qubes-doc/Trusted_parts.md

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Security-critical elements of Qubes OS

Summary

As stated in the architecture document, the threat model for Qubes include:

• a compromised VM
• compromised Internet connectivity (e.g. rogue ISP)

Qubes's goal is to contain an attacker within an already compromised VM. There are a number of system components that are exposed to interaction with untrusted entities, and their compromise is fatal to Qubes security.

Trusted non-Qubes-specific components

• Xen hypervisor
• xenstore
• network PV frontends (exposed to potentially compromised netvm) and backends
• block backend implemented in dom0 kernel
• integrity of Fedora packages (meaning, they are not trojaned)

At the current project stage, we cannot afford to spend time to improve them - all we can do is to limit the number and extent of these components.

Trusted Qubes-specific components

• dom0-side libvchan library
• GUI virtualization code in dom0 (qubes-guid)
• sound virtualization code in dom0 (pacat-simple-vchan); note at the current state, it parses no data from VM (just passes raw audio frames to pulseaudio), so it should be safe
• qrexec-related code in dom0 (qrexec_daemon)
• some Qubes rpc servers. The servers implementing qubes.Filecopy, qubes.ReceiveUpdates? and qubes.SyncAppMenus? must be bullet-proof. In case of qubes.OpenInVM and qubes.VMShell, their incarnation require explicit consent from the user, and such consent basically grants control over the target VM to the source VM, thus they are not critical.

It is the priority of the project to design the system so that the amount of this code is as limited as possible, and to code them securely.