lsusb lists the fingerprint reader in the x240 as follows:

Bus 002 Device 003: ID 138a:0017 Validity Sensors, Inc.

There exists experimental driver support for this in a fork of libfprint for vfs5011 sensors, however you'll need to compile the driver yourself. To get the fingerprint sensor to work for lightdm login, su etc, follow these steps.

1. Install fingerprint-gui:

   sudo add-apt-repository ppa:fingerprint/fingerprint-gui sudo apt-get update sudo apt-get install libbsapi policykit-1-fingerprint-gui fingerprint-gui

2. Get your compilation tools and library dependencies:

   sudo apt-get install build-essential automake checkinstall shtool libtool libxv-dev libmagick++-dev

3. Get the sources, compile and install them:

   git clone https://github.com/abbradar/fprintvfs5011 cd fprintvfs5011 ./autogen.sh make sudo make install

4. Make sure your user has permissions for accessing the fingerprint sensor device. Create a file /lib/udev/rules.d/40-libfprint0-custom.rules with the following contents:

   Device Validity VFS5011

   ATTRS{idVendor}=="138a", ATTRS{idProduct}=="0017", MODE="0664", GROUP="plugdev"

5. Add your user to the plugdev group:

   sudo usermod -a -G plugdev

6. Restart udev:

   sudo service udev restart

7. Enroll your fingerprints with fingerprint-gui and see if a sudo -s now suddenly asks you for your fingerprint.