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 4288CC25B06 for ; Sun, 14 Aug 2022 17:06:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229847AbiHNRGM (ORCPT ); Sun, 14 Aug 2022 13:06:12 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53686 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240546AbiHNRFh (ORCPT ); Sun, 14 Aug 2022 13:05:37 -0400 Received: from mail-ed1-x535.google.com (mail-ed1-x535.google.com [IPv6:2a00:1450:4864:20::535]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B1091DFF7 for ; Sun, 14 Aug 2022 10:05:35 -0700 (PDT) Received: by mail-ed1-x535.google.com with SMTP id z20so7008854edb.9 for ; Sun, 14 Aug 2022 10:05:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date:from:to :cc; bh=tZcCB2gC9m8Y31d7V/x8BlzpzNtqrtS99NVpRNEbvlI=; b=JcE/WoD8y7dDqJAcSlpuM69/kU83laWaeQKBGuNiwIF0NvrRIynb+Yp7ZLVu1Bccl1 tMC+IrB7WMxhHnbLlf1yaXmx601Oj1IhOLaIG5AJ1Zhgd3LoGcs9pS/gZYcFjX1PQ5Yk fdBs6xWWCi5o4N4X4xPhOdJD7NS3D0ZDPurIzfKwCxODzsWqjTpQupdxcEtnac1HYOz5 LIRfT+MPk2BDd/BEyVqcyenDtOI6o20Vjrrk7t8gR3fNiriSIPKyYqDFaZkMwvv3M2iO XwGPhZ7ZAo3K5TZ7yKY53JlssXASHfKiWe4H/Q3vVASyE6L2P8GhVHvXb6iSv5hv+oJk LFrQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date :x-gm-message-state:from:to:cc; bh=tZcCB2gC9m8Y31d7V/x8BlzpzNtqrtS99NVpRNEbvlI=; b=pBqKu0Z+xGL4LDYS7vyag7kPJp2lfenfsH3jxu1SA8SL66FvD053ac0R/7ekguQxuQ s+wNkNehU6wfDwJMhoRVudTh+TEVHYedg+vO/5xiAoXfL9ztnK6+TolwQpjMdIlmhWqg C4pa60uDGW7f1nKxWtevYAVN7Xt7CTZRXasBwEG3nUc7WPbxTU05K1HPgFqqOSBzO/Uq fYtWm+UEnWoCW8opGJ+OYTDETLx0O5JWfmNxInZNO4vSw/B+gfL/nJfTpdp6munCvRgF 71xWqbZBW77TTkMKdFMbOOakZGSya9vFPQQjhKYDOTHuz3t4EXPrUAYdmoIq8SEnzgbE Ztkw== X-Gm-Message-State: ACgBeo2KV5VjQ/BN9iePWBOTxyASug2EchYKU9evdMgNNDwQqyZcSfPY 23FBbeiOohriIOtj7v6QBCc= X-Google-Smtp-Source: AA6agR76XozZpyFxrLhcCMjsDgYGEby6BErVsgHrVsfhbhg+5GmPmp66cGBdAw7ZxEGgLb92rwUOLw== X-Received: by 2002:a05:6402:5388:b0:435:71b:5d44 with SMTP id ew8-20020a056402538800b00435071b5d44mr11151292edb.364.1660496734158; Sun, 14 Aug 2022 10:05:34 -0700 (PDT) Received: from nuc ([2a02:168:633b:1:1e69:7aff:fe05:97e6]) by smtp.gmail.com with ESMTPSA id b6-20020a056402138600b004418c7d633bsm4996808edv.18.2022.08.14.10.05.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 14 Aug 2022 10:05:33 -0700 (PDT) Date: Sun, 14 Aug 2022 19:05:32 +0200 From: =?iso-8859-1?Q?G=FCnther?= Noack To: =?iso-8859-1?Q?Micka=EBl_Sala=FCn?= Cc: linux-security-module@vger.kernel.org, James Morris , Paul Moore , "Serge E . Hallyn" Subject: Re: [PATCH v3 4/4] landlock: Document Landlock's file truncation support Message-ID: References: <20220804193746.9161-1-gnoack3000@gmail.com> <20220804193746.9161-5-gnoack3000@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: On Fri, Aug 12, 2022 at 01:19:47PM +0200, Mickaël Salaün wrote: > > On 04/08/2022 21:37, 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 | 41 ++++++++++++++++++++---- > > 1 file changed, 34 insertions(+), 7 deletions(-) > > > > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst > > index d92e335380d4..9c3c9fa65958 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 +70,24 @@ 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 > > +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("Landlock is not supported with the running kernel"); > > Because there is a distinction between "supported" and "enabled" (as > explained in the sample), let's make this message more generic. The > additional strerror() output would then be enough to distinguish the error > type. > > "The running kernel does not enable to use Landlock" Done, good point. > > > + return 1; > > + case 1: > > This switch/case logic might be a bit confusing; let's explain it for this > doc *and the sample code*: > > /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */ Done, added them to both the sample and the documentation. > > + 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 +136,8 @@ 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 +260,24 @@ 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 > > FYI, I'll send a standalone patch to remove all contractions and get a more > consistent documentation. Please, keep it this way. Acknowledged. > > > > +require the `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`. > > Good! > > nit: you can use a 80-columns limit. Sounds good, applied that in this place and a few others that I touched. > > > + > > Compatibility > > ============= --