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.

It's a pain-and-a-half. I'm not sure any system ever recovers fully, but you basically run chmod -R's on most of the important directories and then fix the many services that will surely fail once you can get a shell.

More specifically, I used a bunch of these links until I could get a shell, then manually-fixed the rest.

https://askubuntu.com/questions/308939/how-to-reset-default-...

https://askubuntu.com/questions/508359/restore-default-syste...

Curious to know what circumstances led up to someone doing that and if they were charged for other violations.