From patchwork Tue Oct 18 18:22:16 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: =?utf-8?q?G=C3=BCnther_Noack?= X-Patchwork-Id: 13010904 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 319FDC433FE for ; Tue, 18 Oct 2022 18:23:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229470AbiJRSXP (ORCPT ); Tue, 18 Oct 2022 14:23:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42614 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229886AbiJRSXH (ORCPT ); Tue, 18 Oct 2022 14:23:07 -0400 Received: from mail-ej1-x632.google.com (mail-ej1-x632.google.com [IPv6:2a00:1450:4864:20::632]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4576D83F04; Tue, 18 Oct 2022 11:22:39 -0700 (PDT) Received: by mail-ej1-x632.google.com with SMTP id ot12so34435706ejb.1; Tue, 18 Oct 2022 11:22:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=BCqE9e12XoBqA5RscNmFvGeITIorfGLdOj2bFbTji74=; b=Xh4SLMDAcfIHODE+7dLbrJE99J02JXGbPmWUTXjbs8GMNN+oGlJDBrBRpQJHJYrE04 Z49RCEvP0T7snGzhSK9WI1YIeujmUc9eC29I8V7SQp6l+SMXe6CsqlL8xsrH9vS5EC2U TiojafYGD1R/cUD+GakOvDyg5fOwaWDZSRdx0GyFhiLUVVOfWqob4E2CsZaBDdHA3rn5 tKB26ndirT9XiJeMs3ZOf30fDdTO2zi5hIvr5kBP04X9j5sl7bARJYbeGJLxOgmwwMu/ IjfbfAsgcc/73eZCs2dHsB65R9ofimrIpPubp3XuxU9R0cjqgLx/NM4UKgbOLFiCh9Ej bQ1Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=BCqE9e12XoBqA5RscNmFvGeITIorfGLdOj2bFbTji74=; b=DElvfuR0pC/NOysKHiHERZyeiytHAk8SBCnUq45nj+jK++LC9pTozaGcXPKzHsj8J6 xtEsInnZGq5d/hWyxGxh8EWWwK0NHqoySjTHrj50pyr31pHUpxJYZdwSMf7yRHxnjsZX D8B72IsMFKpQt32lwgBMm7RjgSSUksyMGWQYXs+DJgoIetgmOLq/joOWIPIhWtneoN13 MXg33PP4MeLDFauO39gsXOr/V3vHm01URPDL9EfMlBbUU4aZ8AP1l2B+HGdSZ4JIGTcK MWl8asskCLVLsdeRbWazlIK9zR9zjsHfA0L05s3vUi9wftPoZrXj7CAlNI3zXsEMXTt3 VOqw== X-Gm-Message-State: ACrzQf24RsLerjrtpvSlSK7pL0Kd+SjbzjOEczgDcRxNYP0QvUrDWNbE fPSwFkJ21udh3+G3VLX2Ftq4Ac9iAjc= X-Google-Smtp-Source: AMsMyM6ngBDuCs9bZkg2u2GgKTojp9eEeHWIMdrCpDs+kXycH9fYKxgTnyNd/cy175OLVRLHlGRJOA== X-Received: by 2002:a17:907:1c28:b0:78d:4616:e6b6 with SMTP id nc40-20020a1709071c2800b0078d4616e6b6mr3356096ejc.257.1666117357707; Tue, 18 Oct 2022 11:22:37 -0700 (PDT) Received: from nuc.i.gnoack.org ([2a02:168:633b:1:1e69:7aff:fe05:97e6]) by smtp.gmail.com with ESMTPSA id i18-20020a0564020f1200b00458a03203b1sm9358395eda.31.2022.10.18.11.22.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 18 Oct 2022 11:22:37 -0700 (PDT) From: =?utf-8?q?G=C3=BCnther_Noack?= To: linux-security-module@vger.kernel.org Cc: =?utf-8?q?Micka=C3=ABl_Sala=C3=BCn?= , James Morris , Paul Moore , "Serge E . Hallyn" , linux-fsdevel@vger.kernel.org, Konstantin Meskhidze , Nathan Chancellor , =?utf-8?q?G=C3=BCnther_Noack?= Subject: [PATCH v10 11/11] landlock: Document Landlock's file truncation support Date: Tue, 18 Oct 2022 20:22:16 +0200 Message-Id: <20221018182216.301684-12-gnoack3000@gmail.com> X-Mailer: git-send-email 2.38.0 In-Reply-To: <20221018182216.301684-1-gnoack3000@gmail.com> References: <20221018182216.301684-1-gnoack3000@gmail.com> MIME-Version: 1.0 Precedence: bulk List-ID: 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 | 67 +++++++++++++++++++++--- 1 file changed, 60 insertions(+), 7 deletions(-) diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst index cec780c2f497..d8cd8cd9ce25 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: September 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 +70,28 @@ 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) { + /* Degrades gracefully if Landlock is not handled. */ + perror("The running kernel does not enable to use Landlock"); + return 0; + } + 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 +140,8 @@ 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 @@ -252,6 +265,37 @@ 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 ============= @@ -398,6 +442,15 @@ 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