Although the vast majority of the software installed on my Linux daily driver is fully free and open source, and in general is trustworthy, over time I've had to make use of some programs that are closed source and created by entities who I wouldn't want to trust on my system.
The quick and easy (lazy) solution I've opted for is to give these programs their own user and home directory and access to my normal user's X session.
For example, we can create a user and home directory for steam (as root):
useradd --create-home steam
We probably want our normal user to be able to launch the program with its user without needing to enter a password every time. To do this, we need to add a line equivalent to this to our sudoers rules:
mcol ALL=(steam) NOPASSWD: /usr/bin/steam
If we were to launch it now, it wouldn't have access to the running X session owned by our normal user. We can use
xhost to give the new steam user access to the X session. This is best wrapped in a script that looks like:
#!/usr/bin/env bash xhost +SI:localuser:steam sudo --set-home -u steam /usr/bin/steam $@
I call this script
steam and put it in a folder on my path inside my home folder. This way, I can just execute
steam as my normal user and the program runs as expected in my X session, but its permissions keep it restricted to its own home folder.
This setup also lets us set firewall rules specific to the program, as nftables and iptables support the configuration of rules that match a user's UID. For example, if we wanted nftables to block connections from steam that weren't going through a VPN we could add this line to our outbound chain:
oifname $vpn_interface skuid steam accept oifname != $vpn_interface skuid steam reject
Simple and great for those who wear tinfoil hats!