1 Once upon a free() Anonymous Phrack 57 Volume 9

2 Outline Introduction Free() Algorithm Free() Source Code (Glibc 2.2.2) Data Structure Exploit Conclusion

3 Introduction Memory allocation Free a empty chunk Synthesize a fake chunk

4 Free() Algorithm free(0) has no effect. If the chunk was allocated via mmap, it is released via munmap(). If a returned chunk borders the current high end of memory, it is consolidated into the top, and if the total unused topmost memory exceeds the trim threshold, malloc_trim is called.

5 Free() Algorithm (Cont.) Other chunks are consolidated as they arrive, and placed in corresponding bins. (This includes the case of consolidating with the current `last_remainder').

6 Free() Source Code (Glibc 2.2.2) 因篇幅過大, 所以在介紹例子時, trace 給 大家看

7 Data Structure unsigned char * mem = malloc (16); chunk = (mem – 8) ;

8 Data Structure ( Cont. ) prev_size If the chunk before the current one is unused (it was free'd), it contains the length of the chunk before the chunk before the current one is used - 'prev_size' is part of the 'data' of it, saving four bytes

9 Data Structure ( Cont. ) size As you call malloc(), four is added to the size you pass to it and afterwards the size is padded up to the next double-word boundary.malloc(6)  malloc(16). malloc(0)  malloc(8). The lowest bit, called PREV_INUSE, indicates whether the previous chunk is used or not. The second least significant bit is set if the memory area is mmap'ed. The third least significant bit is unused.

10 Data Structure ( Cont. ) When free(mem), some check take place and memory is released. If its neighbour blocks are free, too(checked using the PREV_INUSE flag ), they will merge as large as passible.

11 Data Structure ( Cont. ) If a merge is not passible, the next chunk cleared PREV_INUSE bit.

12 Exploit #define unlink(P, BK, FD) { BK = P  bk; FD = P  fd; FD  bk = BK; BK  fd = FD; }

13 Exploit ( Cont. ) 上面的巨集其實就是執行 : *(next  fd + 12 ) = next  bk *(next  bk + 8 ) = next  fd They are not swapped, but the ‘ fd ’ and ‘ bk ’ pointers point to other chunks.

14 Exploit ( Cont. ) Pseudo attack method

15 Exploit ( Cont. ) Prepare to attack the least significant bit of 'size' has to be zero both, 'prev_size' and 'size' should be add- safe to a pointer that is read from. So either use very small values up to a few thousand, or - to avoid NUL bytes - use big values such as 0xfffffffc you have to ensure that at (chunk_boundary + size + 4) the lowest bit is zeroed out (0xfffffffc will work just fine)

16 Exploit ( Cont. ) fd = retloc – 12 bk = retaddr

17 Conclusion MallocGuard Sanity check like StackGuard. Performance For every protection there is an anti- protextion.