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=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 2BD7BC433E9 for ; Wed, 10 Mar 2021 18:14:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0696664FB2 for ; Wed, 10 Mar 2021 18:14:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233604AbhCJSNv (ORCPT ); Wed, 10 Mar 2021 13:13:51 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52126 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233428AbhCJSNn (ORCPT ); Wed, 10 Mar 2021 13:13:43 -0500 Received: from smtp-8fab.mail.infomaniak.ch (smtp-8fab.mail.infomaniak.ch [IPv6:2001:1600:3:17::8fab]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A6446C061761 for ; Wed, 10 Mar 2021 10:13:42 -0800 (PST) Received: from smtp-3-0001.mail.infomaniak.ch (unknown [10.4.36.108]) by smtp-2-3000.mail.infomaniak.ch (Postfix) with ESMTPS id 4DwgDz4NX7zMqMqc; Wed, 10 Mar 2021 19:13:39 +0100 (CET) Received: from ns3096276.ip-94-23-54.eu (unknown [23.97.221.149]) by smtp-3-0001.mail.infomaniak.ch (Postfix) with ESMTPA id 4DwgDt0Ylzzlh8TK; Wed, 10 Mar 2021 19:13:33 +0100 (CET) Subject: Re: [PATCH v1 1/1] fs: Allow no_new_privs tasks to call chroot(2) To: "Eric W. Biederman" 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> From: =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= Message-ID: <5edd8272-a2d5-028d-28da-de76a93f2fa4@digikod.net> Date: Wed, 10 Mar 2021 19:13:33 +0100 User-Agent: MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/03/2021 17:56, Eric W. Biederman wrote: > 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("../../../../../../../../.."). Not with this patch. > > 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. chroot(2) is simple. > > 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. Could you please provide a new attack scenario? > > 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. They are explained in this commit message. > > Eric >