# House of Roman

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## Basic Information

This was a very interesting technique that allowed for RCE without leaks via fake fastbins, the unsorted\_bin attack and relative overwrites. However it has ben [**patched**](https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=b90ddd08f6dd688e651df9ee89ca3a69ff88cd0c).

### Code

* You can find an example in [https://github.com/shellphish/how2heap/blob/master/glibc\_2.23/house\_of\_roman.c](https://github.com/shellphish/how2heap/blob/master/glibc_2.23/house_of_roman.c)

### Goal

* RCE by abusing relative pointers

### Requirements

* Edit fastbin and unsorted bin pointers
* 12 bits of randomness must be brute forced (0.02% chance) of working

## Attack Steps

### Part 1: Fastbin Chunk points to \_\_malloc\_hook

Create several chunks:

* `fastbin_victim` (0x60, offset 0): UAF chunk later to edit the heap pointer later to point to the LibC value.
* `chunk2` (0x80, offset 0x70): For good alignment
* `main_arena_use` (0x80, offset 0x100)
* `relative_offset_heap` (0x60, offset 0x190): relative offset on the 'main\_arena\_use' chunk

Then `free(main_arena_use)` which will place this chunk in the unsorted list and will get a pointer to `main_arena + 0x68` in both the `fd` and `bk` pointers.

Now it's allocated a new chunk `fake_libc_chunk(0x60)` because it'll contain the pointers to `main_arena + 0x68` in `fd` and `bk`.

Then `relative_offset_heap` and `fastbin_victim` are freed.

```c
/*
Current heap layout: 
	0x0:   fastbin_victim       - size 0x70 
	0x70:  alignment_filler     - size 0x90
	0x100: fake_libc_chunk      - size 0x70 (contains a fd ptr to main_arena + 0x68)
	0x170: leftover_main        - size 0x20
	0x190: relative_offset_heap - size 0x70 

	bin layout: 
			fastbin:  fastbin_victim -> relative_offset_heap
			unsorted: leftover_main
*/
```

* `fastbin_victim` has a `fd` pointing to `relative_offset_heap`
* `relative_offset_heap` is an offset of distance from `fake_libc_chunk`, which contains a pointer to `main_arena + 0x68`
* Just changing the last byte of `fastbin_victim.fd` it's possible to make `fastbin_victim points` to `main_arena + 0x68`

For the previous actions, the attacker needs to be capable of modifying the fd pointer of `fastbin_victim`.

Then, `main_arena + 0x68` is not that interesting, so lets modify it so the pointer points to **`__malloc_hook`**.

Note that `__memalign_hook` usually starts with `0x7f` and zeros before it, then it's possible to fake it as a value in the `0x70` fast bin. Because the last 4 bits of the address are **random** there are `2^4=16` possibilities for the value to end pointing where are interested. So a BF attack is performed here so the chunk ends like: **`0x70: fastbin_victim -> fake_libc_chunk -> (__malloc_hook - 0x23)`.**

(For more info about the rest of the bytes check the explanation in the [how2heap](https://github.com/shellphish/how2heap/blob/master/glibc_2.23/house_of_roman.c)[ example](https://github.com/shellphish/how2heap/blob/master/glibc_2.23/house_of_roman.c)). If the BF don't work the program just crashes (so start gain until it works).

Then, 2 mallocs are performed to remove the 2 initial fast bin chunks and the a third one is alloced to get a chunk in the **`__malloc_hook:`**

```c
malloc(0x60);
malloc(0x60);
uint8_t* malloc_hook_chunk = malloc(0x60);
```

### Part 2: Unsorted\_bin attack

For more info you can check:

{% content-ref url="unsorted-bin-attack" %}
[unsorted-bin-attack](unsorted-bin-attack)
{% endcontent-ref %}

But basically it allows to write `main_arena + 0x68` to any location by specified in `chunk->bk`. And for the attack we choose `__malloc_hook`. Then, after overwriting it we will use a relative overwrite) to point to a `one_gadget`.

For this we start getting a chunk and putting it into the **unsorted bin**:

```c
uint8_t* unsorted_bin_ptr = malloc(0x80);	
malloc(0x30); // Don't want to consolidate

puts("Put chunk into unsorted_bin\n");
// Free the chunk to create the UAF
free(unsorted_bin_ptr);
```

Use an UAF in this chunk to point `unsorted_bin_ptr->bk` to the address of `__malloc_hook` (we brute forced this previously).

{% hint style="danger" %}
Note that this attack corrupts the unsorted bin (hence small and large too). So we can only **use allocations from the fast bin now** (a more complex program might do other allocations and crash), and to trigger this we must **alloc the same size or the program will crash.**
{% endhint %}

So, to trigger the write of `main_arena + 0x68` in `__malloc_hook` we perform after setting `__malloc_hook` in `unsorted_bin_ptr->bk` we just need to do: **`malloc(0x80)`**

### Step 3: Set \_\_malloc\_hook to system

In the step one we ended controlling a chunk containing `__malloc_hook` (in the variable `malloc_hook_chunk`) and in the second step we managed to write `main_arena + 0x68` in here.

Now, we abuse a partial overwrite in `malloc_hook_chunk` to use the libc address we wrote there(`main_arena + 0x68`) to **point a `one_gadget` address**.

Here is where it's needed to **bruteforce 12 bits of randomness** (more info in the [how2heap](https://github.com/shellphish/how2heap/blob/master/glibc_2.23/house_of_roman.c)[ example](https://github.com/shellphish/how2heap/blob/master/glibc_2.23/house_of_roman.c)).

Finally, one the correct address is overwritten, **call `malloc` and trigger the `one_gadget`**.

## References

* [https://github.com/shellphish/how2heap](https://github.com/shellphish/how2heap)
* [https://github.com/shellphish/how2heap/blob/master/glibc\_2.23/house\_of\_roman.c](https://github.com/shellphish/how2heap/blob/master/glibc_2.23/house_of_roman.c)
* [https://ctf-wiki.mahaloz.re/pwn/linux/glibc-heap/house\_of\_roman/](https://ctf-wiki.mahaloz.re/pwn/linux/glibc-heap/house_of_roman/)

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{% endhint %}
