Buffer Overflow strcpy()

Question

I would like to know how many bytes do we have to overflow to run a shellcode ?

int fun (char data[256]){
int i;
char *tmp;

strcpy(tmp,data);

}

It is known that:

If string chain *data is larger than *tmp then there will be overflow.
Otherwise there will be no buffer overflow.

That depends entirely on your compiler, its settings, operating system and probably quite a few other factors. — Eric J., Apr 28 '15 at 20:27
`*tmp` is uninitialised, so copying anything to it is undefined behaviour. — Vicky, Apr 28 '15 at 20:27

score 0 · Answer 1 · answered Apr 29 '15 at 08:03

makes general way for the compiler. This is an exam for a computer master. we must explain the 2 cases :

-when *tmp[200] for example and

-when *tmp[300] ie a case or *tmp larger than *data (no overflows) and *tmp smaller than *data (overflow)

how to know the number of bytes that are overwhelmed that the code executes ?

score 0 · Accepted Answer · answered Mar 17 '18 at 03:05

*tmp isn't initialized, so you're usually going to segmentation fault anyway.

A better example would be to change char *tmp; to something like char tmp[64]; and have stuff (over 64 bytes of stuff in this case) from data be copied over to tmp. To answer your question from thereon out, you'll need to fire up a debugger like gdb after changing the code then see how far you can write out until you overwrite the RIP. On my system that's 78 bytes.

marshall@marshall-debian-testbed:~$ cat bof.c
int fun (char data[256]) {
int i;
char tmp[64];
strcpy(tmp,data);
}

int main (int argc, char *argv[]) {
fun(argv[1]);
return(0);
}
marshall@marshall-debian-testbed:~$ gcc bof.c -o bof
bof.c: In function ‘fun’:
bof.c:4:1: warning: implicit declaration of function ‘strcpy’ [-Wimplicit-function-declaration]
 strcpy(tmp,data);
 ^~~~~~
bof.c:4:1: warning: incompatible implicit declaration of built-in function ‘strcpy’
bof.c:4:1: note: include ‘<string.h>’ or provide a declaration of ‘strcpy’
marshall@marshall-debian-testbed:~$ ./bof AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Segmentation fault
marshall@marshall-debian-testbed:~$ gdb ./bof
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Reading symbols from ./bof...(no debugging symbols found)...done.
(gdb) disas main
Dump of assembler code for function main:
   0x00000000000006d2 <+0>:     push   %rbp
   0x00000000000006d3 <+1>:     mov    %rsp,%rbp
   0x00000000000006d6 <+4>:     sub    $0x10,%rsp
   0x00000000000006da <+8>:     mov    %edi,-0x4(%rbp)
   0x00000000000006dd <+11>:    mov    %rsi,-0x10(%rbp)
   0x00000000000006e1 <+15>:    mov    -0x10(%rbp),%rax
   0x00000000000006e5 <+19>:    add    $0x8,%rax
   0x00000000000006e9 <+23>:    mov    (%rax),%rax
   0x00000000000006ec <+26>:    mov    %rax,%rdi
   0x00000000000006ef <+29>:    callq  0x6b0 <fun>
   0x00000000000006f4 <+34>:    mov    $0x0,%eax
   0x00000000000006f9 <+39>:    leaveq
   0x00000000000006fa <+40>:    retq
End of assembler dump.
(gdb) disas fun
Dump of assembler code for function fun:
   0x00000000000006b0 <+0>:     push   %rbp
   0x00000000000006b1 <+1>:     mov    %rsp,%rbp
   0x00000000000006b4 <+4>:     sub    $0x50,%rsp
   0x00000000000006b8 <+8>:     mov    %rdi,-0x48(%rbp)
   0x00000000000006bc <+12>:    mov    -0x48(%rbp),%rdx
   0x00000000000006c0 <+16>:    lea    -0x40(%rbp),%rax
   0x00000000000006c4 <+20>:    mov    %rdx,%rsi
   0x00000000000006c7 <+23>:    mov    %rax,%rdi
   0x00000000000006ca <+26>:    callq  0x560 <strcpy@plt>
   0x00000000000006cf <+31>:    nop
   0x00000000000006d0 <+32>:    leaveq
   0x00000000000006d1 <+33>:    retq
End of assembler dump.
(gdb) r `perl -e 'print "A"x78;'`
Starting program: /home/marshall/bof `perl -e 'print "A"x78;'`

Program received signal SIGSEGV, Segmentation fault.
0x0000414141414141 in ?? ()
(gdb) info registers
rax            0x7fffffffdce0   140737488346336
rbx            0x0      0
rcx            0x4141414141414141       4702111234474983745
rdx            0x414141 4276545
rsi            0x7fffffffe140   140737488347456
rdi            0x7fffffffdd23   140737488346403
rbp            0x4141414141414141       0x4141414141414141
rsp            0x7fffffffdd30   0x7fffffffdd30
r8             0x555555554770   93824992233328
r9             0x7ffff7de99e0   140737351948768
r10            0x5b     91
r11            0x7ffff7b9ab28   140737349528360
r12            0x555555554580   93824992232832
r13            0x7fffffffde20   140737488346656
r14            0x0      0
r15            0x0      0
rip            0x414141414141   0x414141414141
eflags         0x10202  [ IF RF ]
cs             0x33     51
ss             0x2b     43
ds             0x0      0
es             0x0      0
fs             0x0      0
gs             0x0      0
(gdb)

Buffer Overflow strcpy()

2 Answers2