hacktricks/binary-exploitation/libc-heap/heap-memory-functions/free.md

# free

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## Resumo do Pedido Gratuito <a href="#libc_free" id="libc_free"></a>

(Nenhuma verificação é explicada neste resumo e alguns casos foram omitidos por brevidade)

1. Se o endereço for nulo, não faça nada
2. Se o bloco foi mapeado, desmapeie-o e termine
3. Chame `_int_free`:
1. Se possível, adicione o bloco ao tcache
2. Se possível, adicione o bloco ao fast bin
3. Chame `_int_free_merge_chunk` para consolidar o bloco, se necessário, e adicione-o à lista não ordenada

## \_\_libc\_free <a href="#libc_free" id="libc_free"></a>

`Free` chama `__libc_free`.

* Se o endereço passado for Nulo (0), não faça nada.
* Verifique a tag do ponteiro
* Se o bloco estiver `mapeado`, `desmapeie` e é isso
* Se não, adicione a cor e chame `_int_free` sobre ele

<details>

<summary>Código __lib_free</summary>
```c
void
__libc_free (void *mem)
{
mstate ar_ptr;
mchunkptr p;                          /* chunk corresponding to mem */

if (mem == 0)                              /* free(0) has no effect */
return;

/* Quickly check that the freed pointer matches the tag for the memory.
This gives a useful double-free detection.  */
if (__glibc_unlikely (mtag_enabled))
*(volatile char *)mem;

int err = errno;

p = mem2chunk (mem);

if (chunk_is_mmapped (p))                       /* release mmapped memory. */
{
/* See if the dynamic brk/mmap threshold needs adjusting.
Dumped fake mmapped chunks do not affect the threshold.  */
if (!mp_.no_dyn_threshold
&& chunksize_nomask (p) > mp_.mmap_threshold
&& chunksize_nomask (p) <= DEFAULT_MMAP_THRESHOLD_MAX)
{
mp_.mmap_threshold = chunksize (p);
mp_.trim_threshold = 2 * mp_.mmap_threshold;
LIBC_PROBE (memory_mallopt_free_dyn_thresholds, 2,
mp_.mmap_threshold, mp_.trim_threshold);
}
munmap_chunk (p);
}
else
{
MAYBE_INIT_TCACHE ();

/* Mark the chunk as belonging to the library again.  */
(void)tag_region (chunk2mem (p), memsize (p));

ar_ptr = arena_for_chunk (p);
_int_free (ar_ptr, p, 0);
}

__set_errno (err);
}
libc_hidden_def (__libc_free)
```
</details>

## \_int\_free <a href="#int_free" id="int_free"></a>

### Início do \_int\_free <a href="#int_free" id="int_free"></a>

Começa com algumas verificações para garantir que:

* o **ponteiro** está **alinhado**, ou aciona o erro `free(): invalid pointer`
* o **tamanho** não é menor que o mínimo e que o **tamanho** também está **alinhado** ou aciona o erro: `free(): invalid size`

<details>

<summary>Início do \_int\_free</summary>
```c
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4493C1-L4513C28

#define aligned_OK(m) (((unsigned long) (m) &MALLOC_ALIGN_MASK) == 0)

static void
_int_free (mstate av, mchunkptr p, int have_lock)
{
INTERNAL_SIZE_T size;        /* its size */
mfastbinptr *fb;             /* associated fastbin */

size = chunksize (p);

/* Little security check which won't hurt performance: the
allocator never wraps around at the end of the address space.
Therefore we can exclude some size values which might appear
here by accident or by "design" from some intruder.  */
if (__builtin_expect ((uintptr_t) p > (uintptr_t) -size, 0)
|| __builtin_expect (misaligned_chunk (p), 0))
malloc_printerr ("free(): invalid pointer");
/* We know that each chunk is at least MINSIZE bytes in size or a
multiple of MALLOC_ALIGNMENT.  */
if (__glibc_unlikely (size < MINSIZE || !aligned_OK (size)))
malloc_printerr ("free(): invalid size");

check_inuse_chunk(av, p);
```
</details>

### \_int\_free tcache <a href="#int_free" id="int_free"></a>

Ele primeiro tentará alocar esse chunk no tcache relacionado. No entanto, algumas verificações são realizadas anteriormente. Ele irá percorrer todos os chunks do tcache no mesmo índice do chunk liberado e:

- Se houver mais entradas do que `mp_.tcache_count`: `free(): too many chunks detected in tcache`
- Se a entrada não estiver alinhada: `free(): unaligned chunk detected in tcache 2`
- se o chunk liberado já foi liberado e está presente como chunk no tcache: `free(): double free detected in tcache 2`

Se tudo correr bem, o chunk é adicionado ao tcache e a função retorna.

