Buffer Overflow strcpy() - c

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.

Answer 1

*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)

Answer 2

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 ?