this post was submitted on 19 Oct 2023

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Warning: ubuntu tpm recovery key cannot unlock disk outside of boot (lemmy.world)

submitted 11 months ago* (last edited 11 months ago) by ichbinjasokreativ@lemmy.world to c/linux@lemmy.ml

13 comments fedilink hide all child comments

Hi everybody, bit of a warning here: The recovery key generated during the installation of Ubuntu 23.10 (if you select tpm-backed fde) cannot be used to unlock the disk outside of boot, as in any 'cryptsetup' command and so on will not accept the recovery key. unlocking when accessed from different system does not work etc.

You can use it to unlock the disk while booting if your tpm somehow fails, but ONLY in that specific situation.

I kind of purposefully broke my tpm keys to see if it could be restored with 23.10 and ended up having to reinstal, as I ended up having to enter the recovery key at boot every time and no way of adding additional unlock options to the volume, as cryptsetup would not accept the recovery key as passphrase.

This bug could be very bad for new users.

See this bug report: https://bugs.launchpad.net/ubuntu-desktop-installer/+bug/2039741

all 14 comments

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[–] mvirts@lemmy.world 15 points 11 months ago (1 children)

Yet another reason why I wouldn't put anything important on a tpm encrypted volume 😹 I just don't trust it

[–] skullgiver@popplesburger.hilciferous.nl 10 points 11 months ago* (last edited 10 months ago) (2 children)

[This comment has been deleted by an automated system]

[–] ichbinjasokreativ@lemmy.world 3 points 11 months ago (1 children)

That's true, but the issue is that cryptsetup does not accept the recovery key as a passphrase for the disk. Once the tpm gets reset, the user has to always enter the recovery key and cannot implement a new key to luks and the tpm.

[+] skullgiver@popplesburger.hilciferous.nl 4 points 11 months ago* (last edited 10 months ago) (2 children)

[deleted]

[–] ichbinjasokreativ@lemmy.world 2 points 11 months ago

I wasn't blaming cryptsetup, my mistake if it came across as though I did.

Thank you for taking the time to look into a possible fix, I might just reinstall with tpm-backed fde again to see if this really works.

[–] MAFoElffen@lemmy.world 1 points 11 months ago* (last edited 11 months ago) (1 children)

@Skull giver -- I mentioned this above, but couldn't link it to you.

I like the code but the go run recover.go 2> key.text does not redirect the key to a file. It does not get input of the recovery key, so errors. Could you please add a few lines to write it directly to that file, instead of displaying it? (or output to both?)

As someone thought, what is displayed onscreen is jibberish, because console cannot display raw hex characters... I'm thinking the LUKS key in the keyslot is raw or hex.

As I now know, it is translated. And we can get the recovery key (hopefully) translated the other way around...

If I can just find the valid key value, and write it to a file, then I can help @inchbinjasokreativ to write it back to his TPM. I've already written a BASH script to do that, but am just missing that key-file. I have the problem replicated to a VM, so can test it on that first.

[–] skullgiver@popplesburger.hilciferous.nl 2 points 11 months ago* (last edited 10 months ago) (1 children)

[This comment has been deleted by an automated system]

[–] MAFoElffen@lemmy.world 3 points 11 months ago (1 children)

Success! I have a working key file...

root@ubuntu:/home/ubuntu/Downloads# cryptsetup -v luksOpen /dev/sda3 luks-01 --key-file ./key-file.key
No usable token is available.
Warning: keyslot operation could fail as it requires more than available memory.
Key slot 1 unlocked.
Command successful.
root@ubuntu:/home/ubuntu/Downloads# cryptsetup -v luksOpen /dev/sda4 luks-02 --key-file ./key-file.key
No usable token is available.
Warning: keyslot operation could fail as it requires more than available memory.
Key slot 1 unlocked.
Command successful.

Success on the first volume, which I picked as first because it was only 53M in size. Mounted it to /mnt... And guess what I found inside it?

root@ubuntu:/home/ubuntu/Downloads# ls -l /mnt/device/private-keys-v1/
total 4
-rw------- 1 root root 2459 Oct 18 18:29 O8CbAEpnfm7jGKkMqnokmdMBlE1oV6Xma_bUNudlshDYPxE4aJNhbhiGnF360Ze4

That is a key, but not connected to either LUKS container there... I dumped the headers of both LUKS. There are 2 key-slots, and the key translated from the recovery key is in slot one of both containers, The second key-slot's key must be the TPM's key, which is unknown if that is stored anywhere except the TPM...

But is shouldn't matter now... Because that key-file did work to add a new passphrase to both LUKS containers.

Thank you @Skull giver.

[–] skullgiver@popplesburger.hilciferous.nl 2 points 11 months ago* (last edited 10 months ago)

[This comment has been deleted by an automated system]

[–] kautau@lemmy.world 1 points 11 months ago* (last edited 11 months ago)

Yeah I’m hybrid Windows/Linux user, but many of my drives are Bitlocker encrypted. I need to install a bios update. To do so, it requires me to decrypt every bitlocker drive and not just the OS drive. Because the TPM keystore is based on the OS key store for each bitlocker drive. It’s frustrating, but it makes sense, so I’m all for additional security

[–] erik1984@lemmy.world 1 points 11 months ago (1 children)

Is it still possible to add an extra key (in another slot) to unlock it with cryptsetup? Adding this extra key might beat the purpose of using the TPM but if you choose a long random key and store it in a password manager that should still be pretty safe right?

[–] ichbinjasokreativ@lemmy.world 2 points 11 months ago

it's not, because to to so you need to enter a valid passphrase to cryptsetup, which the recovery key is NOT

[–] stealthnerd@lemmy.world 1 points 11 months ago (1 children)

Okay so I just read up on this. It's it true that TPM backed FDE only allows snaps?!?

Debs are completely unsupported?

[–] ichbinjasokreativ@lemmy.world 1 points 11 months ago

The ubuntu implementation is based on snap, but if you want to purge snapd (just install another distro at that point though), you can implement encryption as usual and add the tpm later on. Snap is only neccessary if you enable tpm-backed fde from setup.