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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1D0AEC25B0E for ; Tue, 16 Aug 2022 19:19:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229541AbiHPTTB (ORCPT ); Tue, 16 Aug 2022 15:19:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38536 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236744AbiHPTSj (ORCPT ); Tue, 16 Aug 2022 15:18:39 -0400 Received: from smtp-42a8.mail.infomaniak.ch (smtp-42a8.mail.infomaniak.ch [IPv6:2001:1600:4:17::42a8]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8770354C97 for ; Tue, 16 Aug 2022 12:18:36 -0700 (PDT) Received: from smtp-3-0001.mail.infomaniak.ch (unknown [10.4.36.108]) by smtp-3-3000.mail.infomaniak.ch (Postfix) with ESMTPS id 4M6gt23W4vzMpnZc; Tue, 16 Aug 2022 21:18:34 +0200 (CEST) Received: from ns3096276.ip-94-23-54.eu (unknown [23.97.221.149]) by smtp-3-0001.mail.infomaniak.ch (Postfix) with ESMTPA id 4M6gt15F4pzln8Vj; Tue, 16 Aug 2022 21:18:33 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=digikod.net; s=20191114; t=1660677514; bh=6V4iNwz2tJ/llXSuD4WLyfGpfpIquRyjZkRx97pl93E=; h=Date:To:Cc:References:From:Subject:In-Reply-To:From; b=ezYVnxwC9VWPUFMd3ABY/b/CNsyyvzkBJ1Cp7PIA1bn2fo1MYR8BpWWUJrIchgdBi XXHILmOBjD0jAhYgY7nvYlOXG/dvSDu5t31t2KmPZ1QwyOpdHnoZLfevJMvnnP1q1c fA9aAN7KGbNJK7kQyO3FWR4PTRcDd/fXrpBL4osQ= Message-ID: Date: Tue, 16 Aug 2022 21:18:33 +0200 MIME-Version: 1.0 User-Agent: Content-Language: en-US To: =?UTF-8?Q?G=c3=bcnther_Noack?= , linux-security-module@vger.kernel.org Cc: James Morris , Paul Moore , "Serge E . Hallyn" References: <20220814192603.7387-1-gnoack3000@gmail.com> <20220814192603.7387-5-gnoack3000@gmail.com> From: =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= Subject: Re: [PATCH v4 4/4] landlock: Document Landlock's file truncation support In-Reply-To: <20220814192603.7387-5-gnoack3000@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: On 14/08/2022 21:26, Günther Noack wrote: > Use the LANDLOCK_ACCESS_FS_TRUNCATE flag in the tutorial. > > Adapt the backwards compatibility example and discussion to remove the > truncation flag where needed. > > Point out potential surprising behaviour related to truncate. > > Signed-off-by: Günther Noack > --- > Documentation/userspace-api/landlock.rst | 48 +++++++++++++++++++----- > 1 file changed, 38 insertions(+), 10 deletions(-) > > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst > index 6648e59fabe7..3ceb97cbe9d1 100644 > --- a/Documentation/userspace-api/landlock.rst > +++ b/Documentation/userspace-api/landlock.rst > @@ -60,7 +60,8 @@ the need to be explicit about the denied-by-default access rights. > LANDLOCK_ACCESS_FS_MAKE_FIFO | > LANDLOCK_ACCESS_FS_MAKE_BLOCK | > LANDLOCK_ACCESS_FS_MAKE_SYM | > - LANDLOCK_ACCESS_FS_REFER, > + LANDLOCK_ACCESS_FS_REFER | > + LANDLOCK_ACCESS_FS_TRUNCATE, > }; > > Because we may not know on which kernel version an application will be > @@ -69,16 +70,26 @@ should try to protect users as much as possible whatever the kernel they are > using. To avoid binary enforcement (i.e. either all security features or > none), we can leverage a dedicated Landlock command to get the current version > of the Landlock ABI and adapt the handled accesses. Let's check if we should > -remove the `LANDLOCK_ACCESS_FS_REFER` access right which is only supported > -starting with the second version of the ABI. > +remove the `LANDLOCK_ACCESS_FS_REFER` and `LANDLOCK_ACCESS_FS_TRUNCATE` access s/and/or/ > +rights, which are only supported starting with the second and third version of > +the ABI. > > .. code-block:: c > > int abi; > > abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION); > - if (abi < 2) { > - ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER; > + switch (abi) { > + case -1: > + perror("The running kernel does not enable to use Landlock"); > + return 1; > + case 1: > + /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */ > + ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER; > + __attribute__((fallthrough)); > + case 2: > + /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */ > + ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE; > } > > This enables to create an inclusive ruleset that will contain our rules. > @@ -127,8 +138,8 @@ descriptor. > > It may also be required to create rules following the same logic as explained > for the ruleset creation, by filtering access rights according to the Landlock > -ABI version. In this example, this is not required because > -`LANDLOCK_ACCESS_FS_REFER` is not allowed by any rule. > +ABI version. In this example, this is not required because all of the requested > +``allowed_access`` rights are already available in ABI 1. > > We now have a ruleset with one rule allowing read access to ``/usr`` while > denying all other handled accesses for the filesystem. The next step is to > @@ -251,6 +262,24 @@ To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target > process, a sandboxed process should have a subset of the target process rules, > which means the tracee must be in a sub-domain of the tracer. > > +Truncating files > +---------------- > + > +The operations covered by `LANDLOCK_ACCESS_FS_WRITE_FILE` and > +`LANDLOCK_ACCESS_FS_TRUNCATE` both change the contents of a file and sometimes > +overlap in non-intuitive ways. It is recommended to always specify both of > +these together. > + > +A particularly surprising example is :manpage:`creat(2)`. The name suggests > +that this system call requires the rights to create and write files. However, > +it also requires the truncate right if an existing file under the same name is > +already present. > + > +It should also be noted that truncating files does not necessarily require the I think "necessarily" is superfluous here. > +`LANDLOCK_ACCESS_FS_WRITE_FILE` right. Apart from the obvious > +:manpage:`truncate(2)` system call, this can also be done through > +:manpage:`open(2)` with the flags `O_RDONLY` and `O_TRUNC`. `O_RDONLY | O_TRUNC`. > + > Compatibility > ============= > > @@ -386,9 +415,8 @@ File truncation (ABI < 3) > File truncation could not be denied before the third Landlock ABI, so it is > always allowed when using a kernel that only supports the first or second ABI. > > -Starting with the Landlock ABI version 3, it is now possible to securely > -control truncation thanks to the new `LANDLOCK_ACCESS_FS_TRUNCATE` access > -right. > +Starting with the Landlock ABI version 3, it is now possible to securely control > +truncation thanks to the new `LANDLOCK_ACCESS_FS_TRUNCATE` access right. This is an inconsistent hunk, patching the first patch. Please also move this "File truncation" section below the "File renaming and linking". > > File renaming and linking (ABI 1) > ---------------------------------