I don’t agree.
Using KVM on linux: You can set up a block producer and relay VM on the same host. Both VMs can have their network communication via a private virtual network bridge. The host runs a NFTables firewall with IP forwarding enabled to route the traffic between the bp, relay, and internet.
It would be pretty difficult to “hack” into the relay if you set up your firewall well and don’t run any additional services on the relay VM. The one additional service your relay will need is ssh, so you firewall this to allow access only from within your private network, and use key based authentication. Your host machine only runs ssh and you firewall this from everywhere too except your PC, and again only key based authentication.
Now a “hacker” could gain access via cardano-node somehow and get user level access this way. Difficult, but cardano-node is new software so who knows?
Even if the above “hack” happens and someone gains user access to your relay via cardano-node. This won’t give them automatic access to your bp node. This also won’t give them automatic access to your host machine.
If they can “hack” in via cardano-node to your relay, then they could proceed to “hack” into your bp via its cardano-node since your relay is permitted to connect to your bp via your firewall. However, this is exactly the same as if your relay and bp were running on separate machines, in separate networks, in separate parts of the world, with separate firewalls.
I think you need to consider the threat possibilities and then determine whether your security is compromised by running your relay and bp as separate VMs on the same host.
The threat that everyone assumes will give access to both the relay and bp VMs is if someone can “hack” the host. But if it is only running ssh service and this is firewalled from everywhere except your PC and requires key based authentication, then good luck with that. Plus, I think the latest Debian stable (linux) version of ssh would be one of the most scrutinised pieces of software on the planet.
There are a lot of brilliant eyes looking at the open source KVM software too and its security design. Many large corporations are depending on it and they have a lot of money and reputation at stake. Furthermore, Debian and other distributions will provide updates very quickly if a security vulnerability is found. Just keep your software up to date.
Are you likely to be the world’s first target for a KVM vulnerability so someone can “hack” your cardano-node processes? What will they gain?