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He asked if Joe from VT1 can spy a root on VT2. I think that if everything is properly configured (for example CTRL+ALT+F* in X server are not disabled by administrator) then not. But I am not a hacker.
For example to access other X session (screen, keys activity) you need to know Magic Cookie stored on user who started session home directory. Which is protected.
Of course, I understand that. My question is: can a program running as a _regular, non-root_ user can access another VT?
(assuming that this program cannot su or sudo to become root)
Ah, OK. Simple answer: no due to separation of privileges (UID) and X11 authorization protocol (see xauth). That said X11 server is way ancient and abuses system resources like ioctls in mysterious ways.
Thanks for the link. Yes the X Server is a big and very complex engine that runs as root (SUID) which is not good.
I have read (https://wiki.ubuntu.com/X/Rootless) that X can be run as a non-root user (=> not more need to suid X) which would be good... but the recipe includes giving rw access to /dev/input/* to at least the user, which seems to introduce a bigger risk (maybe a user program could then spy on _any_ input?!?).
Has anyone run X as a non-root user?
I wonder if it is not more systematically setup that way because (a) it doesn't work with non-KMS drivers, (b) it is more complex to setup for a small perceived benefit, or (c) because it is simply less secure than running X as suid root?