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Hallyn" , linux-fsdevel@vger.kernel.org, Konstantin Meskhidze , Nathan Chancellor , =?utf-8?q?G=C3=BCnther_Noack?= Subject: [PATCH v9 10/11] samples/landlock: Extend sample tool to support LANDLOCK_ACCESS_FS_TRUNCATE Date: Sat, 8 Oct 2022 13:13:35 +0200 Message-Id: <20221008111336.74806-1-gnoack3000@gmail.com> X-Mailer: git-send-email 2.38.0 MIME-Version: 1.0 Precedence: bulk List-ID: Update the sandboxer sample to restrict truncate actions. This is automatically enabled by default if the running kernel supports LANDLOCK_ACCESS_FS_TRUNCATE, except for the paths listed in the LL_FS_RW environment variable. Signed-off-by: Günther Noack --- samples/landlock/sandboxer.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) diff --git a/samples/landlock/sandboxer.c b/samples/landlock/sandboxer.c index f29bb3c72230..fd4237c64fb2 100644 --- a/samples/landlock/sandboxer.c +++ b/samples/landlock/sandboxer.c @@ -76,7 +76,8 @@ static int parse_path(char *env_path, const char ***const path_list) #define ACCESS_FILE ( \ LANDLOCK_ACCESS_FS_EXECUTE | \ LANDLOCK_ACCESS_FS_WRITE_FILE | \ - LANDLOCK_ACCESS_FS_READ_FILE) + LANDLOCK_ACCESS_FS_READ_FILE | \ + LANDLOCK_ACCESS_FS_TRUNCATE) /* clang-format on */ @@ -160,11 +161,12 @@ static int populate_ruleset(const char *const env_var, const int ruleset_fd, 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) /* clang-format on */ -#define LANDLOCK_ABI_LAST 2 +#define LANDLOCK_ABI_LAST 3 int main(const int argc, char *const argv[], char *const *const envp) { @@ -234,6 +236,10 @@ int main(const int argc, char *const argv[], char *const *const envp) 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; fprintf(stderr, "Hint: You should update the running kernel " From patchwork Sat Oct 8 11:13:36 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: 13001756 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 30AAFC4332F for ; Sat, 8 Oct 2022 11:13:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229729AbiJHLNn (ORCPT ); Sat, 8 Oct 2022 07:13:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52212 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229758AbiJHLNm (ORCPT ); Sat, 8 Oct 2022 07:13:42 -0400 Received: from mail-ej1-x62b.google.com (mail-ej1-x62b.google.com [IPv6:2a00:1450:4864:20::62b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0DA0E50F9E; Sat, 8 Oct 2022 04:13:41 -0700 (PDT) Received: by mail-ej1-x62b.google.com with SMTP id k2so16156731ejr.2; Sat, 08 Oct 2022 04:13:40 -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=EyjeljUZ0zV6Z8u3UFOaFlCImS4ar8/ivtmgCiAy2D+hgNHEsVluG1yeRxBxm6Kt86 yO6c5hQN8pCJRXutyIny62vlqYvKIAyueApI0CH+XnSwxsNRXSn4vrsyaSdLYMBFPsd4 3m0aHEbbcAEkd40jLmwTQy1WKYcG+/KuEPeYX7RN7FAlusBleTdv6EAZMFhV2tU03iP6 WMC0wgiVQSZDniAyab2KKBv4m9jsukBlpHxmwbWdiRPHvskceTQZuCATMFcI6zm0IpM4 8BGfrV7JPApABaDAkwPCLlQIqr7c5f4AhOHj0JL9TQ0/EP3wl/SpzLl9IYmmPoJWlcDa ZXVQ== 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=zhOq8kyywKZAO6mAll9lZHbUUZfXu43eyI8RK+WoKFOJWdl5qpM8Lm2O2XNEgtmzVB ajHF/mmWTxELLJ5xLuWPxxSyjStx/V2zflnfVRMm0Iu6ZMMW1Ys8m9oOy95VZ9e+krbB ZvLVVq0A2l02FPXBuEpMDCREarlMaUiLnkny5Pcjo0tlFyjpg4+xV7vZ10B1BOexygrh hNoS/ASsrcRolSFk8wSwDuW3RYMMif4nAuhEdw5DK6mGeFrk7S+dfjUjgBsZf+PGH9nk 8O9gEDGlNUYrtoa5HO/5xCttXM+LCvyr6Nqxg790PgHXTjiWkgDcDHh2b80duPjtpHRm suCg== X-Gm-Message-State: ACrzQf1N0E5ghlhFQDYkKJQTGxQRcvIx9FSIsRYdvMHG4lEeEJwY6jsr qkqj8edKoPTVTSUp22Sv0y//bwj8o/Y= X-Google-Smtp-Source: AMsMyM6j0trNAW6VhtifcXgmxCCwsixLUApSU4//Ai6NfudocS11tkyTMzd8dXB/h1HUc5IwAwrHlw== X-Received: by 2002:a17:907:1c98:b0:78d:3b06:dc8f with SMTP id nb24-20020a1709071c9800b0078d3b06dc8fmr7363353ejc.58.1665227619607; Sat, 08 Oct 2022 04:13:39 -0700 (PDT) Received: from nuc.i.gnoack.org ([2a02:168:633b:1:1e69:7aff:fe05:97e6]) by smtp.gmail.com with ESMTPSA id kt25-20020a1709079d1900b0078d886c871bsm1740312ejc.70.2022.10.08.04.13.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 08 Oct 2022 04:13:39 -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 v9 11/11] landlock: Document Landlock's file truncation support Date: Sat, 8 Oct 2022 13:13:36 +0200 Message-Id: <20221008111336.74806-2-gnoack3000@gmail.com> X-Mailer: git-send-email 2.38.0 In-Reply-To: <20221008111336.74806-1-gnoack3000@gmail.com> References: <20221008111336.74806-1-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