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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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Beyond the Sandbox: Signed Code and Java 2
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CHAPTER SECTIONS: 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8
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Section 6 -- Access Control and Stack Inspection
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The idea of access control is not a new one in computer security. For decades, researchers have built on the fundamental concept of grouping and permissions. The idea is to define a logical system in which entities known as principals (often corresponding one to one with code owned by users or groups of users) are authorized to access a number of particular protected objects (often system resources such as files). To make this less esoteric, consider that the familiar JDK 1.0.2 Java sandbox is a primitive kind of access control. In the default case, applets (which serve as principals in our example) are allowed to access all objects inside the sandbox, but none outside the sandbox. So what we're talking about here is a way of setting up logical groupings. Then we can start talking about separating groups from each other and granting groups particular permissions. Security is all about separation. Readers familiar with the Unix or NT file system will see clear similarities to the notion of user IDs and file permissions. Sometimes a Java application (like, say, a Web browser) needs to run untrusted code within itself. In this case, Java system libraries need some way of distinguishing between calls originating in untrusted code and calls originating from the trusted application itself. Clearly, the calls originating in untrusted code need to be restricted to prevent hostile activities. By contrast, calls originating in the application itself should be allowed to proceed (as long as they follow any security rules that the operating system mandates). The question is, how can we implement a system that does this Java implements such a system by allowing security-checking code to examine the runtime stack for frames executing untrusted code. Each thread of execution has its own runtime stack (see Figure 3.5). Security decisions can be made with reference to this check. This is called stack inspection [Wallach, et al., 1997]. All the major vendors have adopted stack inspection to meet the demand for more flexible security policies than those originally allowed under the old sandbox model. Stack inspection is used by
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Netscape Navigator 4.0, Microsoft Internet Explorer 4.0, and Sun Microsystems' Java 2. (Interestingly, Java is thus the most widespread use of stack inspection for security ever. You can think of it as a very big security-critical experiment.)
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Figure 3.5 Each Java program thread includes a runtime stack that tracks method calls. The purpose of the stack is to keep track of which method calls which other method in order to be able to return to the appropriate program location when an invoked method has finished its work. The stack grows and shrinks during typical program operation. Java 2 inspects the stack in order to make access control decisions. In this example, each stack frame includes both a method call and a trust label (T for trusted, U for untrusted).
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Simple Stack Inspection
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Netscape 3.0's stack-inspection-based model (and every other black-and-white security model) is a simple access control system with two principals: system and untrusted. Just to keep things simple, the only privilege available is full. In this model, every stack frame is labeled with a principal (system if the frame is executing code that is part of the VM or the built-in libraries and untrusted otherwise). Each stack frame also includes a flag that specifies whether privilege is full. A system class can set this flag, thus enabling its privilege. This need only be done when something dangerous must occur-something that not every piece of code should be allowed to do. Untrusted code is not allowed to set the flag. Whenever a stack frame completes its work, its flag (if it has one) disappears. Every method about to do something potentially dangerous is forced to submit to a stack inspection. The
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stack inspection is used to decide whether the dangerous activity should be allowed. The stack inspection algorithm searches the frames on the caller's stack in sequence from the newest to the oldest. If the search encounters an untrusted stack frame (which as we know can never get a privilege flag) the search terminates, access is forbidden, and an exception is thrown. The search also terminates if a system stack frame with a privilege flag is encountered. In this case, access is allowed (see Figure 3.6).
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Figure 3.6 Two examples of simple stack inspection. Each stack is made of frames with three parts: a privilege flag (where full privilege is denoted by an X), a principal entry (untrusted or system), and a method. In STACK A, an untrusted applet is attempting to use the url.open() method to access a file in the browser's cache. The VM makes a decision regarding whether to set the privilege flag (which it does) by looking at the parameters in the actual method invocation. Since the file in this case is a cache file, access is allowed. In short, a system-level method is doing something potentially-dangerous on the behalf of untrusted code. In STACK B, an untrusted applet is also attempting to use the url.open() method, however in this case, the file argument is not a browser cache file but a normal file in the filesystem. Untrusted code is not allowed to do this, so the privilege flag is not set by the VM and access is denied.
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