Public bug reported:

Steps to reproduce

1. setup a virtual machine with GPU pass-through to a nvidia Quadro card
(nvidia only supports passthrough to Quadro cards). Nvidia also
recommends that the passed-through card has its ROM disabled in VM
configuration, i.e. <rom bar='off'/> . Attached an example of such VM.

2. install vanilla Xubuntu 18.04, do not enable auto-login

3. install nvidia drivers, e.g.
  $ sudo add-apt-repository ppa:graphics-drivers/ppa 
  $ sudo apt-get install nvidia-driver-410

4. reboot (due to lack of GPU ROM, the screen will be black until the
greeter appears)

5. login

6. lock the screen back to the greeter
  $ light-locker-command --lock

Now you have black screen and unresponsive keyboard, and also one more file in 
/var/crash . The only way to get to the greeter again is to run (from another 
session, e.g. ssh)
  $ sudo systemctl restart lightdm

... which will also kill all the sessions running under the login manger
(including the one where we tried to lock the screen)

I've found that this only happens with stock xfwm4 version 4.12 . If I
upgrade xfwm4 to version 4.13 (from Mint) the screen lock works as
expected, but this package is not in Ubuntu repo.

So perhaps the problem is not with Xorg but with old xfwm4. I also tried
other flavours of Ubuntu and IIRC also seen it in Lubuntu.

At a stretch this could be called security vulnerability, because with
this bug there is no way to lock the screen, hence users will always
leave their session unlocked (or must log off)

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xorg 1:7.7+19ubuntu7.1
ProcVersionSignature: Ubuntu 4.15.0-45.48-generic 4.15.18
Uname: Linux 4.15.0-45-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
.proc.driver.nvidia.gpus.0000.06.00.0: Error: [Errno 21] Is a directory: 
.proc.driver.nvidia.registry: Binary: ""
 NVRM version: NVIDIA UNIX x86_64 Kernel Module  410.78  Sat Nov 10 22:09:04 
CST 2018
 GCC version:  gcc version 7.3.0 (Ubuntu 7.3.0-27ubuntu1~18.04)
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
CompositorRunning: None
Date: Mon Feb 11 09:14:54 2019
DistUpgraded: Fresh install
DistroCodename: bionic
DistroVariant: ubuntu
DkmsStatus: nvidia, 410.78, 4.15.0-45-generic, x86_64: installed
ExtraDebuggingInterest: Yes
 NVIDIA Corporation GM204GL [Quadro M5000] [10de:13f0] (rev a1) (prog-if 00 
[VGA controller])
   Subsystem: NVIDIA Corporation GM204GL [Quadro M5000] [10de:1152]
InstallationDate: Installed on 2019-02-10 (0 days ago)
InstallationMedia: Xubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 
MachineType: QEMU Standard PC (Q35 + ICH9, 2009)
 PATH=(custom, no user)
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-45-generic 
root=UUID=16524009-5b46-4f13-918c-0dee6cce40fe ro console=tty0 
SourcePackage: xorg
Symptom: display
Title: Xorg crash
UpgradeStatus: No upgrade log present (probably fresh install) 02/06/2015
dmi.bios.vendor: EFI Development Kit II / OVMF
dmi.bios.version: 0.0.0
dmi.chassis.type: 1
dmi.chassis.vendor: QEMU
dmi.chassis.version: pc-q35-3.1
dmi:bvnEFIDevelopmentKitII/OVMF:bvr0.0.0:bd02/06/2015:svnQEMU:pnStandardPC(Q35+ICH9,2009):pvrpc-q35-3.1:cvnQEMU:ct1:cvrpc-q35-3.1: Standard PC (Q35 + ICH9, 2009)
dmi.product.version: pc-q35-3.1
dmi.sys.vendor: QEMU
version.compiz: compiz N/A
version.libdrm2: libdrm2 2.4.95-1~18.04.1
version.libgl1-mesa-dri: libgl1-mesa-dri 18.2.2-0ubuntu1~18.04.2
version.libgl1-mesa-glx: libgl1-mesa-glx 18.2.2-0ubuntu1~18.04.2
version.nvidia-graphics-drivers: nvidia-graphics-drivers-* N/A
version.xserver-xorg-core: xserver-xorg-core 2:1.19.6-1ubuntu4.2
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev N/A
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:18.0.1-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.15-2

** Affects: xorg (Ubuntu)
     Importance: Undecided
         Status: New

** Tags: amd64 apport-bug bionic crash ubuntu

** Attachment added: "virtual machine definition"

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