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

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Date:   Tue, 17 Aug 2021 16:13:09 +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

On 17.08.21 15:55, Mimi Zohar wrote:
> On Tue, 2021-08-17 at 15:04 +0200, Ahmad Fatoum wrote:
>> 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?
> 
> With some applications, the indirection is important.   It allows the
> "encrypted" key type to be updated/re-encypted based on a new "trusted"
> key, without affecting the on disk encrypted key usage.

Those applications were already able to use the encrypted key support
in dm-crypt. For those where re-encryption/PCR-sealing isn't required,
direct trusted key support offers a simpler way to integrate.

> As much as I expected, directly using "trusted" keys is a result of the
> new trusted key sources.

More users = more use cases. You make it sound like a negative
thing.

> I have no opinion as to whether this is/isn't a valid usecase.

So you'd be fine with merging trusted key support as is and leave encrypted
key support to someone who has a valid use case and wants to argue
in its favor?

Cheers,
Ahmad

> 
> thanks,
> 
> Mimi
> 
> 


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