Restrict read access on certain folders I am looking for a way to limit read access for certain folders, for privacy reasons.

E.g. I want to have a folder that will be used for sensitive data. It shall only be accessed by a file manager app, office app, and picture gallery. And nobody else shall be allowed to read it (e.g. untrusted apps that may violate my privacy).

I am rooted, with CM11. Is there any existing solution for that? Or will it be a dirty hack?

I am thinking of creating a new user group, assigning the "private" folder to this group, and adding the apps (additionally) to this group. Is this possible? Or will I mess something up when doing this? I have a rough overview how the linux user system works, but I do not really know how the android users work in detail.
Install an app like KeepSafe, which is optimized for pictures, or Super Vault, which you can store different types of files in. They encrypt your data and require a password to unlock, so those will probably be the best choices for you. They don't even require root.

If you want to go a step further, you can even install an app like AppLock to require a password to open a specific app.

Alternative method: You can install a file manager like ES File Explorer and zip a specific folder with a password, so no one can unzip the encrypted data without the password. Keep in mind that you will have to unzip and re-zip every time you want to add/remove/modify a file.