mirror of
https://github.com/carlospolop/hacktricks
synced 2024-12-18 17:16:10 +00:00
396 lines
14 KiB
Markdown
396 lines
14 KiB
Markdown
# free
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{% hint style="success" %}
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Aprenda e pratique Hacking AWS: <img src="/.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="/.gitbook/assets/arte.png" alt="" data-size="line">\
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Aprenda e pratique Hacking GCP: <img src="/.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="/.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
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<details>
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<summary>Apoie o HackTricks</summary>
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* Verifique os [**planos de assinatura**](https://github.com/sponsors/carlospolop)!
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* **Junte-se ao** 💬 [**grupo Discord**](https://discord.gg/hRep4RUj7f) ou ao [**grupo telegram**](https://t.me/peass) ou **siga-nos** no **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
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* **Compartilhe truques de hacking enviando PRs para os repositórios** [**HackTricks**](https://github.com/carlospolop/hacktricks) e [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud).
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</details>
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{% endhint %}
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## Resumo do Pedido Gratuito <a href="#libc_free" id="libc_free"></a>
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(Nenhuma verificação é explicada neste resumo e alguns casos foram omitidos por brevidade)
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1. Se o endereço for nulo, não faça nada
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2. Se o bloco foi mapeado, desmapeie-o e termine
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3. Chame `_int_free`:
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1. Se possível, adicione o bloco ao tcache
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2. Se possível, adicione o bloco ao fast bin
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3. Chame `_int_free_merge_chunk` para consolidar o bloco, se necessário, e adicione-o à lista não ordenada
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## \_\_libc\_free <a href="#libc_free" id="libc_free"></a>
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`Free` chama `__libc_free`.
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* Se o endereço passado for Nulo (0), não faça nada.
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* Verifique a tag do ponteiro
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* Se o bloco estiver `mapeado`, `desmapeie` e é isso
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* Se não, adicione a cor e chame `_int_free` sobre ele
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<details>
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<summary>Código __lib_free</summary>
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```c
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void
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__libc_free (void *mem)
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{
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mstate ar_ptr;
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mchunkptr p; /* chunk corresponding to mem */
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if (mem == 0) /* free(0) has no effect */
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return;
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/* Quickly check that the freed pointer matches the tag for the memory.
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This gives a useful double-free detection. */
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if (__glibc_unlikely (mtag_enabled))
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*(volatile char *)mem;
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int err = errno;
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p = mem2chunk (mem);
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if (chunk_is_mmapped (p)) /* release mmapped memory. */
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{
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/* See if the dynamic brk/mmap threshold needs adjusting.
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Dumped fake mmapped chunks do not affect the threshold. */
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if (!mp_.no_dyn_threshold
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&& chunksize_nomask (p) > mp_.mmap_threshold
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&& chunksize_nomask (p) <= DEFAULT_MMAP_THRESHOLD_MAX)
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{
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mp_.mmap_threshold = chunksize (p);
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mp_.trim_threshold = 2 * mp_.mmap_threshold;
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LIBC_PROBE (memory_mallopt_free_dyn_thresholds, 2,
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mp_.mmap_threshold, mp_.trim_threshold);
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}
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munmap_chunk (p);
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}
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else
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{
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MAYBE_INIT_TCACHE ();
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/* Mark the chunk as belonging to the library again. */
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(void)tag_region (chunk2mem (p), memsize (p));
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ar_ptr = arena_for_chunk (p);
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_int_free (ar_ptr, p, 0);
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}
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__set_errno (err);
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}
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libc_hidden_def (__libc_free)
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```
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</details>
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## \_int\_free <a href="#int_free" id="int_free"></a>
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### Início do \_int\_free <a href="#int_free" id="int_free"></a>
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Começa com algumas verificações para garantir que:
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* o **ponteiro** está **alinhado**, ou aciona o erro `free(): invalid pointer`
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* o **tamanho** não é menor que o mínimo e que o **tamanho** também está **alinhado** ou aciona o erro: `free(): invalid size`
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<details>
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<summary>Início do \_int\_free</summary>
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```c
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// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4493C1-L4513C28
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#define aligned_OK(m) (((unsigned long) (m) &MALLOC_ALIGN_MASK) == 0)
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static void
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_int_free (mstate av, mchunkptr p, int have_lock)
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{
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INTERNAL_SIZE_T size; /* its size */
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mfastbinptr *fb; /* associated fastbin */
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size = chunksize (p);
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/* Little security check which won't hurt performance: the
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allocator never wraps around at the end of the address space.
