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=-3.8 required=3.0 tests=BAD_ENC_HEADER,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,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 769A1C43603 for ; Wed, 10 Mar 2021 16:57:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 50D7D64FF7 for ; Wed, 10 Mar 2021 16:57:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233571AbhCJQ4e convert rfc822-to-8bit (ORCPT ); Wed, 10 Mar 2021 11:56:34 -0500 Received: from out01.mta.xmission.com ([166.70.13.231]:37178 "EHLO out01.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233548AbhCJQ4E (ORCPT ); Wed, 10 Mar 2021 11:56:04 -0500 Received: from in01.mta.xmission.com ([166.70.13.51]) by out01.mta.xmission.com with esmtps (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.93) (envelope-from ) id 1lK27t-00E2Bv-7C; Wed, 10 Mar 2021 09:56:01 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=fess.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1lK27s-0002lw-6T; Wed, 10 Mar 2021 09:56:00 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= Cc: Al Viro , James Morris , Serge Hallyn , Andy Lutomirski , Christian Brauner , Christoph Hellwig , David Howells , Dominik Brodowski , John Johansen , Kees Cook , Kentaro Takeda , Tetsuo Handa , kernel-hardening@lists.openwall.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= References: <20210310161000.382796-1-mic@digikod.net> <20210310161000.382796-2-mic@digikod.net> Date: Wed, 10 Mar 2021 10:56:04 -0600 In-Reply-To: <20210310161000.382796-2-mic@digikod.net> (=?utf-8?Q?=22Micka?= =?utf-8?Q?=C3=ABl_Sala=C3=BCn=22's?= message of "Wed, 10 Mar 2021 17:10:00 +0100") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT X-XM-SPF: eid=1lK27s-0002lw-6T;;;mid=;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX18KLsREVcGmRhumyXmrSYgLMNzBj8iNk6o= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH v1 1/1] fs: Allow no_new_privs tasks to call chroot(2) X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Mickaël Salaün writes: > From: Mickaël Salaün > > Being able to easily change root directories enable to ease some > development workflow and can be used as a tool to strengthen > unprivileged security sandboxes. chroot(2) is not an access-control > mechanism per se, but it can be used to limit the absolute view of the > filesystem, and then limit ways to access data and kernel interfaces > (e.g. /proc, /sys, /dev, etc.). Actually chroot does not so limit the view of things. It only limits the default view. A process that is chrooted can always escape by something like chroot("../../../../../../../../.."). So I don't see the point of allowing chroot once you are in your locked down sandbox. > Users may not wish to expose namespace complexity to potentially > malicious processes, or limit their use because of limited resources. > The chroot feature is much more simple (and limited) than the mount > namespace, but can still be useful. As for containers, users of > chroot(2) should take care of file descriptors or data accessible by > other means (e.g. current working directory, leaked FDs, passed FDs, > devices, mount points, etc.). There is a lot of literature that discuss > the limitations of chroot, and users of this feature should be aware of > the multiple ways to bypass it. Using chroot(2) for security purposes > can make sense if it is combined with other features (e.g. dedicated > user, seccomp, LSM access-controls, etc.). > > One could argue that chroot(2) is useless without a properly populated > root hierarchy (i.e. without /dev and /proc). However, there are > multiple use cases that don't require the chrooting process to create > file hierarchies with special files nor mount points, e.g.: > * A process sandboxing itself, once all its libraries are loaded, may > not need files other than regular files, or even no file at all. > * Some pre-populated root hierarchies could be used to chroot into, > provided for instance by development environments or tailored > distributions. > * Processes executed in a chroot may not require access to these special > files (e.g. with minimal runtimes, or by emulating some special files > with a LD_PRELOADed library or seccomp). > > Allowing a task to change its own root directory is not a threat to the > system if we can prevent confused deputy attacks, which could be > performed through execution of SUID-like binaries. This can be > prevented if the calling task sets PR_SET_NO_NEW_PRIVS on itself with > prctl(2). To only affect this task, its filesystem information must not > be shared with other tasks, which can be achieved by not passing > CLONE_FS to clone(2). A similar no_new_privs check is already used by > seccomp to avoid the same kind of security issues. Furthermore, because > of its security use and to avoid giving a new way for attackers to get > out of a chroot (e.g. using /proc//root), an unprivileged chroot is > only allowed if the new root directory is the same or beneath the > current one. This still allows a process to use a subset of its > legitimate filesystem to chroot into and then further reduce its view of > the filesystem. > > This change may not impact systems relying on other permission models > than POSIX capabilities (e.g. Tomoyo). Being able to use chroot(2) on > such systems may require to update their security policies. > > Only the chroot system call is relaxed with this no_new_privs check; the > init_chroot() helper doesn't require such change. > > Allowing unprivileged users to use chroot(2) is one of the initial > objectives of no_new_privs: > https://www.kernel.org/doc/html/latest/userspace-api/no_new_privs.html > This patch is a follow-up of a previous one sent by Andy Lutomirski, but > with less limitations: > https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1327858005.git.luto@amacapital.net/ Last time I remember talking architecture we agreed that user namespaces would be used for enabling features and that no_new_privs would just be used to lock-down userspace. That way no_new_privs could be kept simple and trivial to audit and understand. You can build your sandbox and use chroot if you use a user namespace at the start. A mount namespace would also help lock things down. Still allowing chroot after the sanbox has been built, a seccomp filter has been installed and no_new_privs has been enabled seems like it is asking for trouble and may weaken existing sandboxes. So I think we need a pretty compelling use case to consider allowing chroot(2). You haven't even mentioned what your usecase is at this point so I don't know why we would tackle that complexity. Eric