Depending on how you define "fix," the common way (if the machine has already been rebooted and you're locked out of the session) is to boot via a live distro, mount the filesystem, and change permissions to get a usable system back. There are various other methods depending on the machine state and requirements, too, so it's definitely recoverable from a working machine standpoint. Changing permissions back to what they used to be is a rabbit hole that varies depending on your distro, machine-specific requirements, and any special permissions setup by you and your admins.