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Therefore we can exclude some size values which might appear
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here by accident or by "design" from some intruder. */
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if (__builtin_expect ((uintptr_t) p > (uintptr_t) -size, 0)
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|| __builtin_expect (misaligned_chunk (p), 0))
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malloc_printerr ("free(): invalid pointer");
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/* We know that each chunk is at least MINSIZE bytes in size or a
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multiple of MALLOC_ALIGNMENT. */
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if (__glibc_unlikely (size < MINSIZE || !aligned_OK (size)))
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malloc_printerr ("free(): invalid size");
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check_inuse_chunk(av, p);
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```
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</details>
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### \_int\_free tcache <a href="#int_free" id="int_free"></a>
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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:
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- Se houver mais entradas do que `mp_.tcache_count`: `free(): too many chunks detected in tcache`
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- Se a entrada não estiver alinhada: `free(): unaligned chunk detected in tcache 2`
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- se o chunk liberado já foi liberado e está presente como chunk no tcache: `free(): double free detected in tcache 2`
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Se tudo correr bem, o chunk é adicionado ao tcache e a função retorna.
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<details>
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<summary>_int_free tcache</summary>
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```c
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// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4515C1-L4554C7
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#if USE_TCACHE
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{
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size_t tc_idx = csize2tidx (size);
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if (tcache != NULL && tc_idx < mp_.tcache_bins)
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{
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/* Check to see if it's already in the tcache. */
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tcache_entry *e = (tcache_entry *) chunk2mem (p);
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/* This test succeeds on double free. However, we don't 100%
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trust it (it also matches random payload data at a 1 in
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2^<size_t> chance), so verify it's not an unlikely
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coincidence before aborting. */
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if (__glibc_unlikely (e->key == tcache_key))
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{
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tcache_entry *tmp;
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size_t cnt = 0;
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LIBC_PROBE (memory_tcache_double_free, 2, e, tc_idx);
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for (tmp = tcache->entries[tc_idx];
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tmp;
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tmp = REVEAL_PTR (tmp->next), ++cnt)
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{
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if (cnt >= mp_.tcache_count)
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malloc_printerr ("free(): too many chunks detected in tcache");
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if (__glibc_unlikely (!aligned_OK (tmp)))
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malloc_printerr ("free(): unaligned chunk detected in tcache 2");
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if (tmp == e)
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malloc_printerr ("free(): double free detected in tcache 2");
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/* If we get here, it was a coincidence. We've wasted a
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few cycles, but don't abort. */
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}
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}
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if (tcache->counts[tc_idx] < mp_.tcache_count)
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{
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tcache_put (p, tc_idx);
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return;
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}
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}
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}
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#endif
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```
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</details>
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### \_int\_free fast bin <a href="#int_free" id="int_free"></a>
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Comece verificando se o tamanho é adequado para o fast bin e verifique se é possível colocá-lo perto do top chunk.
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Em seguida, adicione o chunk liberado no topo do fast bin enquanto realiza algumas verificações:
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- Se o tamanho do chunk for inválido (muito grande ou pequeno), acione: `free(): invalid next size (fast)`
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- Se o chunk adicionado já estava no topo do fast bin: `double free or corruption (fasttop)`
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- Se o tamanho do chunk no topo tiver um tamanho diferente do chunk que estamos adicionando: `invalid fastbin entry (free)`
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<details>
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<summary>_int_free Fast Bin</summary>
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```c
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// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4556C2-L4631C4
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/*
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If eligible, place chunk on a fastbin so it can be found
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and used quickly in malloc.
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*/
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if ((unsigned long)(size) <= (unsigned long)(get_max_fast ())
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#if TRIM_FASTBINS
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/*
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If TRIM_FASTBINS set, don't place chunks
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bordering top into fastbins
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*/
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&& (chunk_at_offset(p, size) != av->top)
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#endif
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) {
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if (__builtin_expect (chunksize_nomask (chunk_at_offset (p, size))
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<= CHUNK_HDR_SZ, 0)
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|| __builtin_expect (chunksize (chunk_at_offset (p, size))
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>= av->system_mem, 0))
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{
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bool fail = true;
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/* We might not have a lock at this point and concurrent modifications
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of system_mem might result in a false positive. Redo the test after
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getting the lock. */
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if (!have_lock)
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{
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__libc_lock_lock (av->mutex);
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fail = (chunksize_nomask (chunk_at_offset (p, size)) <= CHUNK_HDR_SZ
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|| chunksize (chunk_at_offset (p, size)) >= av->system_mem);
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__libc_lock_unlock (av->mutex);
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}
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if (fail)
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malloc_printerr ("free(): invalid next size (fast)");
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}
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free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);
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atomic_store_relaxed (&av->have_fastchunks, true);
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unsigned int idx = fastbin_index(size);
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fb = &fastbin (av, idx);
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/* Atomically link P to its fastbin: P->FD = *FB; *FB = P; */
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mchunkptr old = *fb, old2;
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if (SINGLE_THREAD_P)
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{
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/* Check that the top of the bin is not the record we are going to
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add (i.e., double free). */
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if (__builtin_expect (old == p, 0))
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malloc_printerr ("double free or corruption (fasttop)");
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p->fd = PROTECT_PTR (&p->fd, old);
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*fb = p;
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}
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else
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do
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{
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/* Check that the top of the bin is not the record we are going to
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add (i.e., double free). */
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if (__builtin_expect (old == p, 0))
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malloc_printerr ("double free or corruption (fasttop)");
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old2 = old;
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p->fd = PROTECT_PTR (&p->fd, old);
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}
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while ((old = catomic_compare_and_exchange_val_rel (fb, p, old2))
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!= old2);
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/* Check that size of fastbin chunk at the top is the same as
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size of the chunk that we are adding. We can dereference OLD
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only if we have the lock, otherwise it might have already been
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allocated again. */
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if (have_lock && old != NULL
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&& __builtin_expect (fastbin_index (chunksize (old)) != idx, 0))
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malloc_printerr ("invalid fastbin entry (free)");
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}
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```
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### \_int\_free final <a href="#int_free" id="int_free"></a>
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Se o bloco ainda não foi alocado em nenhum bin, chame `_int_free_merge_chunk`
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</details>
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```c
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/*
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Consolidate other non-mmapped chunks as they arrive.
