From patchwork Thu Mar 26 14:28:23 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: KP Singh X-Patchwork-Id: 11460211 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 4743713A4 for ; Thu, 26 Mar 2020 14:28:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1D7E12082D for ; Thu, 26 Mar 2020 14:28:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="jUN78QlK" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728278AbgCZO2r (ORCPT ); Thu, 26 Mar 2020 10:28:47 -0400 Received: from mail-wm1-f65.google.com ([209.85.128.65]:37623 "EHLO mail-wm1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728248AbgCZO2r (ORCPT ); Thu, 26 Mar 2020 10:28:47 -0400 Received: by mail-wm1-f65.google.com with SMTP id d1so7243085wmb.2 for ; Thu, 26 Mar 2020 07:28:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=W5Ff7Fn5bZi7i9uBczexSq34+wj8TN4wlXIgFvQuXvM=; b=jUN78QlK1G4YyVDyrmrvjDleFBmsR6d6nESaJYDRYFSLHcDfq/imq26Gaqgao6QV8q l9gC9uAsQvY68rUOoR7xf81WyogRP2hg2qA2D/xV9rDsRSpRHRe7CKcqqIob5dV/ENJM ogmbBIKmkvBn4MsaQ/UE0igYWjby2rKWlf2bI= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=W5Ff7Fn5bZi7i9uBczexSq34+wj8TN4wlXIgFvQuXvM=; b=mte9mY2TZtxfcquZdGjKWC57fv/AeWLUi7GZHisjlDBQpyRSLEq8euCQdEDQs5/JpC YXzh9EYZZ8T2FenQqrQBHCuCcI9Fg/SyNFUY07XvT7G2KpSYYpsnzcbXB7E4HoSsGgVC tecM8NsyK5OATB5IOxb7X6R3JQbtsUoWe4e0FulmTrO+JvZ3mPSNXY4FPsOHu0UOicGR 59yjD2OHj/ENVuAPvAYTGUFdrZj5/FwPrqTYqwcDZX0CkMwcQVXG4mEwluxEXKC27JsY 5ycca6n6LzeviydbOuFSHXdgoAhC+Bee3O3UxldumS/+30ummGGUxAwoVPh3Yaxc+pyA ThZw== X-Gm-Message-State: ANhLgQ0yqILpMc1J5sSh1mq2bZOj2rCGHy0GlIi/e5GIt25yGxtYHITx /xVwmfP0aIcv6IALk3stZztDXA== X-Google-Smtp-Source: ADFU+vvt623pdVGLa4tSnvVuUBz4uO2AQOo0BPKA3e1gQU7O41ZRqUIPW5OQRA+Lo9JpGl1YnooR/Q== X-Received: by 2002:a1c:9e97:: with SMTP id h145mr205280wme.61.1585232924119; Thu, 26 Mar 2020 07:28:44 -0700 (PDT) Received: from kpsingh-kernel.localdomain (77-56-209-237.dclient.hispeed.ch. [77.56.209.237]) by smtp.gmail.com with ESMTPSA id q3sm3643971wru.87.2020.03.26.07.28.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 26 Mar 2020 07:28:43 -0700 (PDT) From: KP Singh To: linux-kernel@vger.kernel.org, bpf@vger.kernel.org, linux-security-module@vger.kernel.org Cc: Brendan Jackman , Florent Revest , Thomas Garnier , Alexei Starovoitov , Daniel Borkmann , James Morris , Kees Cook , Paul Turner , Jann Horn , Florent Revest , Brendan Jackman , Greg Kroah-Hartman Subject: [PATCH bpf-next v7 8/8] bpf: lsm: Add Documentation Date: Thu, 26 Mar 2020 15:28:23 +0100 Message-Id: <20200326142823.26277-9-kpsingh@chromium.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20200326142823.26277-1-kpsingh@chromium.org> References: <20200326142823.26277-1-kpsingh@chromium.org> MIME-Version: 1.0 Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: From: KP Singh Document how eBPF programs (BPF_PROG_TYPE_LSM) can be loaded and attached (BPF_LSM_MAC) to the LSM hooks. Signed-off-by: KP Singh Reviewed-by: Brendan Jackman Reviewed-by: Florent Revest Reviewed-by: Thomas Garnier Reviewed-by: James Morris --- Documentation/bpf/bpf_lsm.rst | 150 ++++++++++++++++++++++++++++++++++ Documentation/bpf/index.rst | 1 + 2 files changed, 151 insertions(+) create mode 100644 Documentation/bpf/bpf_lsm.rst diff --git a/Documentation/bpf/bpf_lsm.rst b/Documentation/bpf/bpf_lsm.rst new file mode 100644 index 000000000000..2a2c3b4a74d4 --- /dev/null +++ b/Documentation/bpf/bpf_lsm.rst @@ -0,0 +1,150 @@ +.. SPDX-License-Identifier: GPL-2.0+ +.. Copyright (C) 2020 Google LLC. + +================ +LSM BPF Programs +================ + +These BPF programs allow runtime instrumentation of the LSM hooks by privileged +users to implement system-wide MAC (Mandatory Access Control) and Audit +policies using eBPF. Since these program end up modifying the MAC policies of +the system, they require both ``CAP_MAC_ADMIN`` and also require +``CAP_SYS_ADMIN`` for the loading of BPF programs. + +Structure +--------- + +The example shows an eBPF program that can be attached to the ``file_mprotect`` +LSM hook: + +.. c:function:: int file_mprotect(struct vm_area_struct *vma, unsigned long reqprot, unsigned long prot); + +Other LSM hooks which can be instrumented can be found in +``include/linux/lsm_hooks.h``. + +eBPF programs that use :doc:`/bpf/btf` do