0x01 前言
在2020年09月10日，Ubuntu基金会发布了名为为2.27-3ubuntu1.3的更新，本次更新进行了对于Tcache分配机制的更新，这将导致一批对于Tcache的利用失效，本文从源码的角度分析了变动的源码与保护。
0x02 获取源码
由于此软件包目前还处于测试状态(proposed)，我们无法使用apt-get update && apt-get upgrade直接获取到此版本。
运行以下命令启用获取测试状态软件包源码：
sudo echo 'deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
' >> /etc/apt/sources.list
sudo echo 'deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
' >> /etc/apt/sources.list
接下来运行sudo apt-get update更新软件源
🚫：更新完软件源后，严禁直接使用sudo apt-get upgrade更新软件，部分测试版核心软件(例如glibc)一旦被更新到测试版将无法进行降级。
接下来运行sudo apt-get source libc6/bionic-proposed以获取目标源码
⚠️：接下来请编辑/etc/apt/sources.list，注释掉刚刚添加的源，并运行sudo apt-get update更新软件源
0x03 代码变更
1.Tcache结构体定义变更
原始定义：
/* We overlay this structure on the user-data portion of a chunk when
the chunk is stored in the per-thread cache. */
typedef struct tcache_entry
{
struct tcache_entry *next;
} tcache_entry;
新定义：
/* We overlay this structure on the user-data portion of a chunk when
the chunk is stored in the per-thread cache. */
typedef struct tcache_entry
{
struct tcache_entry *next;
/* This field exists to detect double frees. */
struct tcache_perthread_struct *key;
} tcache_entry;
意义：
加入了额外指针key，可以用来检查链表完整性。
2.加入Tcache数量限制
原始定义：
无
新定义：
#define MAX_TCACHE_COUNT 127 /* Maximum value of counts[] entries. */
意义：
限制Tcache的数量
3.tcache_put加入了新步骤
原始定义：
/* Caller must ensure that we know tc_idx is valid and there's room
for more chunks. */
static __always_inline void tcache_put (mchunkptr chunk, size_t tc_idx)
{
tcache_entry *e = (tcache_entry *) chunk2mem (chunk);
assert (tc_idx < TCACHE_MAX_BINS);
e->next = tcache->entries[tc_idx];
tcache->entries[tc_idx] = e;
++(tcache->counts[tc_idx]);
}
新定义：
/* Caller must ensure that we know tc_idx is valid and there's room
for more chunks. */
static __always_inline void tcache_put (mchunkptr chunk, size_t tc_idx)
{
tcache_entry *e = (tcache_entry *) chunk2mem (chunk);
assert (tc_idx < TCACHE_MAX_BINS);
/* Mark this chunk as "in the tcache" so the test in _int_free will
detect a double free. */
e->key = tcache;
e->next = tcache->entries[tc_idx];
tcache->entries[tc_idx] = e;
++(tcache->counts[tc_idx]);
}
意义：
现在，将chunk置入Tcache中时，将会新增一个指针指向Tcache，用于检查Double Free。
4.tcache_get加入了新步骤
原始定义：
/* Caller must ensure that we know tc_idx is valid and there's
available chunks to remove. */
static __always_inline void *
tcache_get (size_t tc_idx)
{
tcache_entry *e = tcache->entries[tc_idx];
assert (tc_idx < TCACHE_MAX_BINS);
assert (tcache->entries[tc_idx] > 0);
tcache->entries[tc_idx] = e->next;
--(tcache->counts[tc_idx]);
return (void *) e;
}
新定义：
/* Caller must ensure that we know tc_idx is valid and there's
available chunks to remove. */
static __always_inline void *
tcache_get (size_t tc_idx)
{
tcache_entry *e = tcache->entries[tc_idx];
assert (tc_idx < TCACHE_MAX_BINS);
assert (tcache->entries[tc_idx] > 0);
tcache->entries[tc_idx] = e->next;
--(tcache->counts[tc_idx]);
e->key = NULL;
return (void *) e;
}
意义：
现在，将chunk从Tcache中移除时，将会清空新增的指针。
5.int_free中加入了新检查
原始定义：
#if USE_TCACHE
{
size_t tc_idx = csize2tidx (size);
if (tcache
&& tc_idx < mp_.tcache_bins
&& tcache->counts[tc_idx] < mp_.tcache_count)
{
tcache_put (p, tc_idx);
return;
}
}
#endif
新定义：
#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))
{
tcache_entry *tmp;
LIBC_PROBE (memory_tcache_double_free, 2, e, tc_idx);
for (tmp = tcache->entries[tc_idx]; tmp; tmp = tmp->next)
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
意义：
现在，调用int_free时，将会检查整个Tcache链表，如果发现将要释放的chunk已存在于链表中将会报错free(): double free detected in tcache 2。
6._int_realloc中更改了定义
原始定义：
void*
_int_realloc(mstate av, mchunkptr oldp, INTERNAL_SIZE_T oldsize,
INTERNAL_SIZE_T nb)
{
.........
