vbernat parent
I am talking on SELinux in the host which would totally prevent this exploit as SELinux would not allow you to browse the host. You seem to say that nobody does that. libvirt uses SELinux out of the box to do exactly that.
You're talking past me and missing the point then. SELinux would absolutely not prevent this exploit: it allows the guest access to the filesystem seen by the host process, full stop. You're talking about deployment policy[1] that might help to mitigate VM exposure. And that's a fine idea. But it's not equivalent to saying that SELinux could have prevented this, and saying so in a security context is misleading users.
[1] Which has to be developed, after all. Simply saying you're "using SELinux" does nothing to prove to me you have no vulnerabilities or mistakes in your rule set.
Title says "virtfs permits guest to access entire host filesystem". First answer says "Yet another example where SELinux could have mitigated this effect". You say "That's not true".
"Entire host filesystem" is "/" and anything below, right? If your QEMU process is running in a SELinux context, there is no way it could access the "entire host filesystem".
And as your footnote, a mistake in SELinux configuration is highly unlikely. Once your process is running in a context, you have to have an explicit transition to the unconfined context or you have to relabel the "entire host filesystem" with the context of the QEMU process.