I recently wrote about How to Use SSH with FIDO2/U2F Security Keys, which I now use on almost all of my machines.

The last one that needed this was my Raspberry Pi hooked up to my DEC vt510 terminal and IBM mechanical keyboard. Yes I do still use that setup!

To my surprise, generating a key on it failed. I very quickly saw that /dev/hidraw0 had incorrect permissions, accessible only to root.

On other machines, it looks like this:

crw-rw----+ 1 root root 243, 16 May 24 16:47 /dev/hidraw16

And, if I run getfacl on it, I see:

# file: dev/hidraw16
# owner: root
# group: root
user::rw-
user:jgoerzen:rw-
group::---
mask::rw-
other::---

Yes, something was setting an ACL on it. Thus began to saga to figure out what was doing that.

Firing up inotifywatch, I saw it was systemd-udevd or its udev-worker. But cranking up logging on that to maximum only showed me that uaccess was somehow doing this.

I started digging. uaccess turned out to be almost entirely undocumented. People say to use it, but there’s no description of what it does or how. Its purpose appears to be to grant access to devices to those logged in to a machine by dynamically adding them to ACLs for devices. OK, that’s a nice goal, but why was machine A doing this and not machine B?

I dug some more. I came across a hint that uaccess may only do that for a “seat”. A seat? I’ve not heard of that in Linux before.

Turns out there’s some information (older and newer) about this out there. Sure enough, on the machine with KDE, loginctl list-sessions shows me on seat0, but on the machine where I log in from ttyUSB0, it shows an empty seat.

But how to make myself part of the seat? I tried various udev rules to add the “seat” or “master-of-seat” tags, but nothing made any difference.

I finally gave up and did the old-fashioned rule to just make it work already:

TAG=="security-device",SUBSYSTEM=="hidraw",GROUP="mygroup"

I still don’t know how to teach logind to add a seat for ttyUSB0, but oh well. At least I learned something. An annoying something, but hey.

This all had a laudable goal, but when there are so many layers of indirection, poorly documented, with poor logging, it gets pretty annoying.