[systemd-devel] Issues supporting systems with and without TPM and firmware TPM (was Re: Handle device node timeout?)

[Lennart Poettering]

On Mo, 19.02.24 10:36, Mikko Rapeli (mikko.rapeli at linaro.org) wrote:

> > After=dev-tpmrm0.device tee-supplicant at teepriv0.service
> > Wants=dev-tpmrm0.device tee-supplicant at teepriv0.service
>
> I think my problems come from:
>
> After=tee-supplicant at teepriv0.service
> Wants=tee-supplicant at teepriv0.service
>
> Basically tee-supplicant should only be started if /dev/teepriv* device node
> is available. Then in my case with fTPM devices, all TPM using and encrypted
> rootfs creating services need to depend on the service which starts tee-supplicant
> but only if /dev/teepriv0 exists. If teepriv0 doesn't exist, then tee-supplicant
> should not be started and the dependencies to it should not exist
> either.

Is /dev/teepriv* guaranteed to be available when userspace is invoked?
or is it something that itself requires some kmod loading to show up,
i.e. that "udevadm trigger" causes to load?

> How should this dependency be expressed in systemd services?
>
> Can tee-supplicant at .service include:
>
> Before=systemd-pcrphase-initrd.service systemd-pcrphase.service systemd-pcrmachine.service
> WantedBy=systemd-pcrphase-initrd.service systemd-pcrphase.service systemd-pcrmachine.service
>
> In my testing this does not seem to work inside initramfs.
>
> If systemd-pcrphase-initrd.service systemd-pcrphase.service and systemd-pcrmachine.service
> service have After= and Wants= to tee-supplicant at teepriv0.service then things work,
> except on boards which have no optee and no /dev/teepriv0 where tee-supplicant seems
> be started and fails due to missing optee which breaks the initramfs boot.

For your usecase the new tpm2.target available in git main is what you
really should focus on: all TPM using services should order themselves
after that. All stuff needed to make a TPM device appear should be
placed before that.

The systemd-tpm2-generator that now exists in git main analyzes the
uefi/acpi firmware situation and automatically adds a dev-tpm0.device
dependency on tpm2.target if it comes to the conclusion that such a
device will show up. This generator is not going to cover your
specific case, but I think it would be a good blueprint for you:
i.e. write a generator that checks if /dev/teepriv* exists. If not,
just exit. If yes, generate the required deps to pull in
tee-supplicatnt at .service, and add the dev-tpmrm0.device dep just like
systemd-tpm2-generator does.

Lennart

--
Lennart Poettering, Berlin