From patchwork Tue Jun 25 21:52:39 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: =?utf-8?q?Micka=C3=ABl_Sala=C3=BCn?= X-Patchwork-Id: 11016455 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 15B4114BB for ; Tue, 25 Jun 2019 21:55:12 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 0566326253 for ; Tue, 25 Jun 2019 21:55:12 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id ED299284AF; Tue, 25 Jun 2019 21:55:11 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id D2D7226253 for ; Tue, 25 Jun 2019 21:55:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726037AbfFYVzK (ORCPT ); Tue, 25 Jun 2019 17:55:10 -0400 Received: from smtp-sh.infomaniak.ch ([128.65.195.4]:44236 "EHLO smtp-sh.infomaniak.ch" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726574AbfFYVzK (ORCPT ); Tue, 25 Jun 2019 17:55:10 -0400 Received: from smtp5.infomaniak.ch (smtp5.infomaniak.ch [83.166.132.18]) by smtp-sh.infomaniak.ch (8.14.5/8.14.5) with ESMTP id x5PLrSKW019918 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 25 Jun 2019 23:53:28 +0200 Received: from localhost (ns3096276.ip-94-23-54.eu [94.23.54.103]) (authenticated bits=0) by smtp5.infomaniak.ch (8.14.5/8.14.5) with ESMTP id x5PLrRJX038974; Tue, 25 Jun 2019 23:53:27 +0200 From: =?utf-8?q?Micka=C3=ABl_Sala=C3=BCn?= To: linux-kernel@vger.kernel.org Cc: =?utf-8?q?Micka=C3=ABl_Sala=C3=BCn?= , Aleksa Sarai , Alexander Viro , Alexei Starovoitov , Andrew Morton , Andy Lutomirski , Arnaldo Carvalho de Melo , Casey Schaufler , Daniel Borkmann , David Drysdale , "David S . Miller" , "Eric W . Biederman" , James Morris , Jann Horn , John Johansen , Jonathan Corbet , Kees Cook , Michael Kerrisk , =?utf-8?q?Micka=C3=ABl_Sala=C3=BC?= =?utf-8?q?n?= , Paul Moore , Sargun Dhillon , "Serge E . Hallyn" , Shuah Khan , Stephen Smalley , Tejun Heo , Tetsuo Handa , Thomas Graf , Tycho Andersen , Will Drewry , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-security-module@vger.kernel.org, netdev@vger.kernel.org Subject: [PATCH bpf-next v9 10/10] landlock: Add user and kernel documentation for Landlock Date: Tue, 25 Jun 2019 23:52:39 +0200 Message-Id: <20190625215239.11136-11-mic@digikod.net> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20190625215239.11136-1-mic@digikod.net> References: <20190625215239.11136-1-mic@digikod.net> MIME-Version: 1.0 X-Antivirus: Dr.Web (R) for Unix mail servers drweb plugin ver.6.0.2.8 X-Antivirus-Code: 0x100000 Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: X-Virus-Scanned: ClamAV using ClamSMTP This documentation can be built with the Sphinx framework. Signed-off-by: Mickaël Salaün Cc: Alexei Starovoitov Cc: Andy Lutomirski Cc: Daniel Borkmann Cc: David S. Miller Cc: James Morris Cc: Jonathan Corbet Cc: Kees Cook Cc: Serge E. Hallyn --- Changes since v8: * remove documentation related to chaining and tagging according to this patch series Changes since v7: * update documentation according to the Landlock revamp Changes since v6: * add a check for ctx->event * rename BPF_PROG_TYPE_LANDLOCK to BPF_PROG_TYPE_LANDLOCK_RULE * rename Landlock version to ABI to better reflect its purpose and add a dedicated changelog section * update tables * relax no_new_privs recommendations * remove ABILITY_WRITE related functions * reword rule "appending" to "prepending" and explain it * cosmetic fixes Changes since v5: * update the rule hierarchy inheritance explanation * briefly explain ctx->arg2 * add ptrace restrictions * explain EPERM * update example (subtype) * use ":manpage:" --- Documentation/security/index.rst | 1 + Documentation/security/landlock/index.rst | 20 +++ Documentation/security/landlock/kernel.rst | 99 ++++++++++++++ Documentation/security/landlock/user.rst | 148 +++++++++++++++++++++ 4 files changed, 268 insertions(+) create mode 100644 Documentation/security/landlock/index.rst create mode 100644 Documentation/security/landlock/kernel.rst create mode 100644 Documentation/security/landlock/user.rst diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst index aad6d92ffe31..32b4c1db2325 100644 --- a/Documentation/security/index.rst +++ b/Documentation/security/index.rst @@ -12,3 +12,4 @@ Security Documentation SCTP self-protection tpm/index + landlock/index diff --git a/Documentation/security/landlock/index.rst b/Documentation/security/landlock/index.rst new file mode 100644 index 000000000000..d0af868d1582 --- /dev/null +++ b/Documentation/security/landlock/index.rst @@ -0,0 +1,20 @@ +========================================= +Landlock LSM: programmatic access control +========================================= + +Landlock is a stackable Linux Security Module (LSM) that makes it possible to +create security sandboxes, programmable access-controls or safe endpoint +security agents. This kind of sandbox is expected to help mitigate the +security impact of bugs or unexpected/malicious behaviors in user-space +applications. The current version allows only a process with the global +CAP_SYS_ADMIN capability to create such sandboxes but the ultimate goal of +Landlock is to empower any process, including unprivileged ones, to securely +restrict themselves. Landlock is inspired by seccomp-bpf but instead of +filtering syscalls and