<details>

<summary>_int_free tcache</summary>
```c
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4515C1-L4554C7
#if USE_TCACHE
{
size_t tc_idx = csize2tidx (size);
if (tcache != NULL && tc_idx < mp_.tcache_bins)
{
/* Check to see if it's already in the tcache.  */
tcache_entry *e = (tcache_entry *) chunk2mem (p);

/* This test succeeds on double free.  However, we don't 100%
trust it (it also matches random payload data at a 1 in
2^<size_t> chance), so verify it's not an unlikely
coincidence before aborting.  */
if (__glibc_unlikely (e->key == tcache_key))
{
tcache_entry *tmp;
size_t cnt = 0;
LIBC_PROBE (memory_tcache_double_free, 2, e, tc_idx);
for (tmp = tcache->entries[tc_idx];
tmp;
tmp = REVEAL_PTR (tmp->next), ++cnt)
{
if (cnt >= mp_.tcache_count)
malloc_printerr ("free(): too many chunks detected in tcache");
if (__glibc_unlikely (!aligned_OK (tmp)))
malloc_printerr ("free(): unaligned chunk detected in tcache 2");
if (tmp == e)
malloc_printerr ("free(): double free detected in tcache 2");
/* If we get here, it was a coincidence.  We've wasted a
few cycles, but don't abort.  */
}
}

if (tcache->counts[tc_idx] < mp_.tcache_count)
{
tcache_put (p, tc_idx);
return;
}
}
}
#endif
```
</details>

### \_int\_free fast bin <a href="#int_free" id="int_free"></a>

Comece verificando se o tamanho é adequado para o fast bin e verifique se é possível colocá-lo perto do top chunk.

Em seguida, adicione o chunk liberado no topo do fast bin enquanto realiza algumas verificações:

- Se o tamanho do chunk for inválido (muito grande ou pequeno), acione: `free(): invalid next size (fast)`
- Se o chunk adicionado já estava no topo do fast bin: `double free or corruption (fasttop)`
- Se o tamanho do chunk no topo tiver um tamanho diferente do chunk que estamos adicionando: `invalid fastbin entry (free)`

<details>

<summary>_int_free Fast Bin</summary>
```c
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4556C2-L4631C4

/*
If eligible, place chunk on a fastbin so it can be found
and used quickly in malloc.
*/

if ((unsigned long)(size) <= (unsigned long)(get_max_fast ())

#if TRIM_FASTBINS
/*
If TRIM_FASTBINS set, don't place chunks
bordering top into fastbins
*/
&& (chunk_at_offset(p, size) != av->top)
#endif
) {

if (__builtin_expect (chunksize_nomask (chunk_at_offset (p, size))
<= CHUNK_HDR_SZ, 0)
|| __builtin_expect (chunksize (chunk_at_offset (p, size))
>= av->system_mem, 0))
{
bool fail = true;
/* We might not have a lock at this point and concurrent modifications
of system_mem might result in a false positive.  Redo the test after
getting the lock.  */
if (!have_lock)
{
__libc_lock_lock (av->mutex);
fail = (chunksize_nomask (chunk_at_offset (p, size)) <= CHUNK_HDR_SZ
|| chunksize (chunk_at_offset (p, size)) >= av->system_mem);
__libc_lock_unlock (av->mutex);
}

if (fail)
malloc_printerr ("free(): invalid next size (fast)");
}

free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);

atomic_store_relaxed (&av->have_fastchunks, true);
unsigned int idx = fastbin_index(size);
fb = &fastbin (av, idx);

/* Atomically link P to its fastbin: P->FD = *FB; *FB = P;  */
mchunkptr old = *fb, old2;

if (SINGLE_THREAD_P)
{
/* Check that the top of the bin is not the record we are going to
add (i.e., double free).  */
if (__builtin_expect (old == p, 0))
malloc_printerr ("double free or corruption (fasttop)");
p->fd = PROTECT_PTR (&p->fd, old);
*fb = p;
}
else
do
{
/* Check that the top of the bin is not the record we are going to
add (i.e., double free).  */
if (__builtin_expect (old == p, 0))
malloc_printerr ("double free or corruption (fasttop)");
old2 = old;
p->fd = PROTECT_PTR (&p->fd, old);
}
while ((old = catomic_compare_and_exchange_val_rel (fb, p, old2))
!= old2);

/* Check that size of fastbin chunk at the top is the same as
size of the chunk that we are adding.  We can dereference OLD
only if we have the lock, otherwise it might have already been
allocated again.  */
if (have_lock && old != NULL
&& __builtin_expect (fastbin_index (chunksize (old)) != idx, 0))
malloc_printerr ("invalid fastbin entry (free)");
}
```
### \_int\_free final <a href="#int_free" id="int_free"></a>

