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.
Can confirm. I kept getting permission errors on something I was testing, and in desperation ran `chmod -R 777` thinking I was in my project dir. I was actually at root. After a bunch attempts to recover, I ended up saving the data I cared about and doing a fresh install. 0/10 would not recommend.