On Wed, Aug 11, 2021 at 07:34:18AM -0400, Mimi Zohar wrote: > On Wed, 2021-08-11 at 03:17 +0300, Jarkko Sakkinen wrote: > > 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. > > At least for TPM 1.2, "trusted" keys may be sealed to a PCR and then > extended to prevent subsequent usage. For example, in the initramfs > all of the "encrypted" keys could be decrypted by a single "trusted" > key, before extending the PCR. > > Mimi > 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? - Eric
