[LinusU]

Did you read the article? ;)

> From here it’s easy to manually use the TPM to unlock the disk with the Clevis tooling and mount the root volume for hacking (it takes a few tries sometimes, but it gets there in the end):

They use the exploit to get dropped into a root shell, and then asks the TPM to unlock the disk for them, which it promptly does.

[Guvante]

Technically, but really the fault lies in the OS for providing an exploitable prompt that doesn't break the chain of custody of the boot process on failure.

[rlpb]

Ubuntu developer here. I don't think that's fair. The OS, as shipped today, isn't designed with this threat model in mind. It's correct to say that for TPM-based LUKS unlock to be safe, the initramfs must not allow the user to take control of it. But that's a new requirement introduced when the user modified their system to configure TPM-based LUKS unlock. This isn't the responsibility of an OS that doesn't ship with that support, and in fact, prior to TPMs, it was perfectly reasonable to allow the user to take control of the initramfs prior to LUKS unlock!

That's not to say that an improvement can't be made as we move towards a future where TPM+LUKS might be the norm - just that you can't retrospectively claim fault on an OS that doesn't claim to support that.

[formerly_proven]

This is one of those designs where you skim a description and already know it’s going to break in a million different ways.

[betaby]

I did. It says 'From here it’s easy to manually use the TPM to unlock the disk with the Clevis tooling and mount the root volume for hacking (it takes a few tries sometimes, but it gets there in the end)'

However screenshot says 'Unsealing failing'. Yet Ubuntu-lv is mounted. I don't follow how Luks password was guessed for it.

[bri3d]

There wasn't a password necessary, the TPM was an unlocking mechanism.

Secure Boot with TPM-backed disk encryption works off of a series of numbered hashes. The idea of TPM based FDE is that the machine will use Secure Boot to boot only a software chain that the end-user trusts not to contain authentication bypasses. In Secure Boot, the EFI firmware provides hashes of each stage in the boot chain to the TPM, and the TPM only unlocks the full-disk encryption key (really the key encryption key, since the TPM isn't fast enough to actually decrypt the disk) slot if each stage / configuration is valid.

This issue breaks that chain. In some sense it's an illustration of this system being silly conceptually, but it is a real issue IMO.

[j0057]

No LUKS password was guessed, clevis-disk-unlock command in the last screenshot used the TPM to provide a key to a LUKS keyslot for getting at the actual decryption key to decrypt the disk. The TPM should have had information about the boot state to be able to refuse to provide the key, but didn't.