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A recent Linux Unplugged episode went into how one can use the TPM to set up a secure and trusted chain of trust for the booting process on Linux [0] using Clev

A recent Linux Unplugged episode went into how one can use the TPM to set up a secure and trusted chain of trust for the booting process on Linux [0] using Clevis [1], very interesting! [0] https://linuxunplugged.com/572 https://linuxunplugged.com/572 [1] https://fedoramagazine.org/automatically-decrypt-your-disk-using-tpm2/ https://fedoramagazine.org/automatically-decrypt-your-disk-u...

tostiheld 2y ago
The "new" way of doing this would be using systemd-cryptenroll [0]. I did this recently on Ubuntu 24.04. I actually tried the default LUKS+TPM shipped with Ubuntu 24.04 at first [1], but it was a bit disappointing because it locks you into using snap-based kernels. This means you cannot install custom DKMS modules (which I needed). Although Clevis is very interesting software (you can even unlock based on some other computer in your network [2]), it's not absolutely required anymore for LUKS+TPM. [0] https://fedoramagazine.org/use-systemd-cryptenroll-with-fido-u2f-or-tpm2-to-decrypt-your-disk/ https://fedoramagazine.org/use-systemd-cryptenroll-with-fido... [1] https://ubuntu.com/blog/tpm-backed-full-disk-encryption-is-coming-to-ubuntu https://ubuntu.com/blog/tpm-backed-full-disk-encryption-is-c... [2] https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/8/html/security_hardening/configuring-automated-unlocking-of-encrypted-volumes-using-policy-based-decryption_security-hardening https://docs.redhat.com/en/documentation/red_hat_enterprise_...
dathinab 2y ago
I do something similar on all my laptops: - have custom secure boot platform key - use a unified kernel image (UKI) which means I directly boot the kernel from efi (and place it in the efi partition) - sign the image with that platform key (I use sbctrl) - have every thing else including swap partition for hybernation fully disk encrypted, I could set it up to auto unlock using TPM2 but I would recommend using a long password. TPM2+password would be optimal. There had been too many cases of leaky TPMs and especially on a laptop you don't want to fully rely on it (through you in turn could decide to auto login if PCRs are unchanged, or login using only the (often not so secure) fingerprint reader etc.) - efi password, I mean if you don't set that you lose most secure boot benefits... EDIT: Not really most, there is still a bunch of ways it helps but it's anyway a bad idea to rely on secure boot and not have a efi password As bonus tip: - include the vfat in your initramfs (i.e. `MODULES=(vfat)` in `/etc/mkinitcpio.conf`) if your booting kernel and installed kernel modules ever mismatch that is nice to have to fix the issue
- mixmastamyk 2y ago
  What are the details with a custom key?
  - deleted 2y ago
    [deleted]
  - dathinab 2y ago
    sbctl with package manager hook for automatically signing on updates etc. keys are just stored on the device, for the typical laptop use-case this is good enough (platform key only used by a single device, no MDA or anything like that)
- jcalvinowens 2y ago
  > I could set it up to auto unlock using TPM2 but I would recommend using a long password. TPM2+password would be optimal. Personally, I trust LUKS with passphrases far more than I trust some random proprietary hardware implementation nobody can audit... It's also important to me to be able to recover the disk contents with the passphrase on another machine if the motherboard dies. Maybe that's what you meant (backup passphrase), but I think you meant requiring both?
  - dathinab 2y ago
    I meant: - I'm only using a long password - but it would be optimal to require PCR values and password Note that in any case where you use PCR values you always should setup a secondary way to unlock the partition. Or else you will lose your data if some of your hardware measured into a PCR breaks. Requiring both is optimal as it 1. doesn't rely on TPM/PCRs but 2. prevent certain attack vectors possible with password only but not possible with PCRs. Through you now also have to manage a backup unlock method. Which is annoying. And the security benefits are negligible/irrelevant for most people. Which is why I don't use it.
    - 3np 2y ago
      Thanks for sharing your setup! Nit: It's useful to distinguish between passwords (checked against a hash for auth) and passphrases (used for decryption). It's an important practical distinction because a lost password can in general be bypassed out-of-band somehow while a backup strategy for passphrases is essential.

