linux-kernel - Re: [PATCH v2] fscrypt: support trusted keys

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Message-ID: <a6eb6f38-b9f4-c59c-4181-2049f181e67d@pengutronix.de>
Date:   Tue, 17 Aug 2021 15:04:59 +0200
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Jarkko Sakkinen <jarkko@...nel.org>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>, kernel@...gutronix.de,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Sumit Garg <sumit.garg@...aro.org>,
        David Howells <dhowells@...hat.com>,
        linux-fscrypt@...r.kernel.org, linux-crypto@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fscrypt: support trusted keys

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
> 
> 


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