Below is a list of security-critical (AKA trusted) code in Qubes OS. A successful attack against any of those might allow to compromise the Qubes OS security. This code can be thought of as of a Trusted Computing Base (TCB) of Qubes OS. The goal of the project has been to minimize the amount of this trusted code to an absolute minimum. The size of the current TCB is of an order of hundreds thousands of lines of C code, which is several orders of magnitude less than in other OSes, such as Windows, Linux or Mac, where it is of orders of tens of millions of lines of C code.
- The qubes.Filecopy RPC server that runs in a VM -- this one is critical because it might allow one VM to compromise another one if user allows file copy operation to be performed between them
These are the components that we haven't written or designed ourselves, yet we still rely on them. At the current project stage, we cannot afford to spend time to thoroughly review and audit them, so we just more or less "blindly" trust they are secure.
- Xen hypervisor
- The Xen's xenstore backend running in Dom0
- The Xen's block backend running in Dom0 kernel
- The rpm program used in Dom0 for verifying Dom0 updates' signatures
- Somehow optional: log viewing software in dom0 that parses VM-influenced logs
Additionally when we consider attacks that originate through a compromised network domain and target the VMs connected to it. Those attacks do not apply to domains connected to other network domains (Qubes allows more than one network domains), or those with networking disabled (Dom0 is not connected to any network by default).
- Xen network PV frontends
- VM's core networking stacks (core TCP/IP code)
Buggy Code vs. Back-doored Code Distinction
-------------------------------------------
There is an important distinction between the buggy code and maliciously trojaned code. We could have the most secure architecture and the most bulletproof TCB that perfectly isolates all domains from each other, but it still would be pretty useless if all the code used within domains, e.g. the actual email clients, word processors, etc, was somehow trojaned. In that case only network-isolated domains could be somehow trusted, while all others could be not.
The above means that we must trust at least some of the vendors (not all, of course, but at least those few that provide the apps that we use in the most critical domains). In practice in Qubes OS we trust the software provided by Fedora project. This software is signed by Fedora distribution keys and so it is also critical that the tools used in domains for software updates (yum and rpm) be trusted.
Cooperative Covert Channels Between Domains
-------------------------------------------
Qubes does not attempt to eliminate all possible *cooperative* covert channels between domains, i.e. such channels that could be established between two *compromised* domains. We don't believe this is possible to achieve on x86 hardware, and we also doubt it makes any sense in practice for most users -- after all if the two domains are compromised, then it's already (almost) all lost anyway.
