Fun with Assembly 10: Overflowing the stack to mess with system calls

Posted by Hywel Carver on August 25, 2015

This is the tenth in a series. You might want to read the previous post before reading this.

This post is based on the Whitehorse level on Like last time, we’re trying to find an input to open a lock without knowing the correct password, using knowledge of assembly language.

First steps

The login function here is mercifully short. Let’s read through it.

44f4:  3150 f0ff      add   #0xfff0, sp
44f8:  3f40 7044      mov   #0x4470 "Enter the password to continue.", r15
44fc:  b012 9645      call  #0x4596 <puts>
4500:  3f40 9044      mov   #0x4490 "Remember: passwords are between 8 and 16 characters.", r15
4504:  b012 9645      call  #0x4596 <puts>

Increase the size of the stack by 16 bytes, output some strings.

4508:  3e40 3000      mov   #0x30, r14
450c:  0f41           mov   sp, r15
450e:  b012 8645      call  #0x4586 <getsn>

Get input directly onto the stack, up to 48 bytes. That means our input can potentially be written beyond the current stack frame. Useful to know.

4512:  0f41           mov  sp, r15
4514:  b012 4644      call  #0x4446 <conditional_unlock_door>

This passes the input to the function conditional_unlock_door which then passes it to the lock, to check if it’s the right function.

4518:  0f93           tst r15
451a:  0324           jz    #0x4522 <login+0x2e>
451c:  3f40 c544      mov   #0x44c5 "Access granted.", r15
4520:  023c           jmp   #0x4526 <login+0x32>
4522:  3f40 d544      mov   #0x44d5 "That password is not correct.", r15
4526:  b012 9645      call  #0x4596 <puts>
452a:  3150 1000      add   #0x10, sp
452e:  3041           ret

Outputs some text to tell the user whether the door opened or not, then deallocate 16 bytes from the stack and return.

What happens next?

When the function returns, what will happen? Hopefully you’ll remember from before that it will return to the calling function, whose address is stored in the next 2 bytes up along the stack. And those are 2 bytes that we could overwrite with the 17th and 18th bytes of the string we input.

We don’t want to go to conditional_unlock_door - that function just passes the password elsewhere, so it’s hard to exploit. The function it calls to do that is INT - the hardware interrupt. It passes an argument of 0x7e, which tells the lock to open if the password is correct.

However, reading the manual, an argument of 0x7f will simply open the door. So we can force our function to return to 0x4532, so that the processor drops into the INT function. How do we pass in the 0x7f argument?

Reading through the INT function, it looks 2 bytes beyond the stack pointer for its argument. So the input we need, altogether looks like:

<16 bytes of anything><3245 (the address 0x4532 with bytes reversed)><4 bytes of anything><7f>

I used 00000000000000000000000000000000324500007f, and…

The door springs open

This is a more complicated stack overflow than we’ve seen before - we had to drop straight into the interrupt function and also pass it an argument on the stack.

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