Initially developed for mobile device, trust|me sought to run multiple, systematically isolated Android instances on a single device. Motivated by the bring your own device problematic in enterprises, a prevalent use case was running a business and private Android instance on a single smartphone. Opposed to other approaches, trust|me used OS-level virtualization capabilities of the Linux kernel, allowing to run the Android instances in containers (using namespaces and cgroups features). In order to systematically isolate these containers, trust|me leveraged the cgroups devices subsystem, Linux capabilities, SELinux and implemented an LSM stacking to enforce SELinux policies both inside containers and globally.
With this approach, trust|me not only achieved a reasonable level of security, but also a great level of performance, allowing seamless container switches and almost native user experience. We established a usage model where one container was in the foreground at a time, appearing at the display, while others were in the background and could be toggled by a long power button press.
Following this approach, however, several technical challenges requiring intense development efforts arise when using mobile device with trust|me in practice. The first challenge is to determine the hardware devices a container may use, for instance, the Wi-Fi, radio, GPS or bluetooth peripherals. As soon as multiple containers may use hardware devices, access to them must be virtualized. This can become a complex task. Some hardware device drivers may be implemented entirely in the kernel (trust|me leveraged the device namespaces feature not part of a mainline Linux kernel), while other devices may implement parts in user space (such as a wifi supplicant or the radio interface). For virtualizing devices in user space, trust|me implemented multiplexers in the core container. Another challenge was that trust|me required modifications to the Linux kernel and minor changes within the Android OS. With frequent new versions of both kernel and Android OS, this leads to constantly lagging behind when not investing high effort. Further, the short product lifecycles of smartphones and tablets meant a constant effort in porting trust|me with its hardware device virtualization components to the most recent a wide range of devices.
The latest supported device was the Nexus 5X running Android 7, for which we still provide build and deploy instructions. However, due to the high effort, we decided to abandon the line of work with mobile devices. In the meantime, virtualization solutions like LXC and Dockers gained more and more attraction on the market. For that reason, we decided to open source trust|me as an alternative to other containerization solutions on the market. We target x86 and embedded Linux platforms running distributions like Debian inside containers and neither requiring tedious OS modifications, massive hardware device virtualization nor a container foreground-background model.
This makes trust|me more lean, easier to maintain and easier to apply on a wide range of x86 and ARM platforms. As trust|me was always developed as a security-centric solution, we designed a modular privileged virtualization layer and moved all components, critical yet not necessarily required as part of this layer to a less privileged core container (e.g., update functionalities, remote management). Further, we target not only the separation of critical user space execution contexts, but also platform security. As a result, our code base supports features like full disk encryption, secure and measured boot with remote attestation using a TPM. We integrate a customizable PKI into trust|me builds that allow not only kernel and module signing, but also signing the virtualization layer and all containers. We are also developing functionality for two-factor authentication to bind containers and their data encryption to Secure Elements, and for leveraging TEEs for storing cryptographic keys and executing ciphers within hardware-isolated boundaries.
A detailed introduction to the core concepts of the OS-Level virtualization and separation of privileged instances in trust|me is provided in this slideset. The document is centered around the legacy Android-based version of trust|me, however the core concepts apply likewise to the current version.