Thinkpad fingerprint scan only at login

This solution assumes that you only have (or had) to modify /etc/pam.d/common-auth to make fingerprint reader work.

• Make a copy of the file sudo cp -p /etc/pam.d/common-auth /etc/pam.d/common-auth-finger and make the necessary changes to that file instead (if you use pam-auth-update or some other automated tool, just let it make changes, and swap the files afterwards)
• In /etc/pam.d/lightdm replace the line that has @include common-auth to read @include common-auth-finger

This should be all that is needed. If you had changed pam configuration in a different way, please tell how.