This means systemd needs to live on an unencrypted volume though, right? Seems like a bit of a weakness given how much systemd can actually do.
I went with LUKS1 which grub can unlock.
I went with LUKS1 which grub can unlock.
Now you have the choice to use grub for this or linux itself.
In either case both can be modified to do arbitrary things and corrupt later stages.
To prevent this you use secure boot, and if you want to do it right a custom platform key.
So now you either pack the Linux kernel initramfs etc into a single blob and sign it or do something similar with grub.
In both cases you should have a similar secure system.
But in my experience GRUB's functionalities in this area are not very nice to use and lead to slow boot.
On the other hand directly booting the signed linux blob without a bootloader is fast and smooth, but exposes what is in you initramfs, which normally doesn't matter.
You can configure through systemd-enroll to configure the TPM to only unlock the cryptokey if those measurements are consistent.
If somebody changes your systemd binary in the initramfs; this causes the TPM PCR register to change; and will cause the system to not be able to decrypt the stage-2 rootfs