Bug 505222

Summary:	Fingerprint unlock rarely works after upgrade to Plasma 6.3.4
Product:	[Plasma] plasmashell	Reporter:	mcarans <mcarans>
Component:	Screen locking	Assignee:	Plasma Bugs List <plasma-bugs-null>
Status:	RESOLVED DUPLICATE
Severity:	normal	CC:	nate
Priority:	NOR	Keywords:	regression
Version First Reported In:	6.3.4
Target Milestone:	1.0
Platform:	Kubuntu
OS:	Linux
Latest Commit:		Version Fixed/Implemented In:
Sentry Crash Report:

Description mcarans 2025-06-04 21:06:28 UTC

SUMMARY
With Kubuntu 24.10, I could lock the screen, leave it for a while, come back and unlock with fingerprint. After upgrading to Kubuntu 25.04, I can only log in with password in that case, but fingerprint works for sudo and sometimes works for immediate unlock after window-L lock.

STEPS TO REPRODUCE
1. /etc/pam.d/kde looks like this:
auth            sufficient      pam_fprintd.so
auth            sufficient      pam_unix.so try_first_pass likeauth nullok

Actually I have a slightly more sophisticated setup whereby login uses password and does not use the fingerprint (because it won't unlock KWallet) but screen lock and sudo use fingerprint first falling back on password. This is described in detail here: https://www.reddit.com/r/Kubuntu/comments/1aew6t3/comment/kodetid/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button
 
2. Test unlocking after sleep

OBSERVED RESULT
With Kubuntu 25.04 (Plasma 6.3.4), it doesn't work, but fingerprint for sudo works fine and window key-L to lock and immediate unlock sometimes works.

EXPECTED RESULT
With Kubuntu 24.10, it worked reliably ie. fingerprint works for immediate unlock and unlock after sleep as well as sudo.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma:  Kubuntu 25.04
KDE Plasma Version:  6.3.4
KDE Frameworks Version:  6.12.0
Qt Version: 6.8.3

Comment 1 Nate Graham 2025-06-05 15:36:47 UTC

Unfortunately, with the way we implemented fingprint auth, this is an fprint limitation.

We open a fingerprint auth session immediately when the unlocking UI appears, so that you can immediately swipe your finger to unlock. The moment the session begins, fprint starts an internal timer of I believe 30 seconds, and if it doesn't get anything after that, it closes the session and you can't unlock with a fingerprint anymore.

What I imagine changed is that now, the unlocking UI is shown immediately on the lock screen for you, which means fprint's timer starts immediately. In the past, probably you had to wiggle the pointer first to make it appear, which means the timer didn't start yet.

This is non-deterministic due to Bug 499637. You're probably hitting the condition (which has not been found yet) that makes the UI always appear.

*** This bug has been marked as a duplicate of bug 499637 ***