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 64450C433F5 for ; Sat, 8 Oct 2022 08:49:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229802AbiJHItv (ORCPT ); Sat, 8 Oct 2022 04:49:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47976 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229827AbiJHItm (ORCPT ); Sat, 8 Oct 2022 04:49:42 -0400 Received: from mail-ed1-x52e.google.com (mail-ed1-x52e.google.com [IPv6:2a00:1450:4864:20::52e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 81F3EEA; Sat, 8 Oct 2022 01:49:38 -0700 (PDT) Received: by mail-ed1-x52e.google.com with SMTP id s2so9864317edd.2; Sat, 08 Oct 2022 01:49:38 -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:subject:date:message-id:reply-to; bh=ffUY4ExQOOFTvskz6qhPEsEN8497Tuw4sv0ZJSQHczY=; b=R3WJpCR+A/cxjcdvKLICbOMesbwk+RZDoI5TVpdDTRVNO3uQzWVx5FDeCSXBaHMCS+ 3Igpi4CY+YAZXdZjJGtfGXcmhVXRcu6Z2wM8vF8Rxevht3Hk1AUEwQXlFnu4UxLBcRv1 kmV7K0sk0RT2AuL4NjfnSvpy6NNXilnRFAyTULgsn/vuBPZITw7i6KwmolQp1yxrzAgo 6nBBQPg0jNmz8fcR9m1iKeGphXLNA3S1bmzdl+Tv+Ece1HIMXrdX1KeHF78a2BkRHAwM fJY64ObBeFe/yhwq/38RKJRPdk54yrUzS1xHvPVD/XMzAMaM/WvKLBQomBNpHPp4hSsN YrLA== 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:subject:date:message-id:reply-to; bh=ffUY4ExQOOFTvskz6qhPEsEN8497Tuw4sv0ZJSQHczY=; b=KyJUINHFvCNWPexTO14UiKfmk93f5WxBSqsu3ntoo1xCV0Fl/BLYnH2k0yD4GaAIGx 4tMZ54N3gbgsQxusqXkz8F8QFC124UkmnCdckCObsWh6hs83Hqt0y2tv4LdWEekk2uOU f9qhmSoupk0L/CA6yUplPzwTpO1l3Vv0jhLbVH2STOw9prH/6EMebv7PECVBRCOSJJ7V DyWfm1N3oJm32YgmK07egScovsf+nKfjncj6ImT7dwTu7Xsd/mx6kOu657KSA0FmfIW/ VpJpa/UxP5FTwyE1MH96QpSyR0o3yEn20zf6lLCGM+KDLZgni0KhVAHqklS+mG0SiHFy EGqA== X-Gm-Message-State: ACrzQf1FvL6CS0+mv5uyuXV2OSrrVGIqB+aP+bWuunSttbI5z1BWpStn pgxiNIXZg2yaLcexg5yi8X0= X-Google-Smtp-Source: AMsMyM5Ny/0guSNDQPY0d5A726Ncrw8A8BgI1fUqwHKPPHyZQCGrjH7hfQY+per0DK83W5C22ccI2A== X-Received: by 2002:a50:c31b:0:b0:458:cc93:8000 with SMTP id a27-20020a50c31b000000b00458cc938000mr8428472edb.264.1665218977323; Sat, 08 Oct 2022 01:49:37 -0700 (PDT) Received: from nuc ([2a02:168:633b:1:1e69:7aff:fe05:97e6]) by smtp.gmail.com with ESMTPSA id s22-20020a05640217d600b0045af1e70137sm1282356edy.14.2022.10.08.01.49.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 08 Oct 2022 01:49:37 -0700 (PDT) Date: Sat, 8 Oct 2022 10:49:35 +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" , linux-fsdevel@vger.kernel.org, Konstantin Meskhidze Subject: Re: [PATCH v8 9/9] landlock: Document Landlock's file truncation support Message-ID: References: <20221001154908.49665-1-gnoack3000@gmail.com> <20221001154908.49665-10-gnoack3000@gmail.com> <9c340d63-c90b-6049-ac4d-1415a93a5fc6@digikod.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <9c340d63-c90b-6049-ac4d-1415a93a5fc6@digikod.net> Precedence: bulk List-ID: On Wed, Oct 05, 2022 at 08:57:37PM +0200, Mickaël Salaün wrote: > > 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. */ Done, moved the comment to a dedicated line and added the "s". > > + 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 --