# pwn 学习笔记（12）–Chunk Extend and Overlapping
chunk extend 是堆漏洞的一种常见利用手法，通过 extend 可以实现 chunk overlapping（块重叠） 的效果。这种利用方法需要以下的时机和条件：
- 程序中存在基于堆的漏洞
- 漏洞可以控制 chunk header 中的数据
# 1、对 inuse 的 fastbin 进行 extend：
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| int main(void)
{
void *ptr,*ptr1;
ptr=malloc(0x10);
malloc(0x10);
*(long long *)((long long)ptr-0x8)=0x41;
free(ptr);
ptr1=malloc(0x30);
return 0;
}
|
首先进行两次 malloc，之后看看 heap 的状态：
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| In file: /mnt/hgfs/sharedict/ChunkExtend/extend.c
3 void *ptr,*ptr1;
4
5 ptr=malloc(0x10);//分配第一个0x10的chunk
6 malloc(0x10);//分配第二个0x10的chunk
7
► 8 *(long long *)((long long)ptr-0x8)=0x41;// 修改第一个块的size域
9
10 free(ptr);
11 ptr1=malloc(0x30);// 实现 extend，控制了第二个块的内容
12 return 0;
13 }
─────────────────────────────────────────────────────────[ STACK ]─────────────────────────────────────────────────────────
00:0000│ rsp 0x7fffffffde30 —▸ 0x555555758010 ◂— 0x0
01:0008│ 0x7fffffffde38 ◂— 0x0
02:0010│ rbp 0x7fffffffde40 —▸ 0x5555555546e0 (__libc_csu_init) ◂— push r15
03:0018│ 0x7fffffffde48 —▸ 0x7ffff7a2d840 (__libc_start_main+240) ◂— mov edi, eax
04:0020│ 0x7fffffffde50 ◂— 0x1
05:0028│ 0x7fffffffde58 —▸ 0x7fffffffdf28 —▸ 0x7fffffffe2ac ◂— '/mnt/hgfs/sharedict/ChunkExtend/test'
06:0030│ 0x7fffffffde60 ◂— 0x1f7ffcca0
07:0038│ 0x7fffffffde68 —▸ 0x55555555468a (main) ◂— push rbp
───────────────────────────────────────────────────────[ BACKTRACE ]───────────────────────────────────────────────────────
► f 0 0x5555555546aa main+32
f 1 0x7ffff7a2d840 __libc_start_main+240
───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0x21
Allocated chunk | PREV_INUSE
Addr: 0x555555758020
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x555555758040
Size: 0x20fc1
pwndbg> bins
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
|
有地址的话，就去读一下两个堆的内容：
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| pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000021 <======Chunk1
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000021 <======Chunk2
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000020fc1 <======Top Chunk
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000000 0x0000000000000000
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000000
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
下一步开始释放，看一看修改 chunk1 的 size 域大小：
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0x41
Top chunk | PREV_INUSE
Addr: 0x555555758040
Size: 0x20fc1
pwndbg> bins
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
|
发现 chunk2 被修改后增大了的 chunk1 给那占了，heap 里就只有一个 Chunk 了，看看内存：
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| pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000041 <======原Chunk1
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000021 <======原Chunk2
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000020fc1 <======Top Chunk
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000000 0x0000000000000000
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000000
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
除去 chunk1 的 size 域变化了以外，似乎没有其他变化，但是，逻辑上来说，现在的堆里只有一个 chunk 了，之后 free 掉 chunk1 看看：
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| pwndbg> heap
Free chunk (fastbins) | PREV_INUSE
Addr: 0x555555758000
Size: 0x41
fd: 0x00
Top chunk | PREV_INUSE
Addr: 0x555555758040
Size: 0x20fc1
pwndbg> bins
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x555555758000 ◂— 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
|
之后读取下内存：
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| pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000041
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000021
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000020fc1
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000000 0x0000000000000000
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000000
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
下一步是重头戏，试想，如果原 chunk1 的 size 域没有真正变化，那么我们进行 malloc 一个 0x30 大小的堆块的时候，就不会分配到这个地址上，而是从 Top Chunk 里拆分，那么事实上是怎么样的呢？实践出真知，看一下吧：
