It's hard to report this bug on a laptop with this driver installed, so I'm reporting this with on my laptop with a much newer NVIDIA driver (but not recommended).
My laptop, Lenovo Z40, has NVIDIA GeForce GT820M. With nvidia-340 driver installed it won't boot to SDDM (also tried with LightDM). Opening up another TTY is useless because the same message "PKCS#7 signature not signed with a trusted key" keeps getting spammed. I had to go to recovery to switch to Intel graphics.
[CODE]
$ lspci
03:00.0 3D controller: NVIDIA Corporation GF117M [GeForce 610M/710M/810M/820M / GT 620M/625M/630M/720M] (rev a1)
[/CODE]
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ProblemType: Bug
ApportVersion: 2.20.10-0ubuntu13.1
Architecture: amd64
AudioDevicesInUse:
USER PID ACCESS COMMAND
/dev/snd/controlC0: yamiyukisenpai 2601 F.... pulseaudio
/dev/snd/controlC1: yamiyukisenpai 2601 F.... pulseaudio
CurrentDesktop: KDE
Dependencies:
DistroRelease: Ubuntu 18.10
InstallationDate: Installed on 2018-08-05 (121 days ago)
InstallationMedia: Kubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
MachineType: LENOVO 20366
NonfreeKernelModules: wl
Package: nvidia-prime 0.8.10
PackageArchitecture: all
ProcFB: 0 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.18.0-12-generic root=UUID=3ac3d3b6-dc27-4365-a548-82d3be4e5ac9 ro quiet splash vt.handoff=1
ProcVersionSignature: Ubuntu 4.18.0-12.13-generic 4.18.17
RelatedPackageVersions:
linux-restricted-modules-4.18.0-12-generic N/A
linux-backports-modules-4.18.0-12-generic N/A
linux-firmware 1.175.1
Tags: cosmic
Uname: Linux 4.18.0-12-generic x86_64
UpgradeStatus: Upgraded to cosmic on 2018-10-10 (55 days ago)
UserGroups: adm cdrom dip kvm lpadmin plugdev sambashare sudo
_MarkForUpload: True
dmi.bios.date: 07/21/2015
dmi.bios.vendor: LENOVO
dmi.bios.version: 9BCN91WW
dmi.board.asset.tag: No Asset Tag
dmi.board.name: Lancer 4A5
dmi.board.vendor: LENOVO
dmi.board.version: 31900058STD
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: Lenovo Z40-70
dmi.modalias: dmi:bvnLENOVO:bvr9BCN91WW:bd07/21/2015:svnLENOVO:pn20366:pvrLenovoZ40-70:rvnLENOVO:rnLancer4A5:rvr31900058STD:cvnLENOVO:ct10:cvrLenovoZ40-70:
dmi.product.family: IDEAPAD
dmi.product.name: 20366
dmi.product.sku: LENOVO_MT_20366_BU_idea_FM_Lenovo Z40-70
dmi.product.version: Lenovo Z40-70
dmi.sys.vendor: LENOVO
One thing I forgot to mention: nvidia-prime is also broken on nvidia-340. Even with Intel graphics in use instead of NVIDIA, the laptop remains unusable.
It's fine for newer drivers, like nvidia-driver-415