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ROP - Return Oriented Programing

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Basic Information

Return-Oriented Programming (ROP) is an advanced exploitation technique used to circumvent security measures like No-Execute (NX) or Data Execution Prevention (DEP). Instead of injecting and executing shellcode, an attacker leverages pieces of code already present in the binary or in loaded libraries, known as "gadgets". Each gadget typically ends with a ret instruction and performs a small operation, such as moving data between registers or performing arithmetic operations. By chaining these gadgets together, an attacker can construct a payload to perform arbitrary operations, effectively bypassing NX/DEP protections.

How ROP Works

  1. Control Flow Hijacking: First, an attacker needs to hijack the control flow of a program, typically by exploiting a buffer overflow to overwrite a saved return address on the stack.

  2. Gadget Chaining: The attacker then carefully selects and chains gadgets to perform the desired actions. This could involve setting up arguments for a function call, calling the function (e.g., system("/bin/sh")), and handling any necessary cleanup or additional operations.

  3. Payload Execution: When the vulnerable function returns, instead of returning to a legitimate location, it starts executing the chain of gadgets.

Tools

Typically, gadgets can be found using ROPgadget, ropper or directly from pwntools (ROP).

ROP Chain in x86 Example

x86 (32-bit) Calling conventions

  • cdecl: The caller cleans the stack. Function arguments are pushed onto the stack in reverse order (right-to-left). Arguments are pushed onto the stack from right to left.

  • stdcall: Similar to cdecl, but the callee is responsible for cleaning the stack.

Finding Gadgets

First, let's assume we've identified the necessary gadgets within the binary or its loaded libraries. The gadgets we're interested in are:

  • pop eax; ret: This gadget pops the top value of the stack into the EAX register and then returns, allowing us to control EAX.

  • pop ebx; ret: Similar to the above, but for the EBX register, enabling control over EBX.

  • mov [ebx], eax; ret: Moves the value in EAX to the memory location pointed to by EBX and then returns. This is often called a write-what-where gadget.

  • Additionally, we have the address of the system() function available.

ROP Chain

Using pwntools, we prepare the stack for the ROP chain execution as follows aiming to execute system('/bin/sh'), note how the chain starts with:

  1. A ret instruction for alignment purposes (optional)

  2. Address of system function (supposing ASLR disabled and known libc, more info in Ret2lib)

  3. Placeholder for the return address from system()

  4. "/bin/sh" string address (parameter for system function)

from pwn import *

# Assuming we have the binary's ELF and its process
binary = context.binary = ELF('your_binary_here')
p = process(binary.path)

# Find the address of the string "/bin/sh" in the binary
bin_sh_addr = next(binary.search(b'/bin/sh\x00'))

# Address of system() function (hypothetical value)
system_addr = 0xdeadc0de

# A gadget to control the return address, typically found through analysis
ret_gadget = 0xcafebabe  # This could be any gadget that allows us to control the return address

# Construct the ROP chain
rop_chain = [
    ret_gadget,    # This gadget is used to align the stack if necessary, especially to bypass stack alignment issues
    system_addr,   # Address of system(). Execution will continue here after the ret gadget
    0x41414141,    # Placeholder for system()'s return address. This could be the address of exit() or another safe place.
    bin_sh_addr    # Address of "/bin/sh" string goes here, as the argument to system()
]

# Flatten the rop_chain for use
rop_chain = b''.join(p32(addr) for addr in rop_chain)

# Send ROP chain
## offset is the number of bytes required to reach the return address on the stack
payload = fit({offset: rop_chain})
p.sendline(payload)
p.interactive()

ROP Chain in x64 Example

x64 (64-bit) Calling conventions

  • Uses the System V AMD64 ABI calling convention on Unix-like systems, where the first six integer or pointer arguments are passed in the registers RDI, RSI, RDX, RCX, R8, and R9. Additional arguments are passed on the stack. The return value is placed in RAX.

  • Windows x64 calling convention uses RCX, RDX, R8, and R9 for the first four integer or pointer arguments, with additional arguments passed on the stack. The return value is placed in RAX.

