Can't mount ISO to VM: Permission denied

I have uploaded an ISO file to a SMB share, via SMB. When I try to mount the ISO, I get a 'Permission denied' error. My assumption is the VM runs under some 'weird' service user, which I need to allow on the SMB datastore.

Any clue how this works?

Full error log:

Error: Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/middlewared/plugins/vm/supervisor/supervisor_base.py", line 166, in start
if self.domain.create() < 0:
File...

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