tostiheld 2y ago

The "new" way of doing this would be using systemd-cryptenroll [0]. I did this recently on Ubuntu 24.04. I actually tried the default LUKS+TPM shipped with Ubuntu 24.04 at first [1], but it was a bit disappointing because it locks you into using snap-based kernels. This means you cannot install custom DKMS modules (which I needed). Although Clevis is very interesting software (you can even unlock based on some other computer in your network [2]), it's not absolutely required anymore for LUKS+TPM. [0] https://fedoramagazine.org/use-systemd-cryptenroll-with-fido-u2f-or-tpm2-to-decrypt-your-disk/ https://fedoramagazine.org/use-systemd-cryptenroll-with-fido... [1] https://ubuntu.com/blog/tpm-backed-full-disk-encryption-is-coming-to-ubuntu https://ubuntu.com/blog/tpm-backed-full-disk-encryption-is-c... [2] https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/8/html/security_hardening/configuring-automated-unlocking-of-encrypted-volumes-using-policy-based-decryption_security-hardening https://docs.redhat.com/en/documentation/red_hat_enterprise_...

dathinab 2y ago

I do something similar on all my laptops: - have custom secure boot platform key - use a unified kernel image (UKI) which means I directly boot the kernel from efi (and place it in the efi partition) - sign the image with that platform key (I use sbctrl) - have every thing else including swap partition for hybernation fully disk encrypted, I could set it up to auto unlock using TPM2 but I would recommend using a long password. TPM2+password would be optimal. There had been too many cases of leaky TPMs and especially on a laptop you don't want to fully rely on it (through you in turn could decide to auto login if PCRs are unchanged, or login using only the (often not so secure) fingerprint reader etc.) - efi password, I mean if you don't set that you lose most secure boot benefits... EDIT: Not really most, there is still a bunch of ways it helps but it's anyway a bad idea to rely on secure boot and not have a efi password As bonus tip: - include the vfat in your initramfs (i.e. `MODULES=(vfat)` in `/etc/mkinitcpio.conf`) if your booting kernel and installed kernel modules ever mismatch that is nice to have to fix the issue

mixmastamyk 2y ago
What are the details with a custom key?
- deleted 2y ago
  [deleted]
- dathinab 2y ago
  sbctl with package manager hook for automatically signing on updates etc. keys are just stored on the device, for the typical laptop use-case this is good enough (platform key only used by a single device, no MDA or anything like that)
jcalvinowens 2y ago
> I could set it up to auto unlock using TPM2 but I would recommend using a long password. TPM2+password would be optimal. Personally, I trust LUKS with passphrases far more than I trust some random proprietary hardware implementation nobody can audit... It's also important to me to be able to recover the disk contents with the passphrase on another machine if the motherboard dies. Maybe that's what you meant (backup passphrase), but I think you meant requiring both?
- dathinab 2y ago
  I meant: - I'm only using a long password - but it would be optimal to require PCR values and password Note that in any case where you use PCR values you always should setup a secondary way to unlock the partition. Or else you will lose your data if some of your hardware measured into a PCR breaks. Requiring both is optimal as it 1. doesn't rely on TPM/PCRs but 2. prevent certain attack vectors possible with password only but not possible with PCRs. Through you now also have to manage a backup unlock method. Which is annoying. And the security benefits are negligible/irrelevant for most people. Which is why I don't use it.
  - 3np 2y ago
    Thanks for sharing your setup! Nit: It's useful to distinguish between passwords (checked against a hash for auth) and passphrases (used for decryption). It's an important practical distinction because a lost password can in general be bypassed out-of-band somehow while a backup strategy for passphrases is essential.