VirtualBox freezes when Windows 7/10 guests reboot/reset after restoring online snapshots if GA is installed

[wzyboy]

Host information:

• Arch Linux
• linux 5.6.3.arch1-1
• virtualbox 6.1.4-6
• virtualbox-guest-iso 6.1.4-1

Guest Information (both are reproducible)

• Windows 7 SP1
• Windows 10 LTSC 2019, Build Number 17763.1131

Steps to reproduce:

1. Cold boot the guest VM;
2. Take a online snapshot "Snapshot 1";
3. Shutdown the machine and restore to "Snapshot 1";
4. Boot the guest VM from "Snapshot 1" (directly into the Windows desktop);
5. Reboot/Reset the guest VM;
6. The VM resets, and shows a brief Windows Vista-style loading progress bar, and the VirtualBox (on host machine) freezes before crashing.

Notes:

• I have tried Windows 7 and Windows 10 guests (as decribed above), both are reproducible.
• If Guest Addition is not installed / uninstalled, the bug does not occur.
• If guest VM is cold booted (e.g. after step 4, instead of resetting/rebooting it, just shutdown and boot it), the bug does not occur.

Side note (may not relate to this bug):

After VB freezes/crashes, the RAM for guest VM is simply "taken away" from the host and cannot be recovered without a reboot of the host -- My host has 32 GB RAM, and each time a guest with 4 GB RAM crashes, there are 4 GB of RAM taken away from the host. After 7 crashes, 28 GB RAM are taken away from the host. I tried to shutdown Xorg server and there are only a few running processes left (none of them are VirtualBox), the host system still reports there are ~30 GB RAM used (top, htop, free... not cache/buffer, just "used"). I have to reboot the host system to recover the RAM.

[wzyboy]

I tested these versions of VBoxGuestAdditions_X.X.X.iso with VirtualBox 6.1.4:

• 5.2.38
• 6.0.0
• 6.0.4
• 6.0.12
• 6.1.4

Only with GA 5.2.38 the bug does not occur. It seems to me that the bug was introduced with GA 6.0.0.