这问题是群里有朋友发了一张照片看到的：
出现问题第一时间当然是百度或者谷歌，结果还是查到了，原来是gdm作怪，也可以认为是bug
https://forums.fedoraforum.org/showthread.php?283842-who-shows-unknown-user
https://unix.stackexchange.com/questions/375781/who-shows-unknown-user-logged-in-whats-going-on
That user is created (probably due to a bug) by GDM. It's not really that there is a "unknown" user logged into the system, but it's due to GDM writing a bad entry to /var/run/utmp