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From: Jarkko Sakkinen <jarkko@kernel.org>
To: Eric Biggers <ebiggers@kernel.org>
Cc: Ahmad Fatoum <a.fatoum@pengutronix.de>,
	"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>,
	Mimi Zohar <zohar@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
Subject: Re: [PATCH v2] fscrypt: support trusted keys
Date: Wed, 11 Aug 2021 00:21:40 +0300	[thread overview]
Message-ID: <20210810212140.sdq5dq2wy5uaj7h7@kernel.org> (raw)
In-Reply-To: <YRLJmaafp941uOdA@gmail.com>

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.

/Jarkko

  reply	other threads:[~2021-08-10 21:21 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-06 15:09 Ahmad Fatoum
2021-08-09  9:44 ` Jarkko Sakkinen
2021-08-09 10:00   ` Ahmad Fatoum
2021-08-09 10:02     ` Ahmad Fatoum
2021-08-10 18:02     ` Jarkko Sakkinen
2021-08-09 20:52   ` Eric Biggers
2021-08-10 18:06     ` Jarkko Sakkinen
2021-08-10 18:46       ` Eric Biggers
2021-08-10 21:21         ` Jarkko Sakkinen [this message]
2021-08-10 21:27           ` Eric Biggers
2021-08-11  0:17             ` Jarkko Sakkinen
2021-08-11 11:34               ` Mimi Zohar
2021-08-11 17:16                 ` Eric Biggers
2021-08-12  0:54                   ` Mimi Zohar
2021-08-17 13:04                     ` Ahmad Fatoum
2021-08-17 13:55                       ` Mimi Zohar
2021-08-17 14:13                         ` Ahmad Fatoum
2021-08-17 14:24                           ` Mimi Zohar
2021-08-18  2:09                             ` Jarkko Sakkinen
2021-08-18  4:53                             ` Sumit Garg
2021-08-09 21:24 ` Eric Biggers
2021-08-10  7:41   ` Ahmad Fatoum
2021-08-10 17:35     ` Eric Biggers

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