sudo: no valid sudoers sources found, quitting - ubuntu

Issue:

When I tried to add a new user into sudoer file  /etc/sudoers, I need to chmod sudoers
chmod +w /etc/sudoers
Then, system gave this error:
sudo: /etc/sudoers is mode 0640, should be 0440
sudo: no valid sudoers sources found, quitting

Solution:
Enter Ubuntu recovery mode to change /etc/sudoers to 0440