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. 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. |
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)).
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.
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 a way to help prevent such attacks -- run-time bounds checking, which is implemented by several C/C++ compilers.
ProPolice is one such compiler feature, and is integrated into gcc(1) versions 4.1 and later. It replaces and extends the earlier StackGuard gcc(1) extension.
ProPolice helps to protect against stack-based buffer overflows and other attacks by laying pseudo-random numbers in key areas of the stack before calling any function. When a function returns, these “canaries” are checked and if they are found to have been changed the executable is immediately aborted. Thus any attempt to modify the return address or other variable stored on the stack in an attempt to get malicious code to run is unlikely to succeed, as the attacker would have to also manage to leave the pseudo-random canaries untouched.
Recompiling your application with ProPolice is an effective means of stopping most buffer-overflow attacks, but it can still be compromised.
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.