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0x41
Top chunk | PREV_INUSE
Addr: 0x555555758040
Size: 0x20fc1
pwndbg> bins
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000041
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000021
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000020fc1 <======Top Chunk
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000000 0x0000000000000000
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000000
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
显然，Top Chunk 并未被拆分，这里确定了，似乎 malloc (0x30) 得到的堆块是原 Chunk1 的地址，这里说明了，这里的原 chunk1 因为 size 域被修改了之后成为了一个新的更大的堆块，这里也就造成了所谓的堆重叠了，chunk1 因为修改了 size 域后，生成的那个新的 chunk 和 chunk2 部分重叠了，这也就导致了，有的对原 chunk1 的修改可以修改到 chunk2 的地方，如果 chunk2 保留了指针，那就可以对 chunk2 进行伪造，可以结合类似 off by one 和 UAF 形成很多种利用方式。
# 2、对 inuse 的 smallbin 进行 extend：
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|
int main()
{
void *ptr,*ptr1;
ptr=malloc(0x80);
malloc(0x10);
malloc(0x10);
*(long *)((long)ptr-0x8)=0xb1;
free(ptr);
ptr1=malloc(0xa0);
}
|
首先进行三次分配，其中，第三次分配是防止 extend 后，chunk 与 topchunk 进行合并，无需关注。先看看经过三次 malloc 之后的堆空间是啥样的：
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0x91
Allocated chunk | PREV_INUSE
Addr: 0x555555758090
Size: 0x21
Allocated chunk | PREV_INUSE
Addr: 0x5555557580b0
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x5555557580d0
Size: 0x20f31
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
pwndbg> x/40gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000091 <======Chunk1
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000000
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000000
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000000 0x0000000000000021 <======Chunk2
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000021 <======Chunk3
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000020f31 <======Top Chunk
0x5555557580e0: 0x0000000000000000 0x0000000000000000
0x5555557580f0: 0x0000000000000000 0x0000000000000000
0x555555758100: 0x0000000000000000 0x0000000000000000
0x555555758110: 0x0000000000000000 0x0000000000000000
0x555555758120: 0x0000000000000000 0x0000000000000000
0x555555758130: 0x0000000000000000 0x0000000000000000
|
估摸一下，chunk1 的大小似乎有点大，导致 free 掉的 chunk1 并不会进入 fastbin，而是进入 smallbin，那么修改了 size 域后，原本三个 chunk 在 gdb 里的 heap 指令下依旧少了一个:
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0xb1
Allocated chunk | PREV_INUSE
Addr: 0x5555557580b0
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x5555557580d0
Size: 0x20f31
pwndbg> bins
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
pwndbg> x/40gx 0x555555758000
0x555555758000: 0x0000000000000000 0x00000000000000b1 <======Chunk1
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000000
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000000
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000000 0x0000000000000021 <======Chunk2
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000021 <======Chunk3
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000020f31 <======Top Chunk
0x5555557580e0: 0x0000000000000000 0x0000000000000000
0x5555557580f0: 0x0000000000000000 0x0000000000000000
0x555555758100: 0x0000000000000000 0x0000000000000000
0x555555758110: 0x0000000000000000 0x0000000000000000
0x555555758120: 0x0000000000000000 0x0000000000000000
0x555555758130: 0x0000000000000000 0x0000000000000000
|
下一步，free 掉 chunk1：
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| pwndbg> heap
Free chunk (unsortedbin) | PREV_INUSE
Addr: 0x555555758000
Size: 0xb1
fd: 0x7ffff7dd1b78
bk: 0x7ffff7dd1b78
Allocated chunk
Addr: 0x5555557580b0
Size: 0x20
Top chunk | PREV_INUSE
Addr: 0x5555557580d0
Size: 0x20f31
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x555555758000 —▸ 0x7ffff7dd1b78 (main_arena+88) ◂— 0x555555758000
smallbins
empty
largebins
empty
pwndbg> x/40gx 0x555555758000
0x555555758000: 0x0000000000000000 0x00000000000000b1 <======Chunk1
0x555555758010: 0x00007ffff7dd1b78 0x00007ffff7dd1b78
0x555555758020: 0x0000000000000000 0x0000000000000000
