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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 EE11DC32750 for ; Tue, 13 Aug 2019 00:14:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CA24B20665 for ; Tue, 13 Aug 2019 00:14:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726749AbfHMAOZ (ORCPT ); Mon, 12 Aug 2019 20:14:25 -0400 Received: from outgoing-auth-1.mit.edu ([18.9.28.11]:35401 "EHLO outgoing.mit.edu" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726453AbfHMAOZ (ORCPT ); Mon, 12 Aug 2019 20:14:25 -0400 Received: from callcc.thunk.org (guestnat-104-133-9-109.corp.google.com [104.133.9.109] (may be forged)) (authenticated bits=0) (User authenticated as tytso@ATHENA.MIT.EDU) by outgoing.mit.edu (8.14.7/8.12.4) with ESMTP id x7D0E7I4017125 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 12 Aug 2019 20:14:08 -0400 Received: by callcc.thunk.org (Postfix, from userid 15806) id 1BBDD4218EF; Mon, 12 Aug 2019 20:14:07 -0400 (EDT) Date: Mon, 12 Aug 2019 20:14:07 -0400 From: "Theodore Y. Ts'o" To: Eric Biggers Cc: linux-fscrypt@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-mtd@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-crypto@vger.kernel.org, keyrings@vger.kernel.org, linux-api@vger.kernel.org, Satya Tangirala , Paul Crowley , Jaegeuk Kim Subject: Re: [PATCH v8 14/20] fscrypt: allow unprivileged users to add/remove keys for v2 policies Message-ID: <20190813001406.GI28705@mit.edu> References: <20190805162521.90882-1-ebiggers@kernel.org> <20190805162521.90882-15-ebiggers@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190805162521.90882-15-ebiggers@kernel.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Mon, Aug 05, 2019 at 09:25:15AM -0700, Eric Biggers wrote: > From: Eric Biggers > > Allow the FS_IOC_ADD_ENCRYPTION_KEY and FS_IOC_REMOVE_ENCRYPTION_KEY > ioctls to be used by non-root users to add and remove encryption keys > from the filesystem-level crypto keyrings, subject to limitations. > > Motivation: while privileged fscrypt key management is sufficient for > some users (e.g. Android and Chromium OS, where a privileged process > manages all keys), the old API by design also allows non-root users to > set up and use encrypted directories, and we don't want to regress on > that. Especially, we don't want to force users to continue using the > old API, running into the visibility mismatch between files and keyrings > and being unable to "lock" encrypted directories. > > Intuitively, the ioctls have to be privileged since they manipulate > filesystem-level state. However, it's actually safe to make them > unprivileged if we very carefully enforce some specific limitations. > > First, each key must be identified by a cryptographic hash so that a > user can't add the wrong key for another user's files. For v2 > encryption policies, we use the key_identifier for this. v1 policies > don't have this, so managing keys for them remains privileged. > > Second, each key a user adds is charged to their quota for the keyrings > service. Thus, a user can't exhaust memory by adding a huge number of > keys. By default each non-root user is allowed up to 200 keys; this can > be changed using the existing sysctl 'kernel.keys.maxkeys'. > > Third, if multiple users add the same key, we keep track of those users > of the key (of which there remains a single copy), and won't really > remove the key, i.e. "lock" the encrypted files, until all those users > have removed it. This prevents denial of service attacks that would be > possible under simpler schemes, such allowing the first user who added a > key to remove it -- since that could be a malicious user who has > compromised the key. Of course, encryption keys should be kept secret, > but the idea is that using encryption should never be *less* secure than > not using encryption, even if your key was compromised. > > We tolerate that a user will be unable to really remove a key, i.e. > unable to "lock" their encrypted files, if another user has added the > same key. But in a sense, this is actually a good thing because it will > avoid providing a false notion of security where a key appears to have > been removed when actually it's still in memory, available to any > attacker who compromises the operating system kernel. > > Signed-off-by: Eric Biggers Looks good. I'd probably would have used either "mk_secret_sem" or "mk->mk_secret_sem" in the comments, instead of "->mk_securet_sem", but that's just a personal style preference. Since you consistently used the latter, I assume that's a deliberate choice, which is fine. Feel free to add: Reviewed-by: Theodore Ts'o