little-endian-write

Problem

This problem comes from pwn.college: Assembly Crash Course ⤴.

We will now set some values in memory dynamically before each run. On each run, the values will change. This means you will need to do some type of formulaic operation with registers. We will tell you which registers are set beforehand and where you should put the result. In most cases, it’s rax.

In this level, you will be working with memory. This will require you to read or write to things stored linearly in memory. If you are confused, go look at the linear addressing module in ‘ike. You may also be asked to dereference things, possibly multiple times, to things we dynamically put in memory for your use.

It is worth noting, as you may have noticed, that values are stored in reverse order of how we represent them.

As an example, say:

[0x1330] = 0x00000000deadc0de

If you examined how it actually looked in memory, you would see:

[0x1330] = 0xde
[0x1331] = 0xc0
[0x1332] = 0xad
[0x1333] = 0xde
[0x1334] = 0x00
[0x1335] = 0x00
[0x1336] = 0x00
[0x1337] = 0x00

This format of storing things in ‘reverse’ is intentional in x86, and it’s called “Little Endian”.

For this challenge, we will give you two addresses created dynamically each run.

The first address will be placed in rdi. The second will be placed in rsi.

Using the earlier mentioned info, perform the following:

• Set [rdi] = 0xdeadbeef00001337
• Set [rsi] = 0xc0ffee0000

Hint: it may require some tricks to assign a big constant to a dereferenced register. Try setting a register to the constant value, then assigning that register to the dereferenced register.

Solution

.intel_syntax noprefix
.global _start

_start:
  mov rax, 0xdeadbeef00001337
  mov [rdi], rax
  
  mov rax, 0xc0ffee0000
  mov [rsi], rax

Trying to assign the constants directly to the memory addresses didn’t work. So I instead assigned them to a register, then assigned the register to the memory addresses.