Errors in critically important systems may become very expensive. If such systems must provide confidentiality when working with some critically important data such as classified information or private know-how, an error cost may become difficult to evaluate. For these systems, formal verification methods should be used to prove they are error-free. In the paper, a case of formal verification of such system – a Linux kernel security module – is considered the chosen toolset, the verification process workflow are reviewed, along with some auxiliary tools required for this process and developed by the authors.