PKCS#7 signature not signed with a trusted key (after recent kernel updates to Ubuntu 16.04.6 LTS), and login loop.
Bare metal Dell workstation running Uubuntu 16.04.6 LTS xenial without issues for some years, with no hardware changes.
Nvidia Quadro K420 adapter, with latest nvidia proprietary linux driver 430.50 (september 2019).
I’ve always used the nvidia proprietary driver because the open source drivers gave too many issues.
In the past I have regularly updated the nvidia driver manually after stopping lighdm and executing the relevant runfile without difficulty.
But following recent kernel updates that software-update applied , I cannot login at the console (it just flashes and re-displays the login screen in a loop)
and also I cannot get a login prompt from Ctrl-Alt-F1 (just shows the five dots progress bar indefinitely) on an otherwise blank screen.
From grub, if I boot to 4-15.0-64 recovery mode and drop to a root shell, I can see in /var/log/syslog:
[...] nvidia: loading out-of-tree module taints kernel [...] nvidia: module license 'NVIDIA' taints kernel [...] nvidia: nvidia: module verification failed: signature and/or required key missing -tainting kernel [...] NVRM: loading NVIDIA UNIX x86_64 Kernel Module 430.50 ... [...] PKCS#7 signature not signed with a trusted key
Secure-Boot is already permanently disabled in the BIOS (and the machine can successfully dual boot to Win10 via grub).
What must I do to configure to avoid the need to sign the driver?