pkexec freezes graphical environment when /proc is mounted with hidepid != 0

[thican]

Source report: https://bugs.gentoo.org/924621

I discovered using pkexec outside a terminal will freezes most (if not all) graphical processes, as shown by htop with the "T" state (which means traced or suspended (e.g by SIGTSTP)) when /proc is mounted with hidepid=2 or hidepid=1.

It happened when I used OBS Studio to create a virtual webcam, which tries to load kernel module v4l2loopback through pkexec call.

In normal context, with MATE desktop environment, a window will pop asking for root password.

However when hidepid is enable, /usr/libexec/polkit-mate-authentication-agent-1 will issue a warning when opening the session (stored in file ~/.materc-errors), such as:

(polkit-mate-authentication-agent-1:16427): polkit-mate-1-WARNING **: Unable to determine the session we are in: No session for pid 16427

(see GH-80 and GH-121)

I recently added the patch GH-423 on top of release 124, same issue.

To reproduce:

• set hidepid=2 (or =1) option for /proc (mount -o remount,hidepid=2 /proc);
• connect to your graphical environment (startx, graphical display manager),
• call pkexec outside a terminal, such as using the key combination Alt+F2 to run an application, example: pkexec modprobe v4l2loopback video_nr=10,11 card_label="Dummy video device","Second video device";
• the graphical system should freeze, use Ctrl+Alt+F1 to switch to tty1, connect and see the processes are in suspended state.

To leave this broken state, I have to send the signal SIGHUP on process xinit, which closes my whole session.

My objective is to set a working environment with "modern" privileges workflow while keeping the hidepid=2 feature of /proc to ensure privacy and security.

[bluca]

My objective is to set a working environment with "modern" privileges workflow while keeping the hidepid=2 feature of /proc to ensure privacy and security.

Setting hidepid globally is not a "modern privilege workflow", it's security theater that doesn't provide any real value, and will break a great deal of things. Just don't use it, and instead use service sandboxing.

[thican]

Thanks for your feedback.
While it might have no real security value, nothing else is broken for the 6-7 years I use it, both on server and on desktop environment, and its privacy feature is effective.
At least pkexec should not put any other process in suspended state (not 100% sure if it’s pkexec or the desktop environment’s policykit helper), which is the original point of this report.