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I found the following ticket in the VirtualBox issue tracker:

https://www.virtualbox.org/ticket/17987

The important (in the context of this question) part is that the guest OS (Debian) running in VirtualBox 5.2.18 on a patched Debian host thought it was vulnerable to Spectre v4 (Speculative Store Bypass).

A few months have passed and I'm seeing the same behavior in a fully patched Ubuntu 18.04 running in the latest VirtualBox 6.0.8:

$ cat /sys/devices/system/cpu/vulnerabilities/spec_store_bypass
Vulnerable

I believe that the direct cause is that VirtualBox doesn't seem to pass the SSBD flag to the VMs, even though the host OS (in my case macOS Mojave) is fully patched.

My questions are:

  1. In general, does Spectre v4 allow VM guests to read host memory (i.e. break the host/guest barrier)? I couldn't find a definitive answer..
  2. Assuming the host OS (in my case it's macOS Mojave) is fully patched, does that protect the host from the guest?
  3. (If the answer to the above question is negative) Do the available VirtualBox mitigations for other related vulnerabilities(*) fix the problem (or at least significantly decrease its severity)?

(*) VirtualBox 6.0.8 exposes the following mitigations (which can be enabled using VBoxManage modifyvm [1]):

--mds-clear-on-vm-entry on
--l1d-flush-on-vm-entry on
--spec-ctrl on
--ibpb-on-vm-exit on
--ibpb-on-vm-entry on

[1] https://www.virtualbox.org/manual/ch08.html#vboxmanage-cmd-overview

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