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000000
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000000 0x0000000000000021 <======Chunk2
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x00000000000000b0 0x0000000000000020 <======Chunk3
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000020f31 <======Top Chunk
0x5555557580e0: 0x0000000000000000 0x0000000000000000
0x5555557580f0: 0x0000000000000000 0x0000000000000000
0x555555758100: 0x0000000000000000 0x0000000000000000
0x555555758110: 0x0000000000000000 0x0000000000000000
0x555555758120: 0x0000000000000000 0x0000000000000000
0x555555758130: 0x0000000000000000 0x0000000000000000
|
这里发现了一个点需要注意，就是 free 掉 size 域修改了之后的那个 chunk1 之后，chunk3 的 size 域的最低为，也就是 p 位，变成了 0，这也就说明，chunk1 没有放在 fastbin 里，上面也看到了，被放在了 unsortedbin 里。
那么为啥会被放入 unsortedbin 内而不是 smallbin 呢？估计有一下几种可能：
- 当一个较大的 chunk 被分割成两半后，如果剩下的部分大于 MINSIZE，就会被放到 unsortedbin 中。
- 释放一个不属于 fastbin 的 chunk，并且该 chunk 不和 top chunk 紧邻时，该 chunk 就会被放到 unsorted bin 中，当第二次分配的时候，没有在 unsortedbin 中找到合适的，才会被放入到其对应的 bin 中。
之后进行分配，分配 0xa0 大小的堆块，就会发现，原 chunk1 的地址依旧拿去用了：
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0xb1
Allocated chunk | PREV_INUSE
Addr: 0x5555557580b0
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x5555557580d0
Size: 0x20f31
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
pwndbg> x/40gx 0x555555758000
0x555555758000: 0x0000000000000000 0x00000000000000b1
0x555555758010: 0x00007ffff7dd1b78 0x00007ffff7dd1b78
0x555555758020: 0x0000000000000000 0x0000000000000000
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000000
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000000 0x0000000000000021
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x00000000000000b0 0x0000000000000021
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000020f31
0x5555557580e0: 0x0000000000000000 0x0000000000000000
0x5555557580f0: 0x0000000000000000 0x0000000000000000
0x555555758100: 0x0000000000000000 0x0000000000000000
0x555555758110: 0x0000000000000000 0x0000000000000000
0x555555758120: 0x0000000000000000 0x0000000000000000
0x555555758130: 0x0000000000000000 0x0000000000000000
|
# 3、对 free 的 smallbin 进行 extend：
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|
int main()
{
void *ptr,*ptr1;
ptr=malloc(0x80);
malloc(0x10);
free(ptr);
*(long *)((long)ptr-0x8)=0xb1;
ptr1=malloc(0xa0);
}
|
首先是两次 malloc：
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0x91
Allocated chunk | PREV_INUSE
Addr: 0x555555758090
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x5555557580b0
Size: 0x20f51
pwndbg> bins
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000091 <======Chunk1
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000000
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000000
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000000 0x0000000000000021 <======Chunk2
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000020f51 <======Top Chunk
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
之后直接 free 掉 chunk1：
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| pwndbg> heap
Free chunk (unsortedbin) | PREV_INUSE
Addr: 0x555555758000
Size: 0x91
fd: 0x7ffff7dd1b78
bk: 0x7ffff7dd1b78
Allocated chunk
Addr: 0x555555758090
Size: 0x20
Top chunk | PREV_INUSE
Addr: 0x5555557580b0
Size: 0x20f51
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x555555758000 —▸ 0x7ffff7dd1b78 (main_arena+88) ◂— 0x555555758000
smallbins
empty
largebins
empty
pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000091 <======Chunk1
0x555555758010: 0x00007ffff7dd1b78 0x00007ffff7dd1b78
0x555555758020: 0x0000000000000000 0x0000000000000000
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000000
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000090 0x0000000000000020 <======Chunk2
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000020f51 <======Top Chunk
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
这里还是能看出来存在两个 chunk 的，当修改了 size 域大小后：
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| pwndbg> heap
