[PATCH v2] fscrypt: support trusted keys
jarkko at kernel.org
Wed Aug 11 00:17:43 UTC 2021
On Tue, Aug 10, 2021 at 02:27:24PM -0700, Eric Biggers wrote:
> 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?
Looking at dm-crypt implementation, you can choose to use 'encrypted' key
type, which you can seal with a trusted key.
Note: I have not been involved when the feature was added to dm-crypt.
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