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 85795ECAAD3 for ; Fri, 9 Sep 2022 13:52:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231996AbiIINws (ORCPT ); Fri, 9 Sep 2022 09:52:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35470 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231903AbiIINvr (ORCPT ); Fri, 9 Sep 2022 09:51:47 -0400 Received: from smtp-bc08.mail.infomaniak.ch (smtp-bc08.mail.infomaniak.ch [IPv6:2001:1600:4:17::bc08]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1FE9A9411E for ; Fri, 9 Sep 2022 06:51:40 -0700 (PDT) Received: from smtp-3-0000.mail.infomaniak.ch (unknown [10.4.36.107]) by smtp-3-3000.mail.infomaniak.ch (Postfix) with ESMTPS id 4MPHTh1lsHzMqdjs; Fri, 9 Sep 2022 15:51:36 +0200 (CEST) Received: from ns3096276.ip-94-23-54.eu (unknown [23.97.221.149]) by smtp-3-0000.mail.infomaniak.ch (Postfix) with ESMTPA id 4MPHTg4wjlzx3; Fri, 9 Sep 2022 15:51:35 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=digikod.net; s=20191114; t=1662731496; bh=RZPqhRYxUUKvQKUDFVTOjXCFDzQYPDY+U1MkKG6TJQ0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=YuYydORXn7JmVLy6Im+XtTjua+fvKLZZmXMiA/SKWXlGX8/XulXxXFv28EcwgX3Uk ogb0sOI3+0VOvQh/UAU+gx1X1D7wAcvSWLw6+P8arCrbRmDRk5F2CRpD/GQwCftORs 7M8D7CFMb0nWYAjcfQ+rSANg80fBkB0ZuA/URjrw= Message-ID: <2f9c6131-3140-9c47-cf95-f7fa3cf759ee@digikod.net> Date: Fri, 9 Sep 2022 15:51:35 +0200 MIME-Version: 1.0 User-Agent: Subject: Re: [PATCH v6 5/5] landlock: Document Landlock's file truncation support 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: <20220908195805.128252-1-gnoack3000@gmail.com> <20220908195805.128252-6-gnoack3000@gmail.com> Content-Language: en-US From: =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= In-Reply-To: <20220908195805.128252-6-gnoack3000@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 08/09/2022 21:58, 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 | 62 +++++++++++++++++++++--- > 1 file changed, 54 insertions(+), 8 deletions(-) > > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst > index b8ea59493964..57802fd1e09b 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: September 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 +70,26 @@ 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` 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) { > - 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. This fix is correct, but it should not be part of this series. FYI, I have a patch almost ready to fix some documentation style issues. Please remove this hunk for the next series. I'll deal with the merge conflicts if any. > > 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 +262,32 @@ 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 I investigated and in fact we should use double backquotes almost everywhere because it render the same as when using "%" in header files. Please change this for the next series. I'll do the same on the patch I just talk about. > +`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)`. 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 don't have an enforced Landlock ruleset. Good addition. Please do not use contractions ("don't"). > + > Compatibility > ============= > > @@ -397,6 +434,15 @@ 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