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About the Fobnail Token project
Fobnail is a project that aims to provide a reference architecture for building offline integrity measurement verifiers on the USB device (Fobnail Token) and attesters running in Dynamically Launched Measured Environments (DLME). It allows the Fobnail owner to verify the trustworthiness of the running system before performing any sensitive operation. This project was founded by NlNet Foundation. More information about the project can be found in the Fobnail documentation. Also, make sure to read other posts related to this project by visiting fobnail tag.
Scope of current phase
This phase was carried out to look at how the Fobnail Token can be used in everyday use and to analyze other elements of the Fobnail Project, such as minimal operating system. A system image was prepared using the Yocto Project and tested on PC Engines apu2. Its functionalities include:
- commissioning in the Dynamically Launched Measured Environment,
- integration with Fobnail Token,
- kexec integration, the ability to run other operating systems.
Any tests are described in this post or in additional documents to which links are provided. As part of this phase, we also conducted a research on other possible operating systems that could be run in DLME and work with Fobnail Token.
OS researching
We have conducted research to find out which OS/kernel suits our needs the most. We need USB drivers, including USB EEM (Ethernet over USB) driver and network stack. The base choice is Linux which already has all required drivers. However, we researched the usage of microkernels due to increased security. If minimal OS got compromised (through its USB or network stack), an attacker could trick Fobnail that platform is in a trustworthy state, revealing Fobnail’s kept secrets like cryptographic keys.
So we evaluated the feasibility of using microkernel-based OSes. Many microkernel-based OSes lack the required drivers, ability to boot another OS, and ability to run as DLME payload. Also, since they are designed for an embedded environment, they are not portable (across platforms with the same CPU architecture), which is a serious limitation that would force us to ship many versions of OS for each platform.
As a teaser we include here a table that summarize our research.
| OS | USB host driver | USB EEM driver | Network stack | TPM driver | OS portability | Bootloader capabilities | C library | Microkernel | CPU Architecture support | Bootable by SKL | License | Score |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Zephyr | Yes (+2) | Yes (+2) | Yes (+2) | PoC available (0) | Limited (-1) | No (0) | Yes (+2) | No (0) | Good (+1) | No (0) | OK (0) | 8 |
| Xous | No (0) | No (0) | Yes (+2) | No (0) | Limited (-1) | No (0) | No (0) | Yes (+1) | RISC-V only (-1) | No (0) | OK (0) | 1 |
| seL4 | No (0) | No (0) | Yes (+2) | No (0) | Limited (-1) | No (0) | Yes (+2) | Yes (+1) | Good (+1) | Yes (+2) | OK (0) | 7 |
| Linux | Yes (+2) | Yes (+2) | Yes (+2) | Yes (+2) | Yes (+1) | Yes (kexec) (+2) | Yes (+2) | No (0) | Good (+1) | Yes (+2) | OK (0) | 16 |
| LK | No (0) | No (0) | Limited (-2) | No (0) | Yes (+1) | No (0) | Limited (-2) | No (0) | Good (+1) | No (0) | OK (0) | -2 |
| Fuchsia | Limited (0) | No (0) | Yes (+2) | Limited (0) | Yes (+1) | Yes (mexec) (+2) | Yes (+2) | Yes (+1) | Good (+1) | No (0) | OK (0) | 7 |
As we can see, multiple OSes were taken into account and a lot of requirements. If you are interested in the meanings of the numbers here, please check the full report available on Fobnail Project official website.
Why Linux?
We have decided to use Linux for building minimal OS because it already has everything we need, and other OSes would require a significant amount of work. In the future, we may use another OS.
Reference minimal OS for Fobnail Project
We build a minimal OS image for Fobnail Project by using
Yocto Project, our
meta-fobnail layer and
kas container. It is based on
TrenchBoot project and related meta layers:
meta-pcengines to provide board
support package for PC Engines apu2 and
meta-security to use additional
useful software like tpm-tools package.
meta-fobnail layer
Our minimal OS is based on the TrenchBoot project, so we use all of their components to implement fully secured execution flow: GRUB with SKINIT support, Secure Kernel Loader, and Linux with Secure Launch. All of these components we got from TrenchBoot repositories.
We also add Fobnail Attester
application with required dependencies: qcbor and libcoap. The application
itself is automatically started on system startup. After positive result of
attestation, we can run the target system from an external device by using
kexec.
The process of generating minimal OS is not complicated. In our case, we were
using a PC running Ubuntu 20.04. Firstly, we need to download the latest
kas container:
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With that we can build a minimal OS from meta-fobnail repository:
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It may take a while (up to a few hours). If building will be finished, the
system image should be available in build/tmp/deploy/images/fobnail-machine/.
It was prepared to run from SD card. We need to use bmaptool to flash image on
the card:
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Note: We recommend to use debug version as it is passwordless.
We have also published a document outlining our efforts to make our minimal OS run in DLME. We mentioned that the components from TrenchBoot were used, but in practice, using them on the PC Engines apu2 platform was not trivial.
Attestation workflow
The minimum operating system we have prepared includes a set of functionalities that allows to test the following scenario. Imagine we got a Fobnail Token along with a minimal OS. We can now run it on our platform and validate it. If we get a positive result, we have the green light to boot our target system. Otherwise, unfortunately, we receive information that something may have changed on our platform and it is better not to boot anything else. The diagram below shows the whole situation.
In order to run the test we need to perform following steps. In our scenario we
boot to the minimal OS, run attestation and finally boot Ubuntu 20.04 LTS Live
flashed on pendrive.
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Boot meta-fobnail image in DLME - this was described here.
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Log into the platform, the server should be started on boot, to see logs, please run the following command.
1 2 3# journalctl -fu fobnail-attester -- Journal begins at Tue 2022-06-07 15:04:57 UTC. -- Jun 07 15:05:14 tb systemd[1]: Started Fobnail Attester service. -
Connect Fobnail Token to PC Engines apu2. If the device is detected properly system should print following information
1 2 3[ 42.108151] usb 2-2: new full-speed USB device number 2 using xhci_hcd [ 42.334313] cdc_eem 2-2:1.0 usb0: register 'cdc_eem' at usb-0000:00:10.0-2, CDC EEM Device, 5e:f9:bb:9b:dd:06 [ 42.355332] usbcore: registered new interface driver cdc_eem -
Wait a few seconds for provisioning and attestation. During this procedure
Fobnail Attestershould print information about received data1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17Received message: ek Received message: aik Received message: challenge Received message: metadata MAC: 0: D:B9:53:D2:50 SMBIOSv2 Manufacturer >PC Engines< Product Name >apu2< Serial Number >1373268< Received message: rim Received message: metadata MAC: 0: D:B9:53:D2:50 SMBIOSv2 Manufacturer >PC Engines< Product Name >apu2< Serial Number >1373268< Received message: quote -
The attestation was finished successfully when a green LED constantly lit on Fobnail Token for 10 seconds - see image below
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Now we can assume that we are in secure environment, so we will execute Ubuntu from external memory by using
kexec1 2 3 4 5 6 7# mkdir /mnt/usb # mount /dev/sda -o ro /mnt/usb # cd /mnt/usb # kexec -l casper/vmlinuz --initrd=casper/initrd --command-line="$( cat /proc/cmdline )" # kexec -e [ 133.109546] kexec_core: Starting new kernel
After a minute you should be able to login into the Ubuntu shell:
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Summary
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