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Assume that 0x<return> points into the function that called trouble() The second stack frame illustrates a scenario in which trouble() behaves normally It reads the input Hello World! and returns You can see that line is now partially lled with the input string and that the other values stored on the stack are unchanged The third stack frame illustrates a scenario in which an attacker exploits the buffer over ow vulnerability in trouble() and causes it to execute malicious code instead of returning normally In this case, line has been lled with a series of NOPs, the exploit code, and the address of the beginning of the buffer, 0xNN
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Before Execution line a return address
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32 0xNN 0
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0x<return>
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Scenario 1: Normal Execution line a return address
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Hello World! 0xNN
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0x<return>
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Scenario 2: Buffer Overflow Attack (Not drawn to scale) return a address
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NOP NOP NOP <exploit> 0xNN 0xNN jump to exploit
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Figure 61 Three different versions of a simpli ed stack frame for trouble(): one before execution, one after normal execution, and one after a buffer over ow exploit
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One of the most common misconceptions about buffer over ow vulnerabilities is that they are exploitable only when the buffer is on the stack Heap-based buffer over ow exploits can overwrite important data values
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stored on the heap, and attackers have also found ways to change the control ow of the program For example, they might overwrite the value of a function pointer so that when the program invokes the function referenced by the function pointer, it will execute malicious code Finally, even if an attacker cannot inject malicious code onto a system, an exploit technique known as arc injection or return-into-libc (because of its dependency on standard library functions) might allow a buffer over ow to alter the control ow of the program Arc injection attacks use a buffer over ow to overwrite either a return address or the value of a function pointer with the address of a function already de ned on the system, which could allow an attacker to set up a call to an arbitrary system library, such as system("/bin/sh") [Pincus and Baker, 2004] Enough is enough Buffer over ow vulnerabilities give an attacker a lot of latitude Writing just a single byte past the end of an array can result in system compromise In the rest of this chapter, we spend very little energy discussing exploitability Instead, we talk about common mistakes and how to identify them, remediate them, and hopefully avoid them altogether Buffer Allocation Strategies Most tactics for preventing buffer over ows focus on how and when to check for a condition that will lead to an over ow Before we discuss these tactics, we consider what should happen when such a check fails At its core, this is a question of memory allocation There are only two alternatives; when an operation requires a larger buffer than is currently allocated, the program can do one of two things: Retain the current size of the buffer and either prevent the operation from executing (perhaps by doing something as extreme as terminating program execution) or carry out only part of the operation (thereby truncating the data) Dynamically resize the buffer so that it can accommodate the results of the operation Big programs invariably use static allocation in some cases and dynamic allocation in others Programmers choose between them based on the task at hand Regardless of the approach being used for a particular piece of code, a systematic and explicit method for memory allocation makes it easier for a human or a tool to inspect the code and quickly verify its safety Be consistent with the solution you choose for speci c types of operations
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Make the accepted mechanism for allocating memory in a given context clear so that programmers, auditors, and tools understand the expected behavior and are better able to identify unintentional deviations Consistency makes errors easier to spot Example 62 shows a buffer over ow found in RSA s reference implementation of the RSA cryptographic algorithm When this bug was found in 1999, it affected a number of security-related programs, including PGP, OpenSSH, and Apache s ModSSL [Solino, 1999] The problem is that the function shown (and others like it) did not check to make sure that the dynamically sized function parameter input was smaller than the statically sized stack buffer pkcsBlock
Example 62 This buffer over ow in RSAREF is caused by mixing static and dynamic approaches to memory allocation
int RSAPublicEncrypt (output, outputLen, input, inputLen, publicKey, randomStruct) unsigned char *output; /* output block */ unsigned int *outputLen; /* length of output block */ unsigned char *input; /* input block */ unsigned int inputLen; /* length of input block */ R_RSA_PUBLIC_KEY *publicKey; /* RSA public key */ R_RANDOM_STRUCT *randomStruct; /* random structure */ { int status; unsigned char byte, pkcsBlock[MAX_RSA_MODULUS_LEN]; unsigned int i, modulusLen; R_memcpy ((POINTER)&pkcsBlock[i], (POINTER)input, inputLen); }
Static Buffer Allocation Under a static buffer allocation scheme, memory for a buffer is allocated once and the buffer retains its initial size for the duration of its existence The biggest advantage of this approach is simplicity Because a buffer remains the same size throughout its lifetime, it is easier for programmers to keep track of the size of the buffer and ensure that operations performed on it are safe Along these same lines, allocating memory for a buffer only once