Because of Java's ability to dynamically load classes, there is the possibility that a class file being dynamically linked may not be compatible with a referring class. Binary incompatibility problems could occur when a library of Java classes is updated or when a class from a large Java program is not recompiled during development. There are rules of compatibility that govern the ability to change use of classes and methods without breaking binary compatibility [Venners, 1998]. For example, it is okay to add a method to a class that is used by other classes, but not okay to delete methods from a class used by other classes. Compatibility rules are enforced by the Verifier. Binary incompatibility also has security implications. If a malicious programmer can get your VM to accept a set of mutually incompatible classes, the hostile code will probably be able to break out of the sandbox. This problem happened several times with early VM implementations. (See, for example, You're Not My Type, in 5.) Java users and business people investigating the use of Java in their commercial enterprises often complain about the length of time it takes for a Java applet to get started running in a browser. E-commerce system designers paint startup delay as a business-side show stopper, citing the fact that consumers do not react well even to a 20-second delay in their shopping experience. Many people believe falsely that the main delay in starting applets is the time it takes to download the applet itself. But given a reasonably fast connection to the Internet, what takes the longest is not downloading the code, but verifying it. The inherent costs of verification fit the classic tradeoff between functionality and security to a tee. As security researchers, we believe the security that byte code verification provides is well worth the slight delay. We also think it is possible to speed up the verification process so that its execution time is acceptable. In order to work, the Verifier reconstructs type state information by looking through the byte code. The types of all parameters of all byte code instructions must be checked, since the byte code may have come from an untrustworthy source. Because of this possibility, the Verifier provides a first line of defense against external code that may try to break the VM. Only code that passes the Verifier's tests will be run. The process of Verification in Java is defined to allow different implementations of the Java VM a fair amount of flexibility. The VM specification lists what must be checked and what exceptions and errors may result from failing a check, but it does not specify exactly when and how to verify types. Nevertheless, most Java implementations (especially the most widely used commercial VMs) take a
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similar approach to verification. The process is broken into two major steps: internal checks that check everything that can be checked by looking only at the class file itself and runtime checks that confirm the existence and compatibility of symbolically referenced classes, fields, and methods. Through the two kinds of checks, the Verifier assures a number of important properties. Once byte code passes through verification, the following things are guaranteed: The class file has the correct format, including the magic number (0xCAFEBABE) and proper length. (Much of this trivial checking can take place as a class file loads.) Stacks will not be overflowed or underflowed. Overflowing stacks is a common attack on programs written in other languages such as C that has led to several of the most notorious security vulnerabilities. For example, the Internet worm used stack overflow as part of its arsenal [Spafford, 1989]. Java assigns each thread two stacks: a data stack and an operand stack. The data stack is the kind of stack C programmers are all too familiar with. It includes a series of frames that hold local variables and provide some storage needed for method invocation. The Verifier cannot prevent overflow of the data stack, and a denial of service attack that takes advantage of this fact can be trivially implemented with a recursive function. (A demonstration applet that carries out this attack can be found at www.cs.nps.navy.mil/research/languages/DynApplet.html.) The operand stack (which is itself allocated on the data stack) holds the values that each method invocation in Java byte code operates on. This is the stack that the Verifier tracks and evaluates. Byte code instructions all have parameters of the correct type. For example, integers are always used as integers and nothing else. Method descriptors, which include both a return type and the number and types of parameters, are checked with a context-free grammar [Venners, 1998]. No illegal data conversions (casts) occur. For example, treating an integer as a pointer is not allowed. Correctly handling all potential casting combinations is a tricky undertaking. The rules are complex, making a perfect implementation nontrivial. Although many checks are done by the Verifier, some are deferred until runtime. Private, public, protected, and default accesses are legal. In other words, no improper access to restricted classes, interfaces, variables, and methods will be allowed. All register accesses and stores are valid. Computer scientists refer to the meaning of a language as its semantics, and the structure of a language is its syntax. When you are thinking about what a language can do, it is useful to talk about semantics. The semantics of the Java language provide much of Java's built-in security. It is critical that the semantics of Java be enforced in each and every Java program. Byte code is designed to contain enough symbolic information that safety verification (double-checking the
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compiler-enforced safety rules) can occur. Byte code specifies the methods of a class as a set of Java Virtual Machine instructions. These instructions must pass a battery of tests before they can be run.
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