13 Sep, 2020

My submissions for r2wars 2020

If I learnt one thing, it's that ARM is the future

r2wars is a CoreWar-like game thar runs within the radare2 ESIL virtual machine. In short, you have two programs running in a shared memory space (1kb), with the goal of killing the other and surviving as long as possible. r2wars was conducted as a part of r2con2020.

day 1

My first submission was an incredibly simple “bomber”. All it does is write code to a location, jump there, and continue executing the same thing over and over.

mov eax, 0xfeebfeeb; just some bad jumps
mov ebx, eax
mov ecx, eax
mov edx, eax
mov ebp, eax
mov edi, eax
mov esp, 0x3fc
mov esi, 0x3fd
mov [esi], 0xe6ff60
jmp esi

Specifically, it writes 0xe6ff60, which is

pushal
jmp esi

effectively looping over and over. pushal is a very interesting x86 instruction, that pushes all the registers and decrements the stack pointer esp by how many ever bytes were pushed. Nifty, especially if you’re looking for high throughput (to bomb the address space). Here, it starts bombing from 0x3fc - 0x000 (and below, because there’s no bounds checking in place), and ends up killing itself, since writing outside of the arena (0x000 - 0x400) is illegal.

Ultimately, this bot placed 7th out of 9 contestants—an underwhelming outcome. I had to fix this.

day 1

day 2

I sat for a second and recollected the different reasons for my bot getting killed, and the one that occurred the most was my bot insta-dying to bad instructions being written from 0x400—i.e. from near where I’m positioned. Nearly all competing bots write from bottom up, because pushal decrements the stack pointer. So the obvious solution was to reposition my initial payload way above, at 0x000. And of course, it goes without saying that this assumes everyone’s using pushal (they are).

mov eax, 0xffffffff
mov ecx, eax
mov edx, eax
mov ebx, eax
mov ebp, eax
mov esi, eax

check:
    mov edi, 0x000
    cmp [edi], 0
    jne planb
    mov esp, 0x400
    inc edi
    mov [edi], 0xe7ff6060; pushal, jmp edi
    jmp edi

planb:
    mov edi, 0x3fb
    mov [edi], 0xe7ff6060
    mov esp, 0x3fa
    jmp edi

I also added a (pretty redundant) check to see if the stuff at edi was 0, since the entire arena is initially 0x0. My reasoning, albeit flawed, was that if it wasn’t 0, then it was unsafe to go there. In hindsight, it would’ve been safer, since it’s already been written over by somebody. In any case, planb never got executed because of what I’d mentioned earlier—everyone writes from 0x400. Or anywhere above 0x000, for that matter. So I’m relatively safer than I was in day 1.

These changes paid off, though. I placed 4th on day 2, out of 13 contestants! This screenshot was taken on my phone as I was eating dinner.

day 2

All wasn’t well though—I still lost 4 matches, for the reasons below:

  1. I’d get snuffed out before my bomb wave from 0x400 would reach the opponent.
  2. I’d end up bombing myself without hitting anyone on the way up.

day 3

I needed to add some checks to prevent killing myself in the process of bombing.

mov eax, 0xffffffff
mov ecx, eax
mov edx, eax
mov ebx, eax
mov ebp, eax
mov esi, eax

mov edi, 0x000
mov esp, 0x400
mov [edi], 0x20fc8360
mov [edi+4], 0xff600374
mov [edi+8], 0x0400bce7
mov [edi+12], 0xe7ff0000
jmp edi

If you noticed, the initial payload I’m writing to the address at edi is a bit more complex this time—let’s break it down.

0x20fc8360
0xff600374
0x0400bce7
0xe7ff0000

This translates to:

60                pushal 
83 FC 20          cmp    esp, 0x20
74 03             je     9
60                pushal 
FF E7             jmp    edi
BC 04 00 00 00    mov    esp, 0x400; <- 0x9
FF E7             jmp    edi

I check if the stack pointer is 0x20 (decrements from 0x400 due to pushal); if yes, reset to 0x400, else continue looping. This prevented me from writing myself over, and also resets bombing from 0x400—better chance of hitting someone we missed in our first wave.

Sounds good? That’s what I thought too. Day 3 had a bunch of new bot submissions (and some updated submissions), and a lot of them checked 0x000 for existence of a bot, effectively recking me. I placed 8th out of 14 contestants, with 7 wins and 6 losses. Tough day.

day 3

day 4: the finals

I spent a lot of time refactoring my bot. I tried all kinds of things, even reworked it to be entirely mobile using the pushal + jmp esp trick, but I just wasn’t satisfied. In the end, I decided to address the problem in the simplest way possible. You’re checking 0x000? Okay, I’ll reposition my initial payload to 0xd.

And this surprisingly tiny change landed me in 4th place out of 15 contestants, which was way better than I’d anticipated! The top spots were all claimed by ARM, and naturally so—they had a potential throughput of 64 bytes per cycle thanks to stmia, compared to x86’s 32 bytes. Pretty neat!

day 4

closing thoughts

This was my first ever r2wars, and it was an incredible experience. Who woulda thunk staring at colored boxes on the screen would be so much fun?! So much so that my parents walked over to see what all the fuss was about. Shoutout to Abel and pancake for taking the time out to work on this, and especially Abel for dealing with all the last minute updates and bot submissions!

All things said, mine was still the best x86 bot—so that’s a win. ;)

Questions or comments? Send an email.