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Date: Wed, 11 Aug 2021 00:21:40 +0300 From: Jarkko Sakkinen <jarkko@...nel.org> To: Eric Biggers <ebiggers@...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 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
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