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 909A5C433E9 for ; Tue, 16 Mar 2021 20:07:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 636CF64F76 for ; Tue, 16 Mar 2021 20:07:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238892AbhCPUGf (ORCPT ); Tue, 16 Mar 2021 16:06:35 -0400 Received: from smtp-8fad.mail.infomaniak.ch ([83.166.143.173]:55577 "EHLO smtp-8fad.mail.infomaniak.ch" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237942AbhCPUG1 (ORCPT ); Tue, 16 Mar 2021 16:06:27 -0400 Received: from smtp-2-0001.mail.infomaniak.ch (unknown [10.5.36.108]) by smtp-2-3000.mail.infomaniak.ch (Postfix) with ESMTPS id 4F0PSJ3SyNzMqHf6; Tue, 16 Mar 2021 21:06:24 +0100 (CET) Received: from ns3096276.ip-94-23-54.eu (unknown [23.97.221.149]) by smtp-2-0001.mail.infomaniak.ch (Postfix) with ESMTPA id 4F0PSG4YVGzlh8TH; Tue, 16 Mar 2021 21:06:22 +0100 (CET) Subject: Re: [PATCH v4 1/1] fs: Allow no_new_privs tasks to call chroot(2) To: Jann Horn Cc: Al Viro , James Morris , Serge Hallyn , Andy Lutomirski , Casey Schaufler , Christian Brauner , Christoph Hellwig , David Howells , Dominik Brodowski , "Eric W . Biederman" , John Johansen , Kees Cook , Kentaro Takeda , Tetsuo Handa , Kernel Hardening , linux-fsdevel , kernel list , linux-security-module , =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= References: <20210316170135.226381-1-mic@digikod.net> <20210316170135.226381-2-mic@digikod.net> From: =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= Message-ID: Date: Tue, 16 Mar 2021 21:06:25 +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 16/03/2021 20:31, Jann Horn wrote: > On Tue, Mar 16, 2021 at 8:26 PM Mickaël Salaün wrote: >> On 16/03/2021 20:04, Jann Horn wrote: >>> On Tue, Mar 16, 2021 at 6:02 PM Mickaël Salaün wrote: >>>> 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). >>>> >>>> Unprivileged chroot is especially interesting for userspace developers >>>> wishing to harden their applications. For instance, chroot(2) and Yama >>>> enable to build a capability-based security (i.e. remove filesystem >>>> ambient accesses) by calling chroot/chdir with an empty directory and >>>> accessing data through dedicated file descriptors obtained with >>>> openat2(2) and RESOLVE_BENEATH/RESOLVE_IN_ROOT/RESOLVE_NO_MAGICLINKS. >>> >>> I don't entirely understand. Are you writing this with the assumption >>> that a future change will make it possible to set these RESOLVE flags >>> process-wide, or something like that? >> >> No, this scenario is for applications willing to sandbox themselves and >> only use the FDs to access legitimate data. > > But if you're chrooted to /proc/self/fdinfo and have an fd to some > directory - let's say /home/user/Downloads - there is nothing that > ensures that you only use that fd with RESOLVE_BENEATH, right? If the > application is compromised, it can do something like openat(fd, > "../.bashrc", O_RDWR), right? Or am I missing something? You're totally right, I was mistaken, this simple use case doesn't work without a broker. Perhaps when seccomp will be able to check referenced structs, or with a new FD limitation… > >>> As long as that doesn't exist, I think that to make this safe, you'd >>> have to do something like the following - let a child process set up a >>> new mount namespace for you, and then chroot() into that namespace's >>> root: >>> >>> struct shared_data { >>> int root_fd; >>> }; >>> int helper_fn(void *args) { >>> struct shared_data *shared = args; >>> mount("none", "/tmp", "tmpfs", MS_NOSUID|MS_NODEV, ""); >>> mkdir("/tmp/old_root", 0700); >>> pivot_root("/tmp", "/tmp/old_root"); >>> umount("/tmp/old_root", ""); >>> shared->root_fd = open("/", O_PATH); >>> } >>> void setup_chroot() { >>> struct shared_data shared = {}; >>> prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0); >>> clone(helper_fn, my_stack, >>> CLONE_VFORK|CLONE_VM|CLONE_FILES|CLONE_NEWUSER|CLONE_NEWNS|SIGCHLD, >>> NULL); >>> fchdir(shared.root_fd); >>> chroot("."); >>> } >> >> What about this? >> chdir("/proc/self/fdinfo"); >> chroot("."); >> close(all unnecessary FDs); > > That breaks down if you can e.g. get a unix domain socket connected to > a process in a different chroot, right? Isn't that a bit too fragile? This relies on other (trusted) components, and yes it is fragile if the process communicates with a service able send FDs.