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=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 DA8C3C43331 for ; Tue, 16 Mar 2021 19:28:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B51BA64FE4 for ; Tue, 16 Mar 2021 19:28:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240433AbhCPT1m (ORCPT ); Tue, 16 Mar 2021 15:27:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60000 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240436AbhCPT0t (ORCPT ); Tue, 16 Mar 2021 15:26:49 -0400 Received: from smtp-190f.mail.infomaniak.ch (smtp-190f.mail.infomaniak.ch [IPv6:2001:1600:3:17::190f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5F107C061756 for ; Tue, 16 Mar 2021 12:26:46 -0700 (PDT) Received: from smtp-2-0001.mail.infomaniak.ch (unknown [10.5.36.108]) by smtp-2-3000.mail.infomaniak.ch (Postfix) with ESMTPS id 4F0NZY4lYKzMqDZ6; Tue, 16 Mar 2021 20:26:45 +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 4F0NZX3Hbrzlh8TK; Tue, 16 Mar 2021 20:26:44 +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 20:26:46 +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 20:04, 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. > > > 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); > > [...] >> diff --git a/fs/open.c b/fs/open.c > [...] >> +static inline int current_chroot_allowed(void) >> +{ >> + /* >> + * Changing the root directory for the calling task (and its future >> + * children) requires that this task has CAP_SYS_CHROOT in its >> + * namespace, or be running with no_new_privs and not sharing its >> + * fs_struct and not escaping its current root (cf. create_user_ns()). >> + * As for seccomp, checking no_new_privs avoids scenarios where >> + * unprivileged tasks can affect the behavior of privileged children. >> + */ >> + if (task_no_new_privs(current) && current->fs->users == 1 && > > this read of current->fs->users should be using READ_ONCE() Right, I'll fix this. > >> + !current_chrooted()) >> + return 0; >> + if (ns_capable(current_user_ns(), CAP_SYS_CHROOT)) >> + return 0; >> + return -EPERM; >> +} > [...] > > Overall I think this change is a good idea. >