Hyper-V VM fails with grub rescue shell + kernel panic

[fredrik-nti-johanneberg]

GNS3 Version: GNS3 v3.0.0b1
GNS3 VM: Hyper-V v3.0.0b1
Operating System: Windows 10 Education 22H2

After working with a topology using MikroTik CHR (chr-6.49.11.img) QEMU appliances my Hyper-V VM failed to boot up at the next session.

The VM will halt for a while during the initial boot sequence displaying an error message "error: attempt to read or write outside of disk 'hd0'". The boot sequence will then continue but fails with a kernel panic "not syncing: VFS: Unable to mount root fs on unknown-block(0,0)".

I have been able to recover my project files and configuration by mounting GNS3 VM-disk001.vhd and GNS3 VM-disk002.vhd in another virtual machine.

This seems to be related to gns3-gui #2314 but I cannot figure out if it's possible to restore the affected VM to a working state again.

I started a GNS3 forum thread on the same topic.

[fredrik-nti-johanneberg]

I managed to solve this by repairing grub. Here's what you need to do in order to repair grub and get your GNS3 Hyper-V VM booting normally again:

Download Debian Live standard iso Add a new DVD-device to your VM, link it to the Debian Live iso and change the boot-order to CD first. Make sure that your VM has internet connectivity (via Default switch).

Command sequence:

sudo -i
apt update && apt install grub
mount /dev/sda1 /mnt
mount /dev/sda15 /mnt/boot/efi
mount --rbind /dev /mnt/dev
mount --rbind /proc /mnt/proc
mount --rbind /sys /mnt/sys
chroot /mnt
grub-install /dev/sda

It is necessary to re-mount /dev, /proc and /sys from the readonly live system to /mnt. Grub will otherwise fail with "cannot find a device for /boot (is /dev mounted?)" error.

Stop the VM after this, remove the CD/DVD or change the boot order to IDE first. Then restart the VM.

[fredrik-nti-johanneberg]

In the end, repairing grub did not work out for me (boot issue remains). Here's what I did instead:

Overwrite 'GNS3 VM-disk001.vhd' (GNS3 system partition) with an untouched copy from the GNS3 VM zip-file. Start an administrative Powershell, change working directory to your VM path and issue the following command:

$VMId = get-vm 'GNS3 VM' | Select-Object VMID -ExpandProperty VMId
icacls '.\GNS3 VM-disk001.vhd' /grant "$($VMId.Guid):F"

The icacls command will grant the replaced vhd read/write access and register it with the VM.

Your projects reside in the 'GNS3 VM-disk002.vhd' and will be mounted to /opt/gns3 upon boot. You may now restart your GNS3 VM and reconfigure networking if necessary.

[grossmj]

Thanks for the detailed fix. I am not sure why you got into that situation and how we could prevent this to happen for other users. Please let us know if you have an idea. I am closing this for now.