I just installed an Ubuntu 18.04 on my desktop and decided to try connecting each other (my MacBook Pro) via SMB.
To my surprise, when I connected to my Mac, using my Ubuntu desktop, the Macintosh (the disk itself), and the home folder, along with the public folder is completely accessible.
The problem is, aside from enabling the SMB file sharing on my MacBook Pro, I have not added anything to allow for file sharing, aside from the public folder, which is the only entry on the file sharing settings.
How do I prevent access from the disk, and the home folder?