Se o bloco ainda não foi alocado em nenhum bin, chame `_int_free_merge_chunk`

</details>
```c
/*
Consolidate other non-mmapped chunks as they arrive.
*/

else if (!chunk_is_mmapped(p)) {

/* If we're single-threaded, don't lock the arena.  */
if (SINGLE_THREAD_P)
have_lock = true;

if (!have_lock)
__libc_lock_lock (av->mutex);

_int_free_merge_chunk (av, p, size);

if (!have_lock)
__libc_lock_unlock (av->mutex);
}
/*
If the chunk was allocated via mmap, release via munmap().
*/

else {
munmap_chunk (p);
}
}
```
</details>

## \_int\_free\_merge\_chunk

Esta função tentará mesclar o chunk P de SIZE bytes com seus vizinhos. Coloque o chunk resultante na lista de bins não ordenados.

Algumas verificações são realizadas:

* Se o chunk é o chunk superior: `double free or corruption (top)`
* Se o próximo chunk está fora dos limites da arena: `double free or corruption (out)`
* Se o chunk não está marcado como usado (no `prev_inuse` do chunk seguinte): `double free or corruption (!prev)`
* Se o próximo chunk tem um tamanho muito pequeno ou muito grande: `free(): invalid next size (normal)`
* se o chunk anterior não está em uso, ele tentará consolidar. Mas, se o prev\_size difere do tamanho indicado no chunk anterior: `corrupted size vs. prev_size while consolidating`

<details>

<summary>Código \_int\_free\_merge\_chunk</summary>
```c
// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4660C1-L4702C2

/* Try to merge chunk P of SIZE bytes with its neighbors.  Put the
resulting chunk on the appropriate bin list.  P must not be on a
bin list yet, and it can be in use.  */
static void
_int_free_merge_chunk (mstate av, mchunkptr p, INTERNAL_SIZE_T size)
{
mchunkptr nextchunk = chunk_at_offset(p, size);

/* Lightweight tests: check whether the block is already the
top block.  */
if (__glibc_unlikely (p == av->top))
malloc_printerr ("double free or corruption (top)");
/* Or whether the next chunk is beyond the boundaries of the arena.  */
if (__builtin_expect (contiguous (av)
&& (char *) nextchunk
>= ((char *) av->top + chunksize(av->top)), 0))
malloc_printerr ("double free or corruption (out)");
/* Or whether the block is actually not marked used.  */
if (__glibc_unlikely (!prev_inuse(nextchunk)))
malloc_printerr ("double free or corruption (!prev)");

INTERNAL_SIZE_T nextsize = chunksize(nextchunk);
if (__builtin_expect (chunksize_nomask (nextchunk) <= CHUNK_HDR_SZ, 0)
|| __builtin_expect (nextsize >= av->system_mem, 0))
malloc_printerr ("free(): invalid next size (normal)");

free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);

/* Consolidate backward.  */
if (!prev_inuse(p))
{
INTERNAL_SIZE_T prevsize = prev_size (p);
size += prevsize;
p = chunk_at_offset(p, -((long) prevsize));
if (__glibc_unlikely (chunksize(p) != prevsize))
malloc_printerr ("corrupted size vs. prev_size while consolidating");
unlink_chunk (av, p);
}

/* Write the chunk header, maybe after merging with the following chunk.  */
size = _int_free_create_chunk (av, p, size, nextchunk, nextsize);
_int_free_maybe_consolidate (av, size);
}
```
</details>

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