Free chunk (unsortedbin) | PREV_INUSE
Addr: 0x555555758000
Size: 0xb1
fd: 0x7ffff7dd1b78
bk: 0x7ffff7dd1b78
Top chunk | PREV_INUSE
Addr: 0x5555557580b0
Size: 0x20f51
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x555555758000 —▸ 0x7ffff7dd1b78 (main_arena+88) ◂— 0x555555758000
smallbins
empty
largebins
empty
pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x00000000000000b1
0x555555758010: 0x00007ffff7dd1b78 0x00007ffff7dd1b78
0x555555758020: 0x0000000000000000 0x0000000000000000
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000000
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000090 0x0000000000000020
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000020f51
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
原本的三个 chunk 变成了两个，并且 chunk2 还是 allocated 状态，重叠之后，chunk1 是 free 状态，所以整个 chunk 依旧是 free 状态。之后 malloc (0xa0) 试试：
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0xb1
Top chunk | PREV_INUSE
Addr: 0x5555557580b0
Size: 0x20f51
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x00000000000000b1
0x555555758010: 0x00007ffff7dd1b78 0x00007ffff7dd1b78
0x555555758020: 0x0000000000000000 0x0000000000000000
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000000
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000090 0x0000000000000020
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000020f51
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
# 4、extend 前向 overlapping：
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|
int main()
{
void *ptr,*ptr1;
ptr=malloc(0x10);
malloc(0x10);
malloc(0x10);
malloc(0x10);
*(long *)((long)ptr-0x8)=0x61;
free(ptr);
ptr1=malloc(0x50);
}
|
还是老样子，进行 4 次 malloc，看下 heap 和 bin 以及 chunk 的内容：
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0x21
Allocated chunk | PREV_INUSE
Addr: 0x555555758020
Size: 0x21
Allocated chunk | PREV_INUSE
Addr: 0x555555758040
Size: 0x21
Allocated chunk | PREV_INUSE
Addr: 0x555555758060
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x555555758080
Size: 0x20f81
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000021 <======Chunk1
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000021 <======Chunk2
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000021 <======Chunk3
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000021 <======Chunk4
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000020f81 <======Top Chunk
0x555555758090: 0x0000000000000000 0x0000000000000000
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000000
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
之后修改 size 域：
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0x61
Allocated chunk | PREV_INUSE
Addr: 0x555555758060
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x555555758080
Size: 0x20f81
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000061
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000021
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000021
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000021
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000020f81
0x555555758090: 0x0000000000000000 0x0000000000000000
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000000
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
之后 free：
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0x61
Allocated chunk | PREV_INUSE
Addr: 0x555555758060
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x555555758080
Size: 0x20f81
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000061
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000021
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000021
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000021
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000020f81
0x555555758090: 0x0000000000000000 0x0000000000000000
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000000
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
之后重新 malloc：
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0x61
Allocated chunk | PREV_INUSE
Addr: 0x555555758060
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x555555758080