their raw arguments, a Landlock rule can inspect the use +of kernel objects like files and hence make a decision according to the kernel +semantic. + +.. toctree:: + + user + kernel diff --git a/Documentation/security/landlock/kernel.rst b/Documentation/security/landlock/kernel.rst new file mode 100644 index 000000000000..7d1e06d544bf --- /dev/null +++ b/Documentation/security/landlock/kernel.rst @@ -0,0 +1,99 @@ +============================== +Landlock: kernel documentation +============================== + +eBPF properties +=============== + +To get an expressive language while still being safe and small, Landlock is +based on eBPF. Landlock should be usable by untrusted processes and must +therefore expose a minimal attack surface. The eBPF bytecode is minimal, +powerful, widely used and designed to be used by untrusted applications. Thus, +reusing the eBPF support in the kernel enables a generic approach while +minimizing new code. + +An eBPF program has access to an eBPF context containing some fields used to +inspect the current object. These arguments can be used directly (e.g. cookie) +or passed to helper functions according to their types (e.g. inode pointer). It +is then possible to do complex access checks without race conditions or +inconsistent evaluation (i.e. `incorrect mirroring of the OS code and state +`_). + +A Landlock hook describes a particular access type. For now, there is two +hooks dedicated to filesystem related operations: LANDLOCK_HOOK_FS_PICK and +LANDLOCK_HOOK_FS_WALK. A Landlock program is tied to one hook. This makes it +possible to statically check context accesses, potentially performed by such +program, and hence prevents kernel address leaks and ensure the right use of +hook arguments with eBPF functions. Any user can add multiple Landlock +programs per Landlock hook. They are stacked and evaluated one after the +other, starting from the most recent program, as seccomp-bpf does with its +filters. Underneath, a hook is an abstraction over a set of LSM hooks. + + +Guiding principles +================== + +Unprivileged use +---------------- + +* Landlock helpers and context should be usable by any unprivileged and + untrusted program while following the system security policy enforced by + other access control mechanisms (e.g. DAC, LSM). + + +Landlock hook and context +------------------------- + +* A Landlock hook shall be focused on access control on kernel objects instead + of syscall filtering (i.e. syscall arguments), which is the purpose of + seccomp-bpf. +* A Landlock context provided by a hook shall express the minimal and more + generic interface to control an access for a kernel object. +* A hook shall guaranty that all the BPF function calls from a program are + safe. Thus, the related Landlock context arguments shall always be of the + same type for a particular hook. For example, a network hook could share + helpers with a file hook because of UNIX socket. However, the same helpers + may not be compatible for a file system handle and a net handle. +* Multiple hooks may use the same context interface. + + +Landlock helpers +---------------- + +* Landlock helpers shall be as generic as possible while at the same time being + as simple as possible and following the syscall creation principles (cf. + *Documentation/adding-syscalls.txt*). +* The only behavior change allowed on a helper is to fix a (logical) bug to + match the initial semantic. +* Helpers shall be reentrant, i.e. only take inputs from arguments (e.g. from + the BPF context), to enable a hook to use a cache. Future program options + might change this cache behavior. +* It is quite easy to add new helpers to extend Landlock. The main concern + should be about the possibility to leak information from the kernel that may + not be accessible otherwise (i.e. side-channel attack). + + +Questions and answers +===================== + +Why not create a custom hook for each kind of action? +----------------------------------------------------- + +Landlock programs can handle these checks. Adding more exceptions to the +kernel code would lead to more code complexity. A decision to ignore a kind of +action can and should be done at the beginning of a Landlock program. + + +Why a program does not return an errno or a kill code? +------------------------------------------------------ + +seccomp filters can return multiple kind of code, including an errno value or a +kill signal, which may be convenient for access control. Those return codes +are hardwired in the userland ABI. Instead, Landlock's approach is to return a +boolean to allow or deny an action, which is much simpler and more generic. +Moreover, we do not really have a choice because, unlike to seccomp, Landlock +programs are not enforced at the syscall entry point but may be executed at any +point in the kernel (through LSM hooks) where an errno return code may not make +sense. However, with this simple ABI and with the ability to call helpers, +Landlock may gain features similar to seccomp-bpf in the future while being +compatible with previous programs. diff --git a/Documentation/security/landlock/user.rst b/Documentation/security/landlock/user.rst new file mode 