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 2F938C433FE for ; Wed, 5 Oct 2022 18:57:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229661AbiJES5o (ORCPT ); Wed, 5 Oct 2022 14:57:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37924 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230257AbiJES5l (ORCPT ); Wed, 5 Oct 2022 14:57:41 -0400 Received: from smtp-42ae.mail.infomaniak.ch (smtp-42ae.mail.infomaniak.ch [IPv6:2001:1600:4:17::42ae]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2311D7FE5A for ; Wed, 5 Oct 2022 11:57:40 -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 4MjP2p3ZByzMqJ2s; Wed, 5 Oct 2022 20:57:38 +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 4MjP2n6xnMzMpqBh; Wed, 5 Oct 2022 20:57:37 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=digikod.net; s=20191114; t=1664996258; bh=AikZju/a9kBTV3If2Je225YSS4yslrynNtNIxE5ZElY=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=pePWPNm1/x9tCePr+CJGnaCXa4Ilp/ytW4OJvbRay1Ikf6L5xhXKhCfDP/TuSmc3k eVb5yN3ikC2or8wjqjvv/l9iKZn2y+d1hsDEboz/8E7YiZts2zXihOi4Jeu4zu3mtp KwDvuHV4Eg1LKQIM15B7VuzdzJ0/TeJzKd12fbmQ= Message-ID: <9c340d63-c90b-6049-ac4d-1415a93a5fc6@digikod.net> Date: Wed, 5 Oct 2022 20:57:37 +0200 MIME-Version: 1.0 User-Agent: Subject: Re: [PATCH v8 9/9] landlock: Document Landlock's file truncation support 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" , linux-fsdevel@vger.kernel.org, Konstantin Meskhidze References: <20221001154908.49665-1-gnoack3000@gmail.com> <20221001154908.49665-10-gnoack3000@gmail.com> From: =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= In-Reply-To: <20221001154908.49665-10-gnoack3000@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: On 01/10/2022 17:49, 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 | 66 +++++++++++++++++++++--- > 1 file changed, 59 insertions(+), 7 deletions(-) > > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst > index b8ea59493964..44d6f598b63d 100644 > --- a/Documentation/userspace-api/landlock.rst > +++ b/Documentation/userspace-api/landlock.rst > @@ -8,7 +8,7 @@ Landlock: unprivileged access control > ===================================== > > :Author: Mickaël Salaün > -:Date: May 2022 > +:Date: October 2022 > > The goal of Landlock is to enable to restrict ambient rights (e.g. global > filesystem access) for a set of processes. Because Landlock is a stackable > @@ -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 +70,27 @@ 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`` or ``LANDLOCK_ACCESS_FS_TRUNCATE`` > +access 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) { > + if (abi < 0) { > + perror("The running kernel does not enable to use Landlock"); Please insert in a dedicated line this comment: /* Degrades gracefully if Landlock is not handled. */ > + return 0; /* Degrade gracefully if Landlock is not handled. */ > + } > + switch (abi) { > + 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 +139,8 @@ 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 +263,37 @@ 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 require the > +``LANDLOCK_ACCESS_FS_WRITE_FILE`` right. Apart from the :manpage:`truncate(2)` > +system call, this can also be done through :manpage:`open(2)` with the flags > +``O_RDONLY | O_TRUNC``. > + > +When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE`` > +right is associated with the newly created file descriptor and will be used for > +subsequent truncation attempts using :manpage:`ftruncate(2)`. The behavior is > +similar to opening a file for reading or writing, where permissions are checked > +during :manpage:`open(2)`, but not during the subsequent :manpage:`read(2)` and > +:manpage:`write(2)` calls. > + > +As a consequence, it is possible to have multiple open file descriptors for the > +same file, where one grants the right to truncate the file and the other does > +not. It is also possible to pass such file descriptors between processes, > +keeping their Landlock properties, even when these processes do not have an > +enforced Landlock ruleset. > + > Compatibility > ============= > > @@ -397,6 +440,15 @@ Starting with the Landlock ABI version 2, it is now possible to securely > control renaming and linking thanks to the new `LANDLOCK_ACCESS_FS_REFER` > access right. > > +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. > + > .. _kernel_support: > > Kernel support