From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0889BC4320A for ; Wed, 11 Aug 2021 00:17:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DAFDB61019 for ; Wed, 11 Aug 2021 00:17:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235698AbhHKASJ (ORCPT ); Tue, 10 Aug 2021 20:18:09 -0400 Received: from mail.kernel.org ([198.145.29.99]:34146 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235680AbhHKASI (ORCPT ); Tue, 10 Aug 2021 20:18:08 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 3A02360FD9; Wed, 11 Aug 2021 00:17:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1628641065; bh=vS0O2G4UqYCFp5XbO0UvX+Z3o0P/EBLHkKPuQEwAbYc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=m90I8MFYojVSKUZbGrXDKIJU8CpLr0ZkbA+yZJ31ujVzplnqw+lVK5V/WFofFfbtY YJl9Jd8kVgfFQCL9hbNlX9aweGNOlBnrqZIJ9e4oC+6oIVBy1U/VedMB5GddqhD3LM cln6DPrUnDQhiyi5vhboXLklmcr7o5JtK2kL7kOfy0PeZSNFlhfb9Re4Gf4Vihz9mv lHRW2GirAxlHKqUc/U3nW1jlQjlQpWkfZslp8ai02bxlodrclzFuu6nWxKjArgLu7y zDLZrBbiQgbtJxOd4J3SbyVFFzZXbcQASgAcBjkavW6dgY3w08MWp3qldid9INCFEL aFjN4yuBuj1TQ== Date: Wed, 11 Aug 2021 03:17:43 +0300 From: Jarkko Sakkinen To: Eric Biggers Cc: Ahmad Fatoum , "Theodore Y. Ts'o" , Jaegeuk Kim , kernel@pengutronix.de, James Morris , "Serge E. Hallyn" , James Bottomley , Mimi Zohar , Sumit Garg , David Howells , 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 Message-ID: <20210811001743.ofzkwdwa6rcjsf4d@kernel.org> References: <20210806150928.27857-1-a.fatoum@pengutronix.de> <20210809094408.4iqwsx77u64usfx6@kernel.org> <20210810180636.vqwaeftv7alsodgn@kernel.org> <20210810212140.sdq5dq2wy5uaj7h7@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org 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. /Jarkko