100644 index 000000000000..e4bb7dba4aa7 --- /dev/null +++ b/Documentation/security/landlock/user.rst @@ -0,0 +1,148 @@ +================================ +Landlock: userland documentation +================================ + +Landlock programs +================= + +eBPF programs are used to create security programs. They are contained and can +call only a whitelist of dedicated functions. Moreover, they can only loop +under strict conditions, which protects from denial of service. More +information on BPF can be found in *Documentation/networking/filter.txt*. + + +Writing a program +----------------- + +To enforce a security policy, a thread first needs to create a Landlock program. +The easiest way to write an eBPF program depicting a security program is to write +it in the C language. As described in *samples/bpf/README.rst*, LLVM can +compile such programs. Files *samples/bpf/landlock1_kern.c* and those in +*tools/testing/selftests/landlock/* can be used as examples. + +Once the eBPF program is created, the next step is to create the metadata +describing the Landlock program. This metadata includes a subtype which +contains the hook type to which the program is tied and some options. + +.. code-block:: c + + union bpf_prog_subtype subtype = { + .landlock_hook = { + .type = LANDLOCK_HOOK_FS_PICK, + .triggers = LANDLOCK_TRIGGER_FS_PICK_OPEN, + } + }; + +A Landlock hook describes the kind of kernel object for which a program will be +triggered to allow or deny an action. For example, the hook +LANDLOCK_HOOK_FS_PICK can be triggered every time a landlocked thread performs +a set of action related to the filesystem (e.g. open, read, write, mount...). +This actions are identified by the `triggers` bitfield. + +The next step is to fill a :c:type:`union bpf_attr ` with +BPF_PROG_TYPE_LANDLOCK_HOOK, the previously created subtype and other BPF +program metadata. This bpf_attr must then be passed to the :manpage:`bpf(2)` +syscall alongside the BPF_PROG_LOAD command. If everything is deemed correct +by the kernel, the thread gets a file descriptor referring to this program. + +In the following code, the *insn* variable is an array of BPF instructions +which can be extracted from an ELF file as is done in bpf_load_file() from +*samples/bpf/bpf_load.c*. + +.. code-block:: c + + union bpf_attr attr = { + .prog_type = BPF_PROG_TYPE_LANDLOCK_HOOK, + .insn_cnt = sizeof(insn) / sizeof(struct bpf_insn), + .insns = (__u64) (unsigned long) insn, + .license = (__u64) (unsigned long) "GPL", + .prog_subtype = &subtype, + .prog_subtype_size = sizeof(subtype), + }; + int fd = bpf(BPF_PROG_LOAD, &attr, sizeof(attr)); + if (fd == -1) + exit(1); + + +Enforcing a program +------------------- + +Once the Landlock program has been created or received (e.g. through a UNIX +socket), the thread willing to sandbox itself (and its future children) should +perform the following two steps. + +The thread should first request to never be allowed to get new privileges with a +call to :manpage:`prctl(2)` and the PR_SET_NO_NEW_PRIVS option. More +information can be found in *Documentation/prctl/no_new_privs.txt*. + +.. code-block:: c + + if (prctl(PR_SET_NO_NEW_PRIVS, 1, NULL, 0, 0)) + exit(1); + +A thread can apply a program to itself by using the :manpage:`seccomp(2)` syscall. +The operation is SECCOMP_PREPEND_LANDLOCK_PROG, the flags must be empty and the +*args* argument must point to a valid Landlock program file descriptor. + +.. code-block:: c + + if (seccomp(SECCOMP_PREPEND_LANDLOCK_PROG, 0, &fd)) + exit(1); + +If the syscall succeeds, the program is now enforced on the calling thread and +will be enforced on all its subsequently created children of the thread as +well. Once a thread is landlocked, there is no way to remove this security +policy, only stacking more restrictions is allowed. The program evaluation is +performed from the newest to the oldest. + +When a syscall ask for an action on a kernel object, if this action is denied, +then an EACCES errno code is returned through the syscall. + + +.. _inherited_programs: + +Inherited programs +------------------ + +Every new thread resulting from a :manpage:`clone(2)` inherits Landlock program +restrictions from its parent. This is similar to the seccomp inheritance as +described in *Documentation/prctl/seccomp_filter.txt*. + + +Ptrace restrictions +------------------- + +A landlocked process has less privileges than a non-landlocked process and must +then be subject to additional restrictions when manipulating another process. +To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target +process, a landlocked process must have a subset of the target process programs. + + +Landlock structures and constants +================================= + +Hook types +---------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :functions: landlock_hook_type + + +Contexts +-------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :functions: landlock_ctx_fs_pick landlock_ctx_fs_walk landlock_ctx_fs_get + + +Triggers for fs_pick +-------------------- + +.. kernel-doc:: include/uapi/linux/landlock.h + :functions: landlock_triggers + + +Additional documentation +======================== + +See https://landlock.io