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 75837C433DB for ; Fri, 15 Jan 2021 09:12:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 24A0523435 for ; Fri, 15 Jan 2021 09:12:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730252AbhAOJMC (ORCPT ); Fri, 15 Jan 2021 04:12:02 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52094 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730254AbhAOJMA (ORCPT ); Fri, 15 Jan 2021 04:12:00 -0500 Received: from smtp-bc0a.mail.infomaniak.ch (smtp-bc0a.mail.infomaniak.ch [IPv6:2001:1600:4:17::bc0a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 06DA8C0613C1 for ; Fri, 15 Jan 2021 01:11:15 -0800 (PST) Received: from smtp-2-0000.mail.infomaniak.ch (unknown [10.5.36.107]) by smtp-3-3000.mail.infomaniak.ch (Postfix) with ESMTPS id 4DHFl51bMyzMprtq; Fri, 15 Jan 2021 10:10:25 +0100 (CET) Received: from ns3096276.ip-94-23-54.eu (unknown [23.97.221.149]) by smtp-2-0000.mail.infomaniak.ch (Postfix) with ESMTPA id 4DHFl26L60zlppyv; Fri, 15 Jan 2021 10:10:22 +0100 (CET) Subject: Re: [PATCH v26 07/12] landlock: Support filesystem access-control To: Jann Horn Cc: James Morris , "Serge E . Hallyn" , Al Viro , Andy Lutomirski , Anton Ivanov , Arnd Bergmann , Casey Schaufler , Jeff Dike , Jonathan Corbet , Kees Cook , Michael Kerrisk , Richard Weinberger , Shuah Khan , Vincent Dagonneau , Kernel Hardening , Linux API , linux-arch , "open list:DOCUMENTATION" , linux-fsdevel , kernel list , "open list:KERNEL SELFTEST FRAMEWORK" , linux-security-module , the arch/x86 maintainers , =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= References: <20201209192839.1396820-1-mic@digikod.net> <20201209192839.1396820-8-mic@digikod.net> From: =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= Message-ID: <9be6481f-9c03-dd32-378f-20bc7c52315c@digikod.net> Date: Fri, 15 Jan 2021 10:10:36 +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-doc@vger.kernel.org On 14/01/2021 23:43, Jann Horn wrote: > On Thu, Jan 14, 2021 at 7:54 PM Mickaël Salaün wrote: >> On 14/01/2021 04:22, Jann Horn wrote: >>> On Wed, Dec 9, 2020 at 8:28 PM Mickaël Salaün wrote: >>>> Thanks to the Landlock objects and ruleset, it is possible to identify >>>> inodes according to a process's domain. To enable an unprivileged >>>> process to express a file hierarchy, it first needs to open a directory >>>> (or a file) and pass this file descriptor to the kernel through >>>> landlock_add_rule(2). When checking if a file access request is >>>> allowed, we walk from the requested dentry to the real root, following >>>> the different mount layers. The access to each "tagged" inodes are >>>> collected according to their rule layer level, and ANDed to create >>>> access to the requested file hierarchy. This makes possible to identify >>>> a lot of files without tagging every inodes nor modifying the >>>> filesystem, while still following the view and understanding the user >>>> has from the filesystem. >>>> >>>> Add a new ARCH_EPHEMERAL_INODES for UML because it currently does not >>>> keep the same struct inodes for the same inodes whereas these inodes are >>>> in use. >>>> >>>> This commit adds a minimal set of supported filesystem access-control >>>> which doesn't enable to restrict all file-related actions. This is the >>>> result of multiple discussions to minimize the code of Landlock to ease >>>> review. Thanks to the Landlock design, extending this access-control >>>> without breaking user space will not be a problem. Moreover, seccomp >>>> filters can be used to restrict the use of syscall families which may >>>> not be currently handled by Landlock. >>> [...] >>>> +static bool check_access_path_continue( >>>> + const struct landlock_ruleset *const domain, >>>> + const struct path *const path, const u32 access_request, >>>> + u64 *const layer_mask) >>>> +{ >>> [...] >>>> + /* >>>> + * An access is granted if, for each policy layer, at least one rule >>>> + * encountered on the pathwalk grants the access, regardless of their >>>> + * position in the layer stack. We must then check not-yet-seen layers >>>> + * for each inode, from the last one added to the first one. >>>> + */ >>>> + for (i = 0; i < rule->num_layers; i++) { >>>> + const struct landlock_layer *const layer = &rule->layers[i]; >>>> + const u64 layer_level = BIT_ULL(layer->level - 1); >>>> + >>>> + if (!(layer_level & *layer_mask)) >>>> + continue; >>>> + if ((layer->access & access_request) != access_request) >>>> + return false; >>>> + *layer_mask &= ~layer_level; >>> >>> Hmm... shouldn't the last 5 lines be replaced by the following? >>> >>> if ((layer->access & access_request) == access_request) >>> *layer_mask &= ~layer_level; >>> >>> And then, since this function would always return true, you could >>> change its return type to "void". >>> >>> >>> As far as I can tell, the current version will still, if a ruleset >>> looks like this: >>> >>> /usr read+write >>> /usr/lib/ read >>> >>> reject write access to /usr/lib, right? >> >> If these two rules are from different layers, then yes it would work as >> intended. However, if these rules are from the same layer the path walk >> will not stop at /usr/lib but go down to /usr, which grants write >> access. > > I don't see why the code would do what you're saying it does. And an > experiment seems to confirm what I said; I checked out landlock-v26, > and the behavior I get is: There is a misunderstanding, I was responding to your proposition to modify check_access_path_continue(), not about the behavior of landlock-v26. > > user@vm:~/landlock$ dd if=/dev/null of=/tmp/aaa > 0+0 records in > 0+0 records out > 0 bytes copied, 0.00106365 s, 0.0 kB/s > user@vm:~/landlock$ LL_FS_RO='/lib' LL_FS_RW='/' ./sandboxer dd > if=/dev/null of=/tmp/aaa > 0+0 records in > 0+0 records out > 0 bytes copied, 0.000491814 s, 0.0 kB/s > user@vm:~/landlock$ LL_FS_RO='/tmp' LL_FS_RW='/' ./sandboxer dd > if=/dev/null of=/tmp/aaa > dd: failed to open '/tmp/aaa': Permission denied > user@vm:~/landlock$ > > Granting read access to /tmp prevents writing to it, even though write > access was granted to /. > It indeed works like this with landlock-v26. However, with your above proposition, it would work like this: $ LL_FS_RO='/tmp' LL_FS_RW='/' ./sandboxer dd if=/dev/null of=/tmp/aaa 0+0 records in 0+0 records out 0 bytes copied, 0.000187265 s, 0.0 kB/s …which is not what users would expect I guess. :)