Received-SPF: pass (google.com: domain of linux-crypto-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Message-ID: <285cb263d9c1c16f3918c98dd36074ef16568e6d.camel@linux.ibm.com>
Subject: Re: [PATCH v2] fscrypt: support trusted keys
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Ahmad Fatoum <a.fatoum@pengutronix.de>,
        Eric Biggers <ebiggers@kernel.org>,
        Jarkko Sakkinen <jarkko@kernel.org>
Cc:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        Jaegeuk Kim <jaegeuk@kernel.org>, kernel@pengutronix.de,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        James Bottomley <jejb@linux.ibm.com>,
        Sumit Garg <sumit.garg@linaro.org>,
        David Howells <dhowells@redhat.com>,
        linux-fscrypt@vger.kernel.org, linux-crypto@vger.kernel.org,
        linux-integrity@vger.kernel.org,
        linux-security-module@vger.kernel.org, keyrings@vger.kernel.org,
        linux-kernel@vger.kernel.org
Date:   Tue, 17 Aug 2021 09:55:13 -0400
In-Reply-To: <a6eb6f38-b9f4-c59c-4181-2049f181e67d@pengutronix.de>
References: <20210806150928.27857-1-a.fatoum@pengutronix.de>
         <20210809094408.4iqwsx77u64usfx6@kernel.org> <YRGVcaquAJiuc8bp@gmail.com>
         <20210810180636.vqwaeftv7alsodgn@kernel.org> <YRLJmaafp941uOdA@gmail.com>
         <20210810212140.sdq5dq2wy5uaj7h7@kernel.org> <YRLvPJehAeMiYb2Z@gmail.com>
         <20210811001743.ofzkwdwa6rcjsf4d@kernel.org>
         <d4f5c2593380c82ceebae2c8782a1c440b35f165.camel@linux.ibm.com>
         <YRQF09f8st95yrFZ@gmail.com>
         <0e69a0aa394dd20347b06ae4e700aa17d52583ef.camel@linux.ibm.com>
         <a6eb6f38-b9f4-c59c-4181-2049f181e67d@pengutronix.de>
Content-Type: text/plain; charset="ISO-8859-15"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Precedence: bulk

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.

As much as I expected, directly using "trusted" keys is a result of the
new trusted key sources.  I have no opinion as to whether this is/isn't
a valid usecase.

thanks,

Mimi