Tut09: Exploiting Heap Allocators

Freed heap chunk

Revisiting the struct malloc_chunk allocated by malloc():

Layout of malloc_chunk in heap.

When malloc() is called, ptr pointing at the start of the usable payload section is returned, while the previous bytes store metadata information. When the allocated chunk is freed by calling free(ptr), as we have experienced from the previous steps, the first 16 bytes of the payload section are used as fd and bk.

A more detailed view of a freed chunk:

    chunk-> +-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-+
            |             Size of previous chunk, if unallocated (P clear)  |
    `head:' |             Size of chunk, in bytes                     |A|0|P|
      mem-> +-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-+
            |             Forward pointer to next chunk in list             |
            |             Back pointer to previous chunk in list            |
            |             Unused space (may be 0 bytes long)                .
            .                                                               .
            .                                                               |
nextchunk-> +-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-+
    `foot:' |             Size of chunk, in bytes                           |
            |             Size of next chunk, in bytes                |A|0|0|

[NOTE]: Free chunks are maintained in a circular doubly linked list by struct malloc_state.

Now let's take a look at some interesting heap management mechanisms we can abuse to exploit heap.

Unsafe unlink (< GLIBC 2.26)

The main idea of this technique is to trick free() to unlink the second chunk (p2) from free list so that we can achieve arbitrary write.

Heap unsafe unlink attack.

When free(p1) is called, _int_free(mstate av, mchunkptr p, int have_lock) is actually invoked and frees the first chunk. Several checks are applied during this process, which we will not go into details here, but you will be asked to bypass some of them in the lab challenges ;)

The key step during the free(p1) operation is when the freed chunk is put back to unsorted bin (think of unsorted bin as a cache to speed up allocation and deallocation requests). The chunk will first be merged with neighboring free chunks in memory, called consolidation, then added to the unsorted bin as a larger free chunk for future allocations.

Three important phases:

  1. Consolidate backward

    If previous chunk in memory is not in use (PREV_INUSE (P) == 0), unlink is called on the previous chunk to take it off the free list. The previous chunk's size is then added to the current size, and the current chunk pointer points to the previous chunk.

  2. Consolidate forward (in the figure)

    If next chunk (p2) in memory is not the top chunk and not in use, confirmed by next-to-next chunk’s PREV_INUSE (P) bit is unset (PREV_INUSE (P) == 0), unlink is called on the next chunk (p2) to take it off the free list. To navigate to next-to-next chunk, add both the current chunk's (p1) size and the next chunk's (p2) size to the current chunk pointer.

  3. Finally the consolidated chunk is added to the unsorted bin.

The interesting part comes from the unlink process:

#define unlink(P, BK, FD)
    FD = P->fd;
    BK = P->bk;
    FD->bk = BK;
    BK->fd = FD;

unlink is a macro defined to remove a victim chunk from a bin. Above is a simplified version of unlink. Essentially it is adjusting the fd and bk of neighboring chunks to take the victim chunk (p2) off the free list by P->fd->bk = P->bk and P->bk->fd = P->fd.

If we think carefully, the attacker can craft the fd and bk of the second chunk (p2) and achieve arbitrary write when it's unlinked. Here is how this can be performed.

Let's first break down the above unlink operation from the pure C language's point of view. Assuming 32-bit architecture, we get:

BK = *(P + 12);
FD = *(P + 8);
*(FD + 12) = BK;
*(BK + 8) = FD;

Resulting in:

  1. The memory at FD+12 is overwritten with BK.

  2. The memory at BK+8 is overwritten with FD.

Q: What if we can control BK and FD?

Assume that we can overflow the first chunk (p1) freely into the second chunk (p2). In such case, we are free to put any value to BK and FD of the second chunk (p2).

We can achieve arbitrary writing of malicious_addr to target_addr by simply:

  1. Changing FD of the second chunk (p2) to our target_addr-12

  2. Changing BK of the second chunk (p2) to our malicious_addr

Isn't it just amazing? :)

However, life is not easy. To achieve this, the second chunk (p2) has to be free, confirmed by the third chunk’s PREV_INUSE (P) bit is unset (PREV_INUSE (P) == 0). Recall that during unlink consolidation phase, we navigate to the next chunk by adding the current chunk's size to its chunk pointer. In malloc.c, it is checked in _int_free (mstate av, mchunkptr p, int have_lock):

/* check/set/clear inuse bits in known places */
#define inuse_bit_at_offset(p, s)               \
  (((mchunkptr) (((char *) (p)) + (s)))->size & PREV_INUSE)
static void
_int_free (mstate av, mchunkptr p, int have_lock)
  nextsize = chunksize(nextchunk);
  if (nextchunk != av->top) {
      /* get and clear inuse bit */
      nextinuse = inuse_bit_at_offset(nextchunk, nextsize);

      /* consolidate forward */
      if (!nextinuse) {
        unlink(av, nextchunk, bck, fwd);
        size += nextsize;
      } else
        clear_inuse_bit_at_offset(nextchunk, 0);

[TASK]: Can you trick free() to think the second chunk (p2) is free?