not need to include kernel headers +for accessing information from the attached eBPF program's context. They can +simply declare the structures in the eBPF program and only specify the fields +that need to be accessed. + +.. code-block:: c + + struct mm_struct { + unsigned long start_brk, brk, start_stack; + } __attribute__((preserve_access_index)); + + struct vm_area_struct { + unsigned long start_brk, brk, start_stack; + unsigned long vm_start, vm_end; + struct mm_struct *vm_mm; + } __attribute__((preserve_access_index)); + + +.. note:: Only the size and the names of the fields must match the type in the + kernel and the order of the fields is irrelevant. + +This can be further simplified (if one has access to the BTF information at +build time) by generating the ``vmlinux.h`` with: + +.. code-block:: console + + # bpftool dump file format c > vmlinux.h + +.. note:: ``path-to-btf-vmlinux`` can be ``/sys/kernel/btf/vmlinux`` if the + build environment matches the environment the BPF programs are + deployed in. + +The ``vmlinux.h`` can then simply be included in the BPF programs without +requiring the definition of the types. + +The eBPF programs can be declared using the``BPF_PROG`` +macros defined in `tools/lib/bpf/bpf_tracing.h`_. In this +example: + + * ``"lsm/file_mprotect"`` indicates the LSM hook that the program must + be attached to + * ``mprotect_audit`` is the name of the eBPF program + +.. code-block:: c + + SEC("lsm/file_mprotect") + int BPF_PROG(mprotect_audit, struct vm_area_struct *vma, + unsigned long reqprot, unsigned long prot, int ret) + { + /* Ret is the return value from the previous BPF program + * or 0 if it's the first hook. + */ + if (ret != 0) + return ret; + + int is_heap; + + is_heap = (vma->vm_start >= vma->vm_mm->start_brk && + vma->vm_end <= vma->vm_mm->brk); + + /* Return an -EPERM or write information to the perf events buffer + * for auditing + */ + } + +The ``__attribute__((preserve_access_index))`` is a clang feature that allows +the BPF verifier to update the offsets for the access at runtime using the +:doc:`/bpf/btf` information. Since the BPF verifier is aware of the types, it +also validates all the accesses made to the various types in the eBPF program. + +Loading +------- + +eBPF programs can be loaded with the :manpage:`bpf(2)` syscall's +``BPF_PROG_LOAD`` operation or more simply by using the the libbpf helper +``bpf_prog_load_xattr``: + + +.. code-block:: c + + struct bpf_prog_load_attr attr = { + .file = "./prog.o", + }; + struct bpf_object *prog_obj; + struct bpf_program *prog; + int prog_fd; + + bpf_prog_load_xattr(&attr, &prog_obj, &prog_fd); + +Attachment to LSM Hooks +----------------------- + +The LSM allows attachment of eBPF programs as LSM hooks using :manpage:`bpf(2)` +syscall's ``BPF_PROG_ATTACH`` operation or more simply by +using the libbpf helper ``bpf_program__attach_lsm``. In the code shown below +``prog`` is the eBPF program loaded using ``BPF_PROG_LOAD``: + +.. code-block:: c + + struct bpf_link *link; + + link = bpf_program__attach_lsm(prog); + +The program can be detached from the LSM hook by *destroying* the ``link`` +link returned by ``bpf_program__attach_lsm``: + +.. code-block:: c + + link->destroy(); + +Examples +-------- + +An example eBPF programs can be found in +`tools/testing/selftests/bpf/progs/lsm.c`_ and the corresponding +userspace code in `tools/testing/selftests/bpf/prog_tests/test_lsm.c`_ + +.. Links +.. _tools/lib/bpf/bpf_tracing.h: + https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/lib/bpf/bpf_tracing.h +.. _tools/testing/selftests/bpf/progs/lsm.c: + https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/progs/lsm.c +.. _tools/testing/selftests/bpf/progs/lsm_void_hook.c: + https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/progs/lsm_void_hook.c +.. _tools/testing/selftests/bpf/prog_tests/test_lsm.c: + https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/prog_tests/test_lsm.c diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst index 7be43c5f2dcf..f99677f3572f 100644 --- a/Documentation/bpf/index.rst +++ b/Documentation/bpf/index.rst @@ -45,6 +45,7 @@ Program types prog_cgroup_sockopt prog_cgroup_sysctl prog_flow_dissector + bpf_lsm Testing and debugging BPF