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3.3 Buffer Overflows

Buffer Overflows have been around since the very beginnings of the Von-Neuman 1 architecture. They first gained widespread notoriety in 1988 with the Morris Internet worm. Unfortunately, the same basic attack remains effective today. Of the 17 CERT security advisories of 1999, 10 of them were directly caused by buffer-overflow software bugs. By far the most common type of buffer overflow attack is based on corrupting the stack.

Most modern computer systems use a stack to pass arguments to procedures and to store local variables. A stack is a last in first out (LIFO) buffer in the high memory area of a process image. When a program invokes a function a new "stack frame" is created. This stack frame consists of the arguments passed to the function as well as a dynamic amount of local variable space. The "stack pointer" is a register that holds the current location of the top of the stack. Since this value is constantly changing as new values are pushed onto the top of the stack, many implementations also provide a "frame pointer" that is located near the beginning of a stack frame so that local variables can more easily be addressed relative to this value. 1 The return address for function calls is also stored on the stack, and this is the cause of stack-overflow exploits since overflowing a local variable in a function can overwrite the return address of that function, potentially allowing a malicious user to execute any code he or she wants.

Although stack-based attacks are by far the most common, it would also be possible to overrun the stack with a heap-based (malloc/free) attack.

The C programming language does not perform automatic bounds checking on arrays or pointers as many other languages do. In addition, the standard C library is filled with a handful of very dangerous functions.

`strcpy`(char dest, const char src)	May overflow the dest buffer
`strcat`(char dest, const char src)	May overflow the dest buffer
`getwd`(char *buf)	May overflow the buf buffer
`gets`(char *s)	May overflow the s buffer
`[vf]scanf`(const char *format, ...)	May overflow its arguments.
`realpath`(char *path, char resolved_path[])	May overflow the path buffer
`[v]sprintf`(char str, const char format, ...)	May overflow the str buffer.

3.3.1 Example Buffer Overflow

The following example code contains a buffer overflow designed to overwrite the return address and skip the instruction immediately following the function call. (Inspired by 4)

#include <stdio.h>

void manipulate(char *buffer) {
  char newbuffer[80];
  strcpy(newbuffer,buffer);
}

int main() {
  char ch,buffer[4096];
  int i=0;

  while ((buffer[i++] = getchar()) != '\n') {};
  
  i=1;
  manipulate(buffer);
  i=2;
  printf("The value of i is : %d\n",i);
  return 0;
}

Let us examine what the memory image of this process would look like if we were to input 160 spaces into our little program before hitting return.

[XXX figure here!]

Obviously more malicious input can be devised to execute actual compiled instructions (such as exec(/bin/sh)).

3.3.2 Avoiding Buffer Overflows

The most straightforward solution to the problem of stack-overflows is to always use length restricted memory and string copy functions. strncpy and strncat are part of the standard C library. These functions accept a length value as a parameter which should be no larger than the size of the destination buffer. These functions will then copy up to `length' bytes from the source to the destination. However there are a number of problems with these functions. Neither function guarantees NUL termination if the size of the input buffer is as large as the destination. The length parameter is also used inconsistently between strncpy and strncat so it is easy for programmers to get confused as to their proper usage. There is also a significant performance loss compared to strcpy when copying a short string into a large buffer since strncpy NUL fills up the size specified.

In OpenBSD, another memory copy implementation has been created to get around these problem. The strlcpy and strlcat functions guarantee that they will always null terminate the destination string when given a non-zero length argument. For more information about these functions see 6. The OpenBSD strlcpy and strlcat instructions have been in FreeBSD since 3.3.

3.3.2.1 Compiler based run-time bounds checking

Unfortunately there is still a very large assortment of code in public use which blindly copies memory around without using any of the bounded copy routines we just discussed. Fortunately, there is another solution. Several compiler add-ons and libraries exist to do Run-time bounds checking in C/C++.

StackGuard is one such add-on that is implemented as a small patch to the gcc code generator. From the StackGuard website:

"StackGuard detects and defeats stack smashing attacks by protecting the return address on the stack from being altered. StackGuard places a "canary" word next to the return address when a function is called. If the canary word has been altered when the function returns, then a stack smashing attack has been attempted, and the program responds by emitting an intruder alert into syslog, and then halts."

"StackGuard is implemented as a small patch to the gcc code generator, specifically the function_prolog() and function_epilog() routines. function_prolog() has been enhanced to lay down canaries on the stack when functions start, and function_epilog() checks canary integrity when the function exits. Any attempt at corrupting the return address is thus detected before the function returns."

Recompiling your application with StackGuard is an effective means of stopping most buffer-overflow attacks, but it can still be compromised.

3.3.2.2 Library based run-time bounds checking

Compiler-based mechanisms are completely useless for binary-only software for which you cannot recompile. For these situations there are a number of libraries which re-implement the unsafe functions of the C-library (strcpy, fscanf, getwd, etc..) and ensure that these functions can never write past the stack pointer.

libsafe
libverify
libparanoia

Unfortunately these library-based defenses have a number of shortcomings. These libraries only protect against a very small set of security related issues and they neglect to fix the actual problem. These defenses may fail if the application was compiled with -fomit-frame-pointer. Also, the LD_PRELOAD and LD_LIBRARY_PATH environment variables can be overwritten/unset by the user.

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