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=-13.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL 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 91D09C433E9 for ; Wed, 10 Mar 2021 19:34:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 64C1264FD7 for ; Wed, 10 Mar 2021 19:34:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230173AbhCJTdy (ORCPT ); Wed, 10 Mar 2021 14:33:54 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41330 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233950AbhCJTdl (ORCPT ); Wed, 10 Mar 2021 14:33:41 -0500 Received: from mail-lj1-x22f.google.com (mail-lj1-x22f.google.com [IPv6:2a00:1450:4864:20::22f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B6B2CC061761 for ; Wed, 10 Mar 2021 11:33:40 -0800 (PST) Received: by mail-lj1-x22f.google.com with SMTP id q14so27141672ljp.4 for ; Wed, 10 Mar 2021 11:33:40 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc:content-transfer-encoding; bh=C0lOxwjMEr6SWX1hFkOaQGtQdtdGB5FiXwich6XA348=; b=NM0J3hx50vHdC/p5h3lzfZtYNUEYEEEUohjNigiA0KbbFneszGz381/p3GLd6yhcWu 1KuK6TsD+n34eVK+nnQVCLf7Ie1gBju2CVq9ZLZdvETqSzALoCaMBhnofjKv/3sNUuYN Q+U9UAUcm9S3x+gB24/4Dfqmv17Vzecl9QcOThzFQv18IpgBij82+yGRMQkEuNTIH62P CHqkLGTcxXMX4XVmnYWQNqi/lUVZXi/3hSoVBkV7dSo11C67xn4g9eGuGxYzntTDETq0 gZmbcBXsEeXMzgX7wVy0G9U7fO0WveTk3yR14wSE1t3VhTeVYQHPwgmWcjKqCHW98c+d QxrA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc:content-transfer-encoding; bh=C0lOxwjMEr6SWX1hFkOaQGtQdtdGB5FiXwich6XA348=; b=iuvZyklEkpy2/OYbXKvdxtX5VgaiL/bUd0xziNmuda6f6MuPXJOQg8nKbRZG42LApS kS9J9FY2HPIor5sIDmdNIMxFWdlGBiyHCATvGiZt7tlYIE8C2EEB7t+1Vm0FwJugqSEW d46JHBYzJbryH+CZYpy3jAou9pCSUxxrhGyM3lYyZAAuow67BchIvhFsbq04Ic5mU5Hc UvbGXvGBVUOfKH/thrOG2S7FWfeY5TDUP/l/znBrYxvb/BsVdXZhq0AQWu3cojfkJ1xD MuGA0D4/i2935QyIEXiRjXlJioYQGTGGSRlgHZYackszrJg9dnLW6X5viO0TPyGE3laO qedA== X-Gm-Message-State: AOAM532ZcPTY++VDgJVymbrhYke1H1DczjBgOJFUcATb3eq38mq4yYUi UOvDXN9DywEHUaKKEk6mk3lCw6OuSFujrgtu2n1yDQ== X-Google-Smtp-Source: ABdhPJywfvsHOIz0NkR2nMiYoKEhfTkPo9yaeevm6MgBPXJj4NmO1bY3FBXA9+0DF6QnwYxqUt/SZcMZ24QK6nH4d6g= X-Received: by 2002:a2e:9bcd:: with SMTP id w13mr2643039ljj.43.1615404819005; Wed, 10 Mar 2021 11:33:39 -0800 (PST) MIME-Version: 1.0 References: <20210310181857.401675-1-mic@digikod.net> <20210310181857.401675-2-mic@digikod.net> In-Reply-To: From: Jann Horn Date: Wed, 10 Mar 2021 20:33:12 +0100 Message-ID: Subject: Re: [PATCH v2 1/1] fs: Allow no_new_privs tasks to call chroot(2) To: "Eric W. Biederman" Cc: =?UTF-8?B?TWlja2HDq2wgU2FsYcO8bg==?= , Al Viro , James Morris , Serge Hallyn , Andy Lutomirski , Casey Schaufler , Christian Brauner , Christoph Hellwig , David Howells , Dominik Brodowski , John Johansen , Kees Cook , Kentaro Takeda , Tetsuo Handa , Kernel Hardening , linux-fsdevel , kernel list , linux-security-module , =?UTF-8?B?TWlja2HDq2wgU2FsYcO8bg==?= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: On Wed, Mar 10, 2021 at 8:23 PM Eric W. Biederman w= rote: > > Micka=C3=ABl Sala=C3=BCn writes: > > > From: Micka=C3=ABl Sala=C3=BCn > > > > 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.). > > > > 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 discus= s > > 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 specia= l > > 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 no= t > > 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, becaus= e > > 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 i= s > > 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 o= f > > 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; th= e > > 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, bu= t > > with less limitations: > > https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1= 327858005.git.luto@amacapital.net/ [...] > Neither is_path_beneath nor path_is_under really help prevent escapes, > as except for open files and files accessible from proc chroot already > disallows going up. The reason is the path is resolved with the current > root before switching to it. Yeah, this probably should use the same check as the CLONE_NEWUSER logic, current_chrooted() from CLONE_NEWUSER; that check is already used for guarding against the following syscall sequence, which has similar security properties: unshare(CLONE_NEWUSER); // gives the current process namespaced CAP_SYS_ADM= IN chroot("<...>"); // succeeds because of namespaced CAP_SYS_ADMIN The current_chrooted() check in create_user_ns() is for the same purpose as the check you're introducing here, so they should use the same logic.