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*/
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else if (!chunk_is_mmapped(p)) {
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/* If we're single-threaded, don't lock the arena. */
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if (SINGLE_THREAD_P)
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have_lock = true;
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if (!have_lock)
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__libc_lock_lock (av->mutex);
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_int_free_merge_chunk (av, p, size);
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if (!have_lock)
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__libc_lock_unlock (av->mutex);
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}
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/*
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If the chunk was allocated via mmap, release via munmap().
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*/
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else {
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munmap_chunk (p);
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}
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}
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```
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</details>
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## \_int\_free\_merge\_chunk
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Esta função tentará mesclar o chunk P de SIZE bytes com seus vizinhos. Coloque o chunk resultante na lista de bins não ordenados.
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Algumas verificações são realizadas:
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* Se o chunk é o chunk superior: `double free or corruption (top)`
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* Se o próximo chunk está fora dos limites da arena: `double free or corruption (out)`
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* Se o chunk não está marcado como usado (no `prev_inuse` do chunk seguinte): `double free or corruption (!prev)`
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* Se o próximo chunk tem um tamanho muito pequeno ou muito grande: `free(): invalid next size (normal)`
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* 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`
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<details>
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<summary>Código \_int\_free\_merge\_chunk</summary>
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```c
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// From https://github.com/bminor/glibc/blob/f942a732d37a96217ef828116ebe64a644db18d7/malloc/malloc.c#L4660C1-L4702C2
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/* Try to merge chunk P of SIZE bytes with its neighbors. Put the
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resulting chunk on the appropriate bin list. P must not be on a
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bin list yet, and it can be in use. */
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static void
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_int_free_merge_chunk (mstate av, mchunkptr p, INTERNAL_SIZE_T size)
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{
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mchunkptr nextchunk = chunk_at_offset(p, size);
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/* Lightweight tests: check whether the block is already the
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top block. */
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if (__glibc_unlikely (p == av->top))
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malloc_printerr ("double free or corruption (top)");
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/* Or whether the next chunk is beyond the boundaries of the arena. */
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if (__builtin_expect (contiguous (av)
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&& (char *) nextchunk
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>= ((char *) av->top + chunksize(av->top)), 0))
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malloc_printerr ("double free or corruption (out)");
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/* Or whether the block is actually not marked used. */
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if (__glibc_unlikely (!prev_inuse(nextchunk)))
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malloc_printerr ("double free or corruption (!prev)");
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INTERNAL_SIZE_T nextsize = chunksize(nextchunk);
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if (__builtin_expect (chunksize_nomask (nextchunk) <= CHUNK_HDR_SZ, 0)
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|| __builtin_expect (nextsize >= av->system_mem, 0))
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malloc_printerr ("free(): invalid next size (normal)");
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free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);
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/* Consolidate backward. */
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if (!prev_inuse(p))
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{
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INTERNAL_SIZE_T prevsize = prev_size (p);
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size += prevsize;
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p = chunk_at_offset(p, -((long) prevsize));
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if (__glibc_unlikely (chunksize(p) != prevsize))
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malloc_printerr ("corrupted size vs. prev_size while consolidating");
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unlink_chunk (av, p);
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}
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/* Write the chunk header, maybe after merging with the following chunk. */
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size = _int_free_create_chunk (av, p, size, nextchunk, nextsize);
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_int_free_maybe_consolidate (av, size);
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}
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```
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</details>
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{% hint style="success" %}
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Aprenda e pratique AWS Hacking: <img src="/.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Treinamento AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="/.gitbook/assets/arte.png" alt="" data-size="line">\
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Aprenda e pratique GCP Hacking: <img src="/.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Treinamento GCP Red Team Expert (GRTE)**<img src="/.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
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<details>
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<summary>Suporte o HackTricks</summary>
|
|
|
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* Verifique os [**planos de assinatura**](https://github.com/sponsors/carlospolop)!
|
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* **Junte-se ao** 💬 [**grupo Discord**](https://discord.gg/hRep4RUj7f) ou ao [**grupo telegram**](https://t.me/peass) ou **siga-nos** no **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
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* **Compartilhe truques de hacking enviando PRs para os repositórios** [**HackTricks**](https://github.com/carlospolop/hacktricks) e [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud).
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</details>
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{% endhint %}
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