Is this with running virtualbox as root? This is what I was alluding to as a common thread here.

This computer is the host, it' Slackware64-current. I run VirtualBox from my user account. I did test starting virtualbox from a root Konsole and got this. I also logged in as root and tried starting virtualbox from Konsole with same result.
When running virtuialbox as a user I have no issues, this is with five Slackware virtual machines. Two are 14.2 x86 and x86_64 machines. The remaining three are -current, one x86 and two x86_64. All but one are clean installs of Slackware. All machines are with default settings except for disk size, memory and network (set to bridged). The x86 machines have PAE/NX enabled.

I tested the VirtualBox extensions on a x86_64 and a x86 machine with no issues except that the vbox kernel modules are rebuilt on every boot (annoying). It is a possibility the drivers might actually be removed on a reboot or shutdown because of this line in the shutdown messages "VirtualBox Guest Additions: You may need to restart your guest system to finish removing guest drivers." I have not verified this, but that would explain why the kernel modules are built on every boot.

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there are two issues with the latest Virtualbox 6.1.18:
ad1) running host VB as user vs root
ad2) virtualbox client crashing kernel 5.10.x

these are independent but somehow mixed up:

@Jeebizz refers to ad2)
he has a problem (as I do on antiX, Gentoo, Artix, Venom, Slackware clients) with virtualbox crashing VB client kernel 5.10.x
It does not matter what kernel version VB host is using:
in my case my host VB runs Slackware-current-64 kernel 5.10.x and Virtualbox 6.1.18 without any issues

my solution to ad2) use kernel 5.4.x
This way I have current kernel without crashing VB client

ad1) I have no idea, how to fix this I don't have the problem

My issue is as a user - I have no reason to try it as root.

Okay thanks @Aeterna and @Jeebizz, just trying to sort this out in my head.

With the exception of that 6.1.16 version, after upgrading to a testing version and now the 6.1.18 version I have had no issues with kernel 5.10.x, currently at 5.10.11 running on the host and also -current guest machines. The only problem (more of an annoyance) I've had with the guest additions is the rebuild the vbox modules on every boot I mentioned above.

@Jeebizz, I too have no reason to use virtualbox as root. Sort of puzzled me at first why anyone would, then I realized that there are a few who seem prefer to use root for everything. I'm okay with that.

Strange as in puzzling to me, this issue.

I don't normally install guest additions to my slackware virtual machines anyway. I've been testing on one of my virtual machines since I first noticed this issue over in The Latest Kernel Release thread.

I do install the guest additions to my Windows 10 virtual machine, no issues there.

I can install VirtualBox client with kernel 5.10.x and this works fine. However after installing Vbox client additions kernels 5.10.x crash with the error messages as posted by Jeebizz.

Without VboxClient additions I can't use copy/paste between clients or client and host which is quite convenient for me. So the trade off is Vbox client additions installed with kernel 5.4.x

As far as the guest rebuilding, I have always seen that no matter the version and it doesn't bother me; but I have not seen anything from Oracle officially about a fix for this issue, as I am still experiencing this instability when installing guest additions. If I don't install them, things are fine but then the graphics are screwed.

For a number of years on one of my machines I've been running Slackware Stable (14.2), Kernel 4.4.240 with the Host VirtualBox 5.0.40. The Guests having been running Slackware Current (14.2+) up to and including the 5.4.68 kernel (VBoxGuestAddions-6.0.22) without issue.

Then came kernel 5.10.x. The host is still Slackware 14.2 Kernel 4.4.240 with VirtualBox 5.0.4. One of the guests is Slackware Current with Kernel 5.10.x and VBoxGuestAdditions 6.1.18. Getting all sorts of stability issues on the Guest side including segmentation faults all similar to those already reported.

I've now dual booting with Host Slackware Current (14.2+), Kernel 5.10.x with VBoxGuestAdditions 6.1.18. The Guest is Slackware Current (14.2+). When I install VBoxGuestAdditions-6.1.18 on the guest I get no stability problems.

I noted somewhere in the VirtualBox documentation a recommendation that where possible the host and guests should be running the same versions of Virtualbox..

Alex

I sure like to see were in the documentation you read this. I run host/guest with different kernel combinations all the time. This make no sense.

I've always worked on the basis that VBoxGuestAdditions aren't tied to the VirtualBox version running on the host, until I read https://www.virtualbox.org/manual/ch04.html this statement

That statement pertains to "VBoxGuestAdditions"

They recommend VBoxGuestAdditions be of the same version as the version of VirtualBox the host is running

It has nothing to do with "kernels"

This is a given and well known. Yes you should match the Guest Additions with the version of VirtualBox you are using. You said kernels should match.

My apologies I've just realised what I wrote was wrong and what I meant to write was

No problem, we all been there. I certainly have anyway.

New test builds have been posted at,
https://www.virtualbox.org/wiki/Testbuilds
Version Linux 64-bit, 6.1.x revision 142673 and its companion extension pack work as they should as both user and root.
Ditto for the development snapshot 142300 and the 142298 extension pack.

Are you sure that the development snapshot 142300 is new? That's what I am running right now and I downloaded that weeks ago? Thanks for the heads up on the test builds though.