Here is how we can achieve it while overflowing the first chunk (p1):

  1. Set size of nextchunk to -sizeof(void*) (-4 in 32-bit arch). Note that it also achieves PREV_INUSE (P) == 0 in this case.

  2. Set size of previous chunk to 0.

Therefore, inuse_bit_at_offset(p, s) will get the address of the third chunk by adding -4 bytes to the second chunk's (p2) address, which will return the second chunk (p2) itself. As we have crafted that PREV_INUSE (P) == 0, we can successfully bypass if (!nextinuse) and enter unlink!

Off-by-one (< GLIBC 2.26)

Heap off-by-one attack.

Off-by-one means that when data is written to a buffer, the number of bytes written exceeds the size of the buffer by only one byte. The most common case is that one extra NULL byte is written (e.g. recall strcpy from previous labs), which makes PREV_INUSE (P) == 0 so the previous block is considered a fake free chunk. You can now launch unsafe unlink attack introduced in the previous section.

/* extract inuse bit of previous chunk */
#define prev_inuse(p)       ((p)->size & PREV_INUSE)
static void
_int_free (mstate av, mchunkptr p, int have_lock)
  /* consolidate backward */
  if (!prev_inuse(p)) {
    prevsize = p->prev_size;
    size += prevsize;
    p = chunk_at_offset(p, -((long) prevsize));
    unlink(av, p, bck, fwd);

Here we can try to trigger backward consolidation. When free(p2), since the first chunk (p1) is "free" (PREV_INUSE (P) == 0), _int_free() will try to consolidate the first chunk (p1) backward and invoke unlink. We can therefore launch unlink attack by preparing malicious FD and BK in the first chunk (p1).

Double-free (>= glibc 2.26, FLAG HERE!)

Now let's get our hand dirty and get a flag using another heap exploit technique called double-free. Specifically, we are going to talk about double-free in tcache.

A new heap caching mechanism called tcache (thread local caching) was introduced in glibc 2.26 back in 2017. Tcache offers significant performance gains by creating per-thread caches for chunks up to a certain size. Operations on tcache bins require no locking, hence the speed improvements. The malloc algorithms will first look into tcache bins before traversing fast, small, large or unsorted bins, whenever a chunk is allocated or freed.

A singly linked list is used to manage tcache bins as chunks in tcache are never removed from the middle of the list, but follow LIFO (last-in-first-out) order. Two particular structures are introduced: tcache_perthread_struct and tcache_entry.


/* We overlay this structure on the user-data portion of a chunk when
   the chunk is stored in the per-thread cache.  */
typedef struct tcache_entry
  struct tcache_entry *next;
} tcache_entry;

/* There is one of these for each thread, which contains the
   per-thread cache (hence "tcache_perthread_struct").  Keeping
   overall size low is mildly important.  Note that COUNTS and ENTRIES
   are redundant (we could have just counted the linked list each
   time), this is for performance reasons.  */
typedef struct tcache_perthread_struct
  char counts[TCACHE_MAX_BINS];
  tcache_entry *entries[TCACHE_MAX_BINS];
} tcache_perthread_struct;

There are 64 singly-linked bins per thread by default, for chunksizes from 24 to 1032 (12 to 516 on x86) bytes, in 16 (8) byte increments. A single tcache bin contains at most 7 chunks by default.

/* This is another arbitrary limit, which tunables can change.  Each
   tcache bin will hold at most this number of chunks.  */
/* We want 64 entries.  This is an arbitrary limit, which tunables can reduce.  */
# define TCACHE_MAX_BINS                64

Two functions are added to modern libc for tcache management: tcache_put and tcache_get.