Size: 0x20f81
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
pwndbg> x/30gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000061
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000021
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000021
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000021
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000020f81
0x555555758090: 0x0000000000000000 0x0000000000000000
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000000
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000000
0x5555557580e0: 0x0000000000000000 0x0000000000000000
|
# 5、通过 extend 前向 overlapping：
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|
int main(void)
{
void *ptr1,*ptr2,*ptr3,*ptr4;
ptr1=malloc(128);
ptr2=malloc(0x10);
ptr3=malloc(0x10);
ptr4=malloc(128);
malloc(0x10);
free(ptr1);
*(int *)((long long)ptr4-0x8)=0x90;
*(int *)((long long)ptr4-0x10)=0xd0;
free(ptr4);
malloc(0x150);
}
|
经过五次 malloc 之后：
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68
| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0x91
Allocated chunk | PREV_INUSE
Addr: 0x555555758090
Size: 0x21
Allocated chunk | PREV_INUSE
Addr: 0x5555557580b0
Size: 0x21
Allocated chunk | PREV_INUSE
Addr: 0x5555557580d0
Size: 0x91
Allocated chunk | PREV_INUSE
Addr: 0x555555758160
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x555555758180
Size: 0x20e81
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
pwndbg> x/54gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000091
0x555555758010: 0x0000000000000000 0x0000000000000000
0x555555758020: 0x0000000000000000 0x0000000000000000
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000000
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000000 0x0000000000000021
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000021
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000091
0x5555557580e0: 0x0000000000000000 0x0000000000000000
0x5555557580f0: 0x0000000000000000 0x0000000000000000
0x555555758100: 0x0000000000000000 0x0000000000000000
0x555555758110: 0x0000000000000000 0x0000000000000000
0x555555758120: 0x0000000000000000 0x0000000000000000
0x555555758130: 0x0000000000000000 0x0000000000000000
0x555555758140: 0x0000000000000000 0x0000000000000000
0x555555758150: 0x0000000000000000 0x0000000000000000
0x555555758160: 0x0000000000000000 0x0000000000000021
0x555555758170: 0x0000000000000000 0x0000000000000000
0x555555758180: 0x0000000000000000 0x0000000000020e81
0x555555758190: 0x0000000000000000 0x0000000000000000
0x5555557581a0: 0x0000000000000000 0x0000000000000000
|
free 了 chunk1 之后，chunk2 的 p 位已经变成 0 了：
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| pwndbg> heap
Free chunk (unsortedbin) | PREV_INUSE
Addr: 0x555555758000
Size: 0x91
fd: 0x7ffff7dd1b78
bk: 0x7ffff7dd1b78
Allocated chunk
Addr: 0x555555758090
Size: 0x20
Allocated chunk | PREV_INUSE
Addr: 0x5555557580b0
Size: 0x21
Allocated chunk | PREV_INUSE
Addr: 0x5555557580d0
Size: 0x91
Allocated chunk | PREV_INUSE
Addr: 0x555555758160
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x555555758180
Size: 0x20e81
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x555555758000 —▸ 0x7ffff7dd1b78 (main_arena+88) ◂— 0x555555758000
smallbins
empty
largebins
empty
pwndbg> x/54gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000091 <======Chunk1
0x555555758010: 0x00007ffff7dd1b78 0x00007ffff7dd1b78
0x555555758020: 0x0000000000000000 0x0000000000000000
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000000
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000090 0x0000000000000020
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000021 <======Chunk2
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x0000000000000000 0x0000000000000091 <======Chunk3
0x5555557580e0: 0x0000000000000000 0x0000000000000000
0x5555557580f0: 0x0000000000000000 0x0000000000000000
0x555555758100: 0x0000000000000000 0x0000000000000000
0x555555758110: 0x0000000000000000 0x0000000000000000
