Re: [PATCH v2] fscrypt: support trusted keys

[Ahmad Fatoum]

Hi,

On 12.08.21 02:54, Mimi Zohar wrote:
> On Wed, 2021-08-11 at 10:16 -0700, Eric Biggers wrote:
> 
>> Neither of you actually answered my question, which is whether the support for
>> trusted keys in dm-crypt is a mistake.  I think you're saying that it is?  That
>> would imply that fscrypt shouldn't support trusted keys, but rather encrypted
>> keys -- which conflicts with Ahmad's patch which is adding support for trusted
>> keys.  Note that your reasoning for this is not documented at all in the
>> trusted-encrypted keys documentation; it needs to be (email threads don't really
>> matter), otherwise how would anyone know when/how to use this feature?
> 
> True, but all of the trusted-encrypted key examples in the
> documentation are "encrypted" type keys, encrypted/decrypted based on a
> "trusted" type key.  There are no examples of using the "trusted" key
> type directly.  Before claiming that adding "trusted" key support in
> dm-crypt was a mistake, we should ask Ahmad why he felt dm-crypt needed
> to directly support "trusted" type keys.

I wanted to persist the dm-crypt key as a sealed blob. With encrypted keys,
I would have to persist and unseal two blobs (load trusted key blob, load
encrypted key blob rooted to trusted key) with no extra benefit.

I thus added direct support for trusted keys. Jarkko even commented on the
thread, but didn't voice objection to the approach (or agreement for that
matter), so I assumed the approach is fine.

I can see the utility of using a single trusted key for TPMs, but for CAAM,
I see none and having an encrypted key for every trusted key just makes
it more cumbersome.

In v1 here, I added encrypted key support as well, but dropped it for v2,
because I am not in a position to justify its use. Now that you and Eric
discussed it, should I send v3 with support for both encrypted and trusted
keys like with dm-crypt or how should we proceed?

Cheers,
Ahmad

> 
> Mimi
> 
> 


-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |