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Free Order Summary

(Hakuna ukaguzi ulioelezwa katika muhtasari huu na baadhi ya kesi zimeachwa kwa kifupi)

1. Ikiwa anwani ni null, usifanye chochote

2. Ikiwa kipande kilikuwa kimemmaped, mummap na maliza

3. Piga _int_free:

4. Ikiwezekana, ongeza kipande kwenye tcache

5. Ikiwezekana, ongeza kipande kwenye bakuli la haraka

6. Piga _int_free_merge_chunk ili kuunganisha kipande ikiwa ni lazima na kuongeza kwenye orodha isiyopangwa

__libc_free

Free inaita __libc_free.

• Ikiwa anwani iliyopitishwa ni Null (0) usifanye chochote.

• Angalia lebo ya kidude

• Ikiwa kipande ni kimemmaped, mummap na hiyo yote

• Ikiwa sivyo, ongeza rangi na piga _int_free juu yake

Kificho cha __lib_free

```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>

### _int_free kuanza <a href="#int_free" id="int_free"></a>

Inaanza na ukaguzi fulani kuhakikisha:

* **kipekee** kime **pangiliwa,** au kuzua kosa `free(): pointer batili`
* **ukubwa** si mdogo kuliko kiwango cha chini na kwamba **ukubwa** pia umepangiliwa au kuzua kosa: `free(): ukubwa batili`
```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);

_int_free tcache

Kwanza itajaribu kutenga kipande hiki katika tcache inayohusiana. Walakini, ukaguzi fulani hufanywa hapo awali. Itapita kupitia vikundi vyote vya tcache katika index sawa na kipande kilichofutwa na:

• Ikiwa kuna viingilio zaidi ya mp_.tcache_count: free(): too many chunks detected in tcache

• Ikiwa kuingilio sio sawa: free(): unaligned chunk detected in tcache 2

• ikiwa kipande kilichofutwa tayari kimefutwa na kipo kama kipande katika tcache: free(): double free detected in tcache 2

Ikiwa mambo yote yanaenda vizuri, kipande kitawekwa kwenye tcache na kazi itarudi.

// 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

_int_free kikasha cha haraka

Anza kwa kuangalia kwamba ukubwa unafaa kwa kikasha cha haraka na angalia ikiwa inawezekana kuweka karibu na kipande cha juu.

Kisha, ongeza kipande kilichofutwa juu ya kikasha cha haraka wakati ukifanya ukaguzi fulani:

• Ikiwa ukubwa wa kipande ni batili (kubwa sana au dogo sana) itasababisha: free(): ukubwa usiofaa (haraka)

• Ikiwa kipande kilichoongezwa kilikuwa tayari kilele cha kikasha cha haraka: uhuru maradufu au uharibifu (kilele cha haraka)

• Ikiwa ukubwa wa kipande juu una ukubwa tofauti na kipande tunachoongeza: ingizo batili la kikasha cha haraka (uhuru)

// 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 mwisho

Ikiwa kipande hakijatawiwa bado kwenye bakuli lolote, piga simu _int_free_merge_chunk

/*
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);
}
}

_int_free_merge_chunk

Hii function itajaribu kuunganisha chunk P ya SIZE bytes na majirani zake. Weka chunk iliyopatikana kwenye orodha ya bin isiyo na mpangilio.

Baadhi ya ukaguzi unafanywa:

• Ikiwa chunk ni top chunk: double free or corruption (top)

• Ikiwa chunk inayofuata iko nje ya mipaka ya uwanja: double free or corruption (out)

• Ikiwa chunk haijatambulishwa kama iliyotumika (katika prev_inuse kutoka kwa chunk inayofuata): double free or corruption (!prev)

• Ikiwa chunk inayofuata ina ukubwa mdogo sana au mkubwa sana: free(): invalid next size (normal)

• ikiwa chunk iliyotangulia haiko katika matumizi, itajaribu kuunganisha. Lakini, ikiwa prev_size inatofautiana na ukubwa ulioonyeshwa kwenye chunk iliyotangulia: corrupted size vs. prev_size while consolidating

_int_free_merge_chunk code

```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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Jifunze na zoea AWS Hacking:<img src="/.gitbook/assets/arte.png" alt="" data-size="line">[**Mafunzo ya HackTricks ya Mtaalam wa Timu Nyekundu ya AWS (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="/.gitbook/assets/arte.png" alt="" data-size="line">\
Jifunze na zoea GCP Hacking: <img src="/.gitbook/assets/grte.png" alt="" data-size="line">[**Mafunzo ya HackTricks ya Mtaalam wa Timu Nyekundu ya GCP (GRTE)**<img src="/.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)

<details>

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</details>

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