0x555555758120: 0x0000000000000000 0x0000000000000000
0x555555758130: 0x0000000000000000 0x0000000000000000
0x555555758140: 0x0000000000000000 0x0000000000000000
0x555555758150: 0x0000000000000000 0x0000000000000000
0x555555758160: 0x0000000000000000 0x0000000000000021 <======Chunk4
0x555555758170: 0x0000000000000000 0x0000000000000000
0x555555758180: 0x0000000000000000 0x0000000000020e81 <======Top Chunk
0x555555758190: 0x0000000000000000 0x0000000000000000
0x5555557581a0: 0x0000000000000000 0x0000000000000000
|
之后修改了 chunk3 的 pre_inuse，也就是 size 的最低为 P 位为 0，然后修改 pre_size 位为 0xd8，
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61
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63
64
65
66
67
68
69
70
| pwndbg> heap
Free chunk (unsortedbin) | PREV_INUSE
Addr: 0x555555758000
Size: 0x91
fd: 0x7ffff7dd1b78
bk: 0x7ffff7dd1b78
Allocated chunk
Addr: 0x555555758090
Size: 0x20
Allocated chunk | PREV_INUSE
Addr: 0x5555557580b0
Size: 0x21
Allocated chunk
Addr: 0x5555557580d0
Size: 0x90
Allocated chunk | PREV_INUSE
Addr: 0x555555758160
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x555555758180
Size: 0x20e81
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x555555758000 —▸ 0x7ffff7dd1b78 (main_arena+88) ◂— 0x555555758000
smallbins
empty
largebins
empty
pwndbg> x/54gx 0x555555758000
0x555555758000: 0x0000000000000000 0x0000000000000091 <======Chunk1
0x555555758010: 0x00007ffff7dd1b78 0x00007ffff7dd1b78
0x555555758020: 0x0000000000000000 0x0000000000000000
0x555555758030: 0x0000000000000000 0x0000000000000000
0x555555758040: 0x0000000000000000 0x0000000000000000
0x555555758050: 0x0000000000000000 0x0000000000000000
0x555555758060: 0x0000000000000000 0x0000000000000000
0x555555758070: 0x0000000000000000 0x0000000000000000
0x555555758080: 0x0000000000000000 0x0000000000000000
0x555555758090: 0x0000000000000090 0x0000000000000020
0x5555557580a0: 0x0000000000000000 0x0000000000000000
0x5555557580b0: 0x0000000000000000 0x0000000000000021 <======Chunk2
0x5555557580c0: 0x0000000000000000 0x0000000000000000
0x5555557580d0: 0x00000000000000d0 0x0000000000000090 <======Chunk3
0x5555557580e0: 0x0000000000000000 0x0000000000000000
0x5555557580f0: 0x0000000000000000 0x0000000000000000
0x555555758100: 0x0000000000000000 0x0000000000000000
0x555555758110: 0x0000000000000000 0x0000000000000000
0x555555758120: 0x0000000000000000 0x0000000000000000
0x555555758130: 0x0000000000000000 0x0000000000000000
0x555555758140: 0x0000000000000000 0x0000000000000000
0x555555758150: 0x0000000000000000 0x0000000000000000
0x555555758160: 0x0000000000000000 0x0000000000000021 <======Chunk4
0x555555758170: 0x0000000000000000 0x0000000000000000
0x555555758180: 0x0000000000000000 0x0000000000020e81 <======Top Chunk
0x555555758190: 0x0000000000000000 0x0000000000000000
0x5555557581a0: 0x0000000000000000 0x0000000000000000
|
可以看出来，chunk3 的 pre_size 域的大小刚好能够包含到完 chunk1 和 chunk2。之后 free 掉了 chunk3：
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18
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20
21
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25
26
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28
29
30
| pwndbg> heap
Free chunk (unsortedbin) | PREV_INUSE
Addr: 0x555555758000
Size: 0x161
fd: 0x7ffff7dd1b78
bk: 0x7ffff7dd1b78
Allocated chunk
Addr: 0x555555758160
Size: 0x20
Top chunk | PREV_INUSE
Addr: 0x555555758180
Size: 0x20e81
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x555555758000 —▸ 0x7ffff7dd1b78 (main_arena+88) ◂— 0x555555758000
smallbins
empty
largebins
empty
|
会发现，前面的三个 chunk 都被合并成了一个，这里主要是因为 unlink 的原因，导致了 chunk3 和前面的两个（主要是 pre_size 指定的大小范围内的）chunk 发生了合并。之后再进行 malloc，会分配走新的那个 chunk1：
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| pwndbg> heap
Allocated chunk | PREV_INUSE
Addr: 0x555555758000
Size: 0x161
Allocated chunk | PREV_INUSE
Addr: 0x555555758160
Size: 0x21
Top chunk | PREV_INUSE
Addr: 0x555555758180
Size: 0x20e81
pwndbg> bin
fastbins
0x20: 0x0
0x30: 0x0
0x40: 0x0
0x50: 0x0
0x60: 0x0
0x70: 0x0
0x80: 0x0
unsortedbin
all: 0x0
smallbins
empty
largebins
empty
|
