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

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Date:   Tue, 10 Aug 2021 14:27:24 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Ahmad Fatoum <a.fatoum@...gutronix.de>,
        "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>,
        Mimi Zohar <zohar@...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 Wed, Aug 11, 2021 at 12:21:40AM +0300, Jarkko Sakkinen wrote:
> On Tue, Aug 10, 2021 at 11:46:49AM -0700, Eric Biggers wrote:
> > On Tue, Aug 10, 2021 at 09:06:36PM +0300, Jarkko Sakkinen wrote:
> > > > > 
> > > > > I don't think this is right, or at least it does not follow the pattern
> > > > > in [*]. I.e. you should rather use trusted key to seal your fscrypt key.
> > > > 
> > > > What's the benefit of the extra layer of indirection over just using a "trusted"
> > > > key directly?  The use case for "encrypted" keys is not at all clear to me.
> > > 
> > > Because it is more robust to be able to use small amount of trusted keys,
> > > which are not entirely software based.
> > > 
> > > And since it's also a pattern on existing kernel features utilizing trusted
> > > keys, the pressure here to explain why break the pattern, should be on the
> > > side of the one who breaks it.
> > 
> > This is a new feature, so it's on the person proposing the feature to explain
> > why it's useful.  The purpose of "encrypted" keys is not at all clear, and the
> > documentation for them is heavily misleading.  E.g.:
> > 
> >     "user space sees, stores, and loads only encrypted blobs"
> >     (Not necessarily true, as I've explained previously.)
> > 
> >     "Encrypted keys do not depend on a trust source" ...  "The main disadvantage
> >     of encrypted keys is that if they are not rooted in a trusted key"
> >     (Not necessarily true, and in fact it seems they're only useful when they
> >     *do* depend on a trust source.  At least that's the use case that is being
> >     proposed here, IIUC.)
> > 
> > I do see a possible use for the layer of indirection that "encrypted" keys are,
> > which is that it would reduce the overhead of having lots of keys be directly
> > encrypted by the TPM/TEE/CAAM.  Is this the use case?  If so, it needs to be
> > explained.
> 
> If trusted keys are used directly, it's an introduction of a bottleneck.
> If they are used indirectly, you can still choose to have one trusted
> key per fscrypt key.
> 
> Thus, it's obviously a bad idea to use them directly.
> 

So actually explain that in the documentation.  It's not obvious at all.  And
does this imply that the support for trusted keys in dm-crypt is a mistake?

- Eric