Looks like someone had already reported this issue:
https://bugs.launchpad.net/ubuntu/+source/shadow/+bug/1812095
On there they identify a potential fix:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1813873
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Bugs,
I just experienced the same issue on 18.04, after upgrading to
4.15.0-45-generic and rebooting, gdm hung after logging in, I had this
issue when trying to log in to tty as described here. I found if I
didn't type a username, the cursor returned and i could type again, if I
entered a username it con
