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Comment by blastrock | Hacker News Reader

Very clever!

I am the author of one of the older guides https://blastrock.github.io/fde-tpm-sb.html .

I was wondering about the solution you propose which seems a bit complicated to me. Here's my idea, please tell me if I'm completely wrong here.

What if I put a file on the root filesystem with some random content (say 32 bytes), let's name it /prehash. I hash this file (sha256, blake2, whatever). Then, in the signed initrd, just after mounting the filesystem, I assert that hash(/prehash) == expected_hash or crash the system otherwise. Do you think it would be enough to fix the issue?

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I was reading another web page (I don't have the link unfortunately) several days ago where another reader pointed out to the author the same type of attack mentioned in this article. To address that attack the author came up with the same solution you proposed and I do believe that is sufficient for preventing the type of attack mentioned in this article. There still are other types of attacks (cold boot attack, sniffing TPM traffic, etc...) that can be done though so it still is a good idea to use a PIN/password, network bound disk encryption, etc... in addition to the the TPM.

I'm currently working on setting up disk encryption for a new home server and as an additional precaution I'm also working on getting the initrd to do a few additional sanity checks prior to decrypting a LUKS partition and prior to mounting the root file system within. One check which I think will be highly effective is that prior to decrypting the LUKS partition I have the initrd hash the entire LUKS header and make sure it has the expected value before allowing the boot to continue. So far it seems to be working OK but hashing the entire LUKS header is overkill which will require some care to make sure the expected hash value is kept updated if the LUKS header changes for some reason (like changing encryption passwords). I can not recommend this idea for everyone consequently.

Found the page I had mentioned earlier: https://pawitp.medium.com/the-correct-way-to-use-secure-boot... . In the comments Aleksandar mentioned the possibility of using the attack mentioned in this article and the author replied back with the same solution of verifying a secret file on the root partition.

Thanks for the link!

Funny to see that someone already thought of this attack in 2022

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