kscreenlocker kicks out user sessions

[Max Nikulin]

Hi,

I have faced an issue with kscreenlocker: session may be aborted on
attempt to unlock it. I am unsure if some package is missed in my case
(I am trying to avoid applications that I have never used) or the bug
may be reproduced by others.

- Plasma X11 login
- Activate screen locker
- Switch to another user
- Select the same user in the desktop manager.
I have tried SDDM and LightDM with lightdm-gtk-greeter.
- Type password

In my case the created earlier session is terminated and desktop manager
screen appears again. The following line appears in logs:

kscreenlocker_greet[2446]: pam_unix(kde:auth): authentication failure;
logname= uid=1001 euid=1001 tty= ruser= rhost= user=test

If the password typed incorrectly then the display manager reports an
error and control is not passed to kscreenlocker.

Session may be successfully unlocked from kscreenlocker.

Session may be unlocked from the display manager if kscreenlocker has
not activated yet. E.g. "switch user" action from krunner sends directly
to the display manager, so success of unlock depends on switch to the VT
where the session is running.

I have noticed the issue on my laptop and to find exact conditions I
have used a qemu VM. Set of packages are not identical, but still quite
similar.

Debian 12 bookworm.

There is a similar to some extent bug
https://bugs.debian.org/858596
but .xsession-errors is rather small in my case.

I have never bitten by this bug on Kubuntu-20.04 and lightdm-gtk-greeter
on another laptop despite temporary switching to another user is routine.

[Max Nikulin]

On 07/11/2023 21:53, Max Nikulin wrote:
>
> - Plasma X11 login
> - Activate screen locker
> - Switch to another user
> - Select the same user in the desktop manager.
>   I have tried SDDM and LightDM with lightdm-gtk-greeter.
> - Type password
>
> In my case the created earlier session is terminated and desktop manager
> screen appears again.

This recipe to get crash works rater reliable inside a VM, but not on
the host.

> The following line appears in logs:
>
> kscreenlocker_greet[2446]: pam_unix(kde:auth): authentication failure;
> logname= uid=1001 euid=1001 tty= ruser= rhost=  user=test

This message appears even in cases when the session is successfully
unlocked. So it is a misleading error.

kscreenlocker significantly increases probability of session
termination, but with more repetitions it may be experienced even with
fluxbox.

Actually I have seen 2 scenarios of crash as they appear in Xorg logs.

1. Perhaps something involved with polkit and a race with granting
permissions to current user:

[ 4721.881] (WW) qxl(0): drmSetMaster failed: Device or resource busy
[ 4721.881] (EE) qxl(0): failed to set mode: Permission denied(EE)
Fatal server error:
[ 4721.881] (EE) EnterVT failed for screen 0
[ 4721.881] (EE)

2. Segfault

[ 3444.301] (EE) Backtrace:
[ 3444.301] (EE) 0: /usr/lib/xorg/Xorg (OsLookupColor+0x139)
[0x55fc14832d29]
[ 3444.302] (EE) 1: /lib/x86_64-linux-gnu/libc.so.6 (__sigaction+0x40)
[0x7f983dc5afd0]
[ 3444.302] (EE) unw_get_proc_name failed: no unwind info found [-10]
[ 3444.302] (EE) 2: /usr/lib/xorg/modules/drivers/qxl_drv.so (?+0x0)
[0x7f983d65e0e7]
[ 3444.302] (EE) unw_get_proc_name failed: no unwind info found [-10]
[ 3444.302] (EE) 3: /usr/lib/xorg/modules/drivers/qxl_drv.so (?+0x0)
[0x7f983d65f1a7]
[ 3444.302] (EE) 4: /usr/lib/xorg/Xorg (xf86CrtcSetModeTransform+0x27f)
[0x55fc1473561f]
[ 3444.302] (EE) 5: /usr/lib/xorg/Xorg (xf86SetDesiredModes+0xec)
[0x55fc14735b4c]
[ 3444.302] (EE) unw_get_proc_name failed: no unwind info found [-10]
[ 3444.303] (EE) 6: /usr/lib/xorg/modules/drivers/qxl_drv.so (?+0x0)
[0x7f983d65ddc4]
[ 3444.303] (EE) 7: /usr/lib/xorg/Xorg (xf86CVTMode+0x23ae)
[0x55fc1473c52e]
[ 3444.303] (EE) 8: /usr/lib/xorg/Xorg (xf86VTEnter+0x81) [0x55fc146fee31]
[ 3444.303] (EE) 9: /usr/lib/xorg/Xorg (WakeupHandler+0xb3)
[0x55fc146c4243]
[ 3444.303] (EE) 10: /usr/lib/xorg/Xorg (WaitForSomething+0x198)
[0x55fc1482c6e8]
[ 3444.303] (EE) 11: /usr/lib/xorg/Xorg (SendErrorToClient+0x113)
[0x55fc146bf473]
[ 3444.303] (EE) 12: /usr/lib/xorg/Xorg (InitFonts+0x3bc) [0x55fc146c36cc]
[ 3444.303] (EE) 13: /lib/x86_64-linux-gnu/libc.so.6
(__libc_init_first+0x8a) [0x7f983dc461ca]
[ 3444.304] (EE) 14: /lib/x86_64-linux-gnu/libc.so.6
(__libc_start_main+0x85) [0x7f983dc46285]
[ 3444.304] (EE) 15: /usr/lib/xorg/Xorg (_start+0x21) [0x55fc146acb71]
[ 3444.304] (EE)
[ 3444.304] (EE) Segmentation fault at address 0x55f94a6d9e35
[ 3444.304] (EE)
Fatal server error:
[ 3444.304] (EE) Caught signal 11 (Segmentation fault). Server aborting

I have seen some bugs with OsLookupColor, but "xf86CrtcSetModeTransform"
does not appear in search results.

So likely it is something related with (deprecated?) QXL video driver.
kscreenlocker just makes the issue more apparent.

I am less sure now that I have really seen the bug on the host.