  • Registers: 64-bit registers include RAX, RBX, RCX, RDX, RSI, RDI, RBP, RSP, and R8 to R15.

Finding Gadgets

For our purpose, let's focus on gadgets that will allow us to set the RDI register (to pass the "/bin/sh" string as an argument to system()) and then call the system() function. We'll assume we've identified the following gadgets:

  • pop rdi; ret: Pops the top value of the stack into RDI and then returns. Essential for setting our argument for system().

  • ret: A simple return, useful for stack alignment in some scenarios.

And we know the address of the system() function.

ROP Chain

Below is an example using pwntools to set up and execute a ROP chain aiming to execute system('/bin/sh') on x64:

from pwn import *

# Assuming we have the binary's ELF and its process
binary = context.binary = ELF('your_binary_here')
p = process(binary.path)

# Find the address of the string "/bin/sh" in the binary
bin_sh_addr = next(binary.search(b'/bin/sh\x00'))

# Address of system() function (hypothetical value)
system_addr = 0xdeadbeefdeadbeef

# Gadgets (hypothetical values)
pop_rdi_gadget = 0xcafebabecafebabe  # pop rdi; ret
ret_gadget = 0xdeadbeefdeadbead     # ret gadget for alignment, if necessary

# Construct the ROP chain
rop_chain = [
    ret_gadget,        # Alignment gadget, if needed
    pop_rdi_gadget,    # pop rdi; ret
    bin_sh_addr,       # Address of "/bin/sh" string goes here, as the argument to system()
    system_addr        # Address of system(). Execution will continue here.
]

# Flatten the rop_chain for use
rop_chain = b''.join(p64(addr) for addr in rop_chain)

# Send ROP chain
## offset is the number of bytes required to reach the return address on the stack
payload = fit({offset: rop_chain})
p.sendline(payload)
p.interactive()

In this example:

  • We utilize the pop rdi; ret gadget to set RDI to the address of "/bin/sh".

  • We directly jump to system() after setting RDI, with system()'s address in the chain.

  • ret_gadget is used for alignment if the target environment requires it, which is more common in x64 to ensure proper stack alignment before calling functions.

Stack Alignment

The x86-64 ABI ensures that the stack is 16-byte aligned when a call instruction is executed. LIBC, to optimize performance, uses SSE instructions (like movaps) which require this alignment. If the stack isn't aligned properly (meaning RSP isn't a multiple of 16), calls to functions like system will fail in a ROP chain. To fix this, simply add a ret gadget before calling system in your ROP chain.

x86 vs x64 main difference

Since x64 uses registers for the first few arguments, it often requires fewer gadgets than x86 for simple function calls, but finding and chaining the right gadgets can be more complex due to the increased number of registers and the larger address space. The increased number of registers and the larger address space in x64 architecture provide both opportunities and challenges for exploit development, especially in the context of Return-Oriented Programming (ROP).

ROP chain in ARM64 Example

ARM64 Basics & Calling conventions

Check the following page for this information:

pageIntroduction to ARM64v8

Protections Against ROP

  • ASLR & PIE: These protections makes harder the use of ROP as the addresses of the gadgets changes between execution.

  • Stack Canaries: In of a BOF, it's needed to bypass the stores stack canary to overwrite return pointers to abuse a ROP chain

  • Lack of Gadgets: If there aren't enough gadgets it won't be possible to generate a ROP chain.

ROP based techniques

Notice that ROP is just a technique in order to execute arbitrary code. Based in ROP a lot of Ret2XXX techniques were developed:

  • Ret2lib: Use ROP to call arbitrary functions from a loaded library with arbitrary parameters (usually something like system('/bin/sh').

pageRet2lib

  • Ret2Syscall: Use ROP to prepare a call to a syscall, e.g. execve, and make it execute arbitrary commands.

pageRet2syscall

  • EBP2Ret & EBP Chaining: The first will abuse EBP instead of EIP to control the flow and the second is similar to Ret2lib but in this case the flow is controlled mainly with EBP addresses (although t's also needed to control EIP).

pageStack Pivoting - EBP2Ret - EBP chaining

Other Examples & References

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