__free_hook 劫持原理 | Lantern's 小站

前言

本文学习自Ex师傅的__free_hook 劫持原理, 根据学习情况略有补充和修改

例子

先从一个简单的例子来讲, 下面代码直接队全局变量__free_hook进行修改

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

extern void (*__free_hook) (void *__ptr,const void *);

int main() {
        char *str = malloc(160);
        strcpy(str,"/bin/sh");

        printf("__free_hook: %p\n",__free_hook);

        // 劫持__free_hook

        puts(" __free_hook abduction");
        __free_hook = (void*)system;

        free(str);
        printf("__free_hook: %p\n",__free_hook);
        return 0;
}

编译运行:

➜  work gcc __free_hook.c -o __free_hook
➜  work ./__free_hook
__free_hook: (nil)
 __free_hook abduction
$ echo hello world
hello world
$
__free_hook: 0x7f545a07f4e0

源码分析

源码来自 glibc 2.23, 后面是行号

glibc-2.23/malloc/malloc.c: 1851

void weak_variable (*__free_hook) (void *__ptr,
                                   const void *) = NULL;

这里定义为的__free_hook定义为全局变量，所以可以直接被修改。再来看看__free_hook的引用:

void
__libc_free (void *mem)
{
  mstate ar_ptr;
  mchunkptr p;                          /* chunk corresponding to mem */

  void (*hook) (void *, const void *)
    = atomic_forced_read (__free_hook);
  if (__builtin_expect (hook != NULL, 0))
    {
      (*hook)(mem, RETURN_ADDRESS (0));
      return;
    }
// ...
}