/* Caller must ensure that we know tc_idx is valid and there's room
   for more chunks.  */
static __always_inline void
tcache_put (mchunkptr chunk, size_t tc_idx)
  tcache_entry *e = (tcache_entry *) chunk2mem (chunk);
  assert (tc_idx < TCACHE_MAX_BINS);
  e->next = tcache->entries[tc_idx];
  tcache->entries[tc_idx] = e;

/* Caller must ensure that we know tc_idx is valid and there's
   available chunks to remove.  */
static __always_inline void *
tcache_get (size_t tc_idx)
  tcache_entry *e = tcache->entries[tc_idx];
  assert (tc_idx < TCACHE_MAX_BINS);
  assert (tcache->entries[tc_idx] > 0);
  tcache->entries[tc_idx] = e->next;
  return (void *) e;

When a chunk is freed, __int_free() is invoked and based on certain condition, tcache_put() will be called to put the chunk into tcache.

    size_t tc_idx = csize2tidx (size);

    if (tcache
        && tc_idx < mp_.tcache_bins
        && tcache->counts[tc_idx] < mp_.tcache_count)
        tcache_put (p, tc_idx);

And when a chunk is requested, malloc algorithm will first check whether the chunk of the requested size is available in tcache bins. If yes, tcache_get() will be called to retrieve it.

  /* int_free also calls request2size, be careful to not pad twice.  */
  size_t tbytes;
  checked_request2size (bytes, tbytes);
  size_t tc_idx = csize2tidx (tbytes);


  if (tc_idx < mp_.tcache_bins
      /*&& tc_idx < TCACHE_MAX_BINS*/ /* to appease gcc */
      && tcache
      && tcache->entries[tc_idx] != NULL)
      return tcache_get (tc_idx);

Let's take a look at tcache-example binary to get a better sense of how tcache actually works. This binary allocates in total 9 chunks, where 4 are of size 0x30, 2 are of 0x40, 2 are of 0x50 and 1 is 0x430. At the end of the program, all of the first 8 are freed. As we can imagine, the first 7 should be recycled to tcache bins and the 8th one will be recycled to unsorted bin due to it's large size.

Let's confirm it in pwndbg:

pwndbg> b * main + 296
Breakpoint 1 at 0x80485de: file tcache-example.c, line 32.
pwndbg> r
Starting program: /home/lab09/tut09-advheap/tcache-example
Breakpoint * main + 296
pwndbg> bins
0x20 [  3]: 0x804b1c0 —▸ 0x804b190 —▸ 0x804b160 ◂— 0x0
0x28 [  2]: 0x804b230 —▸ 0x804b1f0 ◂— 0x0
0x30 [  2]: 0x804b2c0 —▸ 0x804b270 ◂— 0x0
0x10: 0x0
0x18: 0x0
0x20: 0x0
0x28: 0x0
0x30: 0x0
0x38: 0x0
0x40: 0x0
all: 0x804b308 —▸ 0xf7fb17d8 (main_arena+56) ◂— 0x804b308

Now, let's talk about double-free.

A double-free vulnerability occurs when a variable is free()'d twice. The implications of a double-free are often memory leaks and arbitrary writes, but the possibilities are endless.

In an old libc (before tcache was added), when a chunk is freed, malloc algorithm checks whether that chunk is already at the top of the bin (already freed). If yes, an ERROR "double free or corruption (fa.ttop)" will be thrown, causing SIGABRT. However, such check is not conducted along the code path involving tcache, which makes double-free exploit even easier.

In new libc where tcache is introduced (>= 2.26):

#include <stdlib.h>
#include <stdio.h>

int main()
  char *a = malloc(0x38);
  printf("%p\n", malloc(0x38));
  printf("%p\n", malloc(0x38));

No surprise, we get the same pointer twice from the last two mallocs.

In old libc (< 2.26):

#include <stdlib.h>
#include <stdio.h>

int main()
  char *a = malloc(0x38);
  char *b = malloc(0x38);
  printf("%p\n", malloc(0x38));
  printf("%p\n", malloc(0x38));
  printf("%p\n", malloc(0x38));



The extra effort required to trigger double-free in old libc is exactly due to the integrity check introduced above.

So, what are the interesting things we can do using double-free vulnerability? As you can imagine, using double-free, we can assign the same memory location to two variables. Operations on those two variable can vary, depending on the program logic, yet they are affecting the same memory location! By carefully choosing the two controlling variables, we can achieve unintended behaviors, such as arbitrary write.

Now let's try to exploit the target program to spit out the flag for you! The target program is a simple notebook kept by an Admin. You can add, delete or edit a note. You can also call the Admin for privileged requests. Do you spot the design flaw? It will be a double-free vulnerability, of course ;)

[TASK]: In target, can you call the Admin back and bring you the flag?

Real world heap

Luckily in modern libc heap implementations various security checks are applied to detect and prevent such vulnerabilities. A curated list of applied security checks can be found here.

Our adventure ends here. In fact, a lot of interesting facts about the glibc heap implementation have not been covered but you have already gained enough basic knowledge to move forward. Check the references for further information.

Last but not least, the source of the glibc heap is always your best helper in this lab (it is available online at here). There is no magic or secret behind the heap!