Arch Linux

musbur

Member

Registered: 2022-01-14

Posts: 46

Secure Boot: Wouldn't initramfs have to be signed, too?

Hello,

after much back and forth I've finally managed to set up Secure Boot (SB). My motivation isn't so much that I really need full security -- I just don't want people browsing my stuff after finding my stolen or lost laptop, and the encrypted SSD takes care of that. But this is a company Windows laptop with mandatory SB and Bitlocker, and I had to switch off SB in the BIOS set up each time I wanted to boot Linux.

Anyway, that's taken care of using this documentation.

But I have a question: The setup with signed shim / GRUB loader / kernel is supposed to ensure that nobody could smuggle a doctored kernel or boot loader onto my machine lest it get compromised the next time I enter my LUKS passphrase on booting. But what about initramfs? Couldn't that be tampered with, or is it "married" to the signed kernel in some other clever way?

Raynman

Member

Registered: 2011-10-22

Posts: 1,539

Re: Secure Boot: Wouldn't initramfs have to be signed, too?

Isn't that why it says to use a unified kernel image? Condition 3 in https://wiki.archlinux.org/title/Unifie … ecure_Boot

musbur

Member

Registered: 2022-01-14

Posts: 46

Re: Secure Boot: Wouldn't initramfs have to be signed, too?

Yes, you're probably right. My grasp of the whole subject is a tad shaky, which is why I stuck to the lower part where it talks about GRUB and shim -- stuff I'm familiar with from my Debian experience.

Head_on_a_Stick

Member

From: London

Registered: 2014-02-20

Posts: 7,732

Website

Re: Secure Boot: Wouldn't initramfs have to be signed, too?

For a unified kernel image the boot can be verified by checking

tpm2_pccread sha256:7

^ That shows the sha256sum for the SecureBoot PCR variable in the TPM chip.

A systemd .timer could be run to check this at boot and notify the user (or even just power off the machine) if the checksum doesn't match.

EDIT: for encrypted systems see these useful blog posts:

https://pawitp.medium.com/full-disk-enc … 892cab9704

https://pawitp.medium.com/the-correct-w … 421796eade

Last edited by Head_on_a_Stick (2022-01-25 16:22:19)

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