Btnx is a small software that will let you configure your mouse's extra button on Linux.

For example with Debian or Ubuntu, the btnx package can be installed using the command:

sudo apt get install btnx

Once the software is installed, btnx must be configured using administrator privileges:

sudo btnx-config

The configuration window will let you affect a command or a keyboard shortcut to each of your mouse buttons.

Once the setup is finished, btnx must be restarted in order to load the new configuration. This can be done on the config window or with the command:

sudo btnx

I have a "Logitech Performance MX" mouse with plenty of extra buttons and this software let me use them all.