unsigned long copysize; /* bytes to copy */
unsigned int ncopies; /* INTERNAL_SIZE_T words to copy */
INTERNAL_SIZE_T* s; /* copy source */
INTERNAL_SIZE_T* d; /* copy destination */
.........
{
/*
Unroll copy of <= 36 bytes (72 if 8byte sizes)
We know that contents have an odd number of
INTERNAL_SIZE_T-sized words; minimally 3.
*/
copysize = oldsize - SIZE_SZ;
s = (INTERNAL_SIZE_T *) (chunk2mem (oldp));
d = (INTERNAL_SIZE_T *) (newmem);
ncopies = copysize / sizeof (INTERNAL_SIZE_T);
assert (ncopies >= 3);
if (ncopies > 9)
memcpy (d, s, copysize);
else
{
*(d + 0) = *(s + 0);
*(d + 1) = *(s + 1);
*(d + 2) = *(s + 2);
if (ncopies > 4)
{
*(d + 3) = *(s + 3);
*(d + 4) = *(s + 4);
if (ncopies > 6)
{
*(d + 5) = *(s + 5);
*(d + 6) = *(s + 6);
if (ncopies > 8)
{
*(d + 7) = *(s + 7);
*(d + 8) = *(s + 8);
}
}
}
}
_int_free (av, oldp, 1);
check_inuse_chunk (av, newp);
return chunk2mem (newp);
}
}
}
.........
}
新定义：
void* _int_realloc(mstate av, mchunkptr oldp, INTERNAL_SIZE_T oldsize,
INTERNAL_SIZE_T nb)
{
.........
/*
Avoid copy if newp is next chunk after oldp.
*/
if (newp == next)
{
newsize += oldsize;
newp = oldp;
}
else
{
memcpy (newmem, chunk2mem (oldp), oldsize - SIZE_SZ);
_int_free (av, oldp, 1);
check_inuse_chunk (av, newp);
return chunk2mem (newp);
}
}
}
.........
}
意义：
此处由原来的逐位复制直接变更为使用memcpy，实现上更加简洁。
7.do_set_tcache_count发生了变更
原始定义：
static inline int __always_inline do_set_tcache_count (size_t value)
{
LIBC_PROBE (memory_tunable_tcache_count, 2, value, mp_.tcache_count);
mp_.tcache_count = value;
return 1;
}
新定义：
static inline int __always_inline do_set_tcache_count (size_t value)
{
if (value <= MAX_TCACHE_COUNT)
{
LIBC_PROBE (memory_tunable_tcache_count, 2, value, mp_.tcache_count);
mp_.tcache_count = value;
}
return 1;
}
意义：
限制tcache_count的数目必须小于MAX_TCACHE_COUNT(即127)，防止发生溢出。
0x04 变化总结
以上几大变化基本上都是针对的Tcache进行的更新，且更新后的代码基本100%照搬Glibc 2.31的代码，因此我们可以直接利用2.31的思路，直接篡改key指针就可以完成利用了。
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