Looking Glass Permission Error

Software & Operating Systems Looking Glass
1

Hi guys,
I’m running a win10 VM on manjaro using libvirt and qemu with VFIO passthrough.
In order to use it on the same monitor I’m using looking-glass.
This entire setup worked fine, until earlier today.
Everytime I try to start the VM, I get the following error:

Error starting domain: internal error: qemu unexpectedly closed the monitor: 2022-03-18T19:58:30.087433Z qemu-system-x86_64: can't open backing store /dev/shm/looking-glass for guest RAM: Permission denied

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 72, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 108, in tmpcb
    callback(*args, **kwargs)
  File "/usr/share/virt-manager/virtManager/object/libvirtobject.py", line 57, in newfn
    ret = fn(self, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/object/domain.py", line 1384, in startup
    self._backend.create()
  File "/usr/lib/python3.10/site-packages/libvirt.py", line 1353, in create
    raise libvirtError('virDomainCreate() failed')
libvirt.libvirtError: internal error: qemu unexpectedly closed the monitor: 2022-03-18T19:58:30.087433Z qemu-system-x86_64: can't open backing store /dev/shm/looking-glass for guest RAM: Permission denied

What did I try already?
All the things that come up in other forum topics with this issue (like this

I’ve checked the apparmor rules:

/etc/apparmor.d/abstractions/libvirt-qemu

dev/shm/looking-glass rw,

I also disabled apparmor completely, no dice.
I tried changing user and group to root in

/etc/libvirt/qemu.conf

still to no avail. And to be honest not my preferred solution, as it worked fine without root and with apparmor

Any help would be appreciated!

2

Make sure to follow Installation — Looking Glass B5.0.1 documentation. If QEMU has issues with permissions, just delete /dev/shm/looking-glass and touch a new one with the proper owner/group/permissions (which is essentially whatever the systemd-tmpfiles facility does every time you boot the host).

3

I followed the documentation you linked, everything was already setup like it should be. For now I bruteforced by it giving it 666 permissions, which gets the VM working again, but I’d love to understand what happened here. My own user/group used to run the VM and looking-glass. I checked out /etc/libvirt/qemu.conf and its configured to use my user and group. this might be a stupid question, but how do I figure out which user:group is trying to access the shared memory device and to change it back?

4

you can find out which user runs qemu by running ps -ef | grep qemu-system-x86_64 | awk '{print $1}' | head -1 while the VM is running. Now run ls -la /dev/shm/looking-glass and see if that user has r+w permissions.

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5

Thanks, that helped! Still no idea how the user got changed, but all is well that ends well.

2 Likes