I cannot reproduce this. I compiled the reproducer program and tested on
a Eoan VM running in KVM, with the same 5.3.0-29-generic kernel:

ubuntu@ubuntu:~$ ./socktest 
Segmentation fault (core dumped)
ubuntu@ubuntu:~$ tail /var/log/kern.log
Feb 14 04:00:41 ubuntu kernel: [  134.951620] socktest[1598]: segfault at 21 ip 
000055892e4cb2a3 sp 00007ffed8905de0 error 6 in socktest[55892e4cb000+1000]
Feb 14 04:00:41 ubuntu kernel: [  134.951634] Code: 48 8d 4d e0 8b 45 dc ba 10 
00 00 00 48 89 ce 89 c7 e8 71 fe ff ff 8b 45 dc be 0a 00 00 00 89 c7 e8 52 fe 
ff ff b8 21 00 00 00 <c7> 00 21 00 00 00 b8 00 00 00 00 48 8b 4d f8 64 48 33 0c 
25 28 00
ubuntu@ubuntu:~$ ps ax | grep socktest
 1602 pts/0    S+     0:00 grep --color=auto socktest
ubuntu@ubuntu:~$ uname -rv
5.3.0-29-generic #31-Ubuntu SMP Fri Jan 17 17:27:26 UTC 2020

The segfault happens as expected, but there is no null pointer
dereference and no stack trace in dmesg. The process is terminated and I
can still bind to port 6000 with netcat.

I see that you have Virtual Box kernel modules loaded. Can you try
reproduce this on a fresh Eoan VM with no Virtual Box drivers installed?

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https://bugs.launchpad.net/bugs/1863086

Title:
  unkillable process (kernel NULL pointer dereference)

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