qrexec

qrexec is a framework within the Qubes OS operating system that enables secure inter-process communication (IPC) between different virtual machines (VMs), known as qubes. Its primary purpose is to allow a trusted qube to execute specific commands in a less trusted qube, retrieve their results, and handle exceptions. This mechanism is crucial for Qubes OS's security model, as it prevents direct access between qubes and channels operations through a controlled interface.

The qrexec framework operates by defining a set of services. A service is essentially a command that can be executed remotely in another qube. When a qube requests to execute a service in another qube, the qrexec daemon in the target qube receives the request. It then verifies the request against access control policies and, if permitted, executes the specified command. The results or any errors are then sent back to the requesting qube.

This architecture allows Qubes OS to implement features like file copy-paste, opening documents from one qube in another, or running specific applications with limited privileges in a separate, isolated environment. The security of qrexec relies on strong isolation between qubes and the careful management of which services are exposed and to whom. It is a fundamental component that underpins the secure and granular control of interactions within the Qubes OS ecosystem.