bpf: lsm: Add Documentation

.. 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.

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:: 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 btf dump file <path-to-btf-vmlinux> 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

		 */

		if (is_heap)

			return -EPERM;

	}

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:

.. code-block:: c

	struct bpf_object *obj;

	obj = bpf_object__open("./my_prog.o");

	bpf_object__load(obj);

This can be simplified by using a skeleton header generated by ``bpftool``:

.. code-block:: console

	# bpftool gen skeleton my_prog.o > my_prog.skel.h

and the program can be loaded by including ``my_prog.skel.h`` and using

the generated helper, ``my_prog__open_and_load``.

Attachment to LSM Hooks

-----------------------

The LSM allows attachment of eBPF programs as LSM hooks using :manpage:`bpf(2)`

syscall's ``BPF_RAW_TRACEPOINT_OPEN`` operation or more simply by

using the libbpf helper ``bpf_program__attach_lsm``.

The program can be detached from the LSM hook by *destroying* the ``link``

link returned by ``bpf_program__attach_lsm`` using ``bpf_link__destroy``.

One can also use the helpers generated in ``my_prog.skel.h`` i.e.

``my_prog__attach`` for attachment and ``my_prog__destroy`` for cleaning up.

Examples

--------

An example eBPF program 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/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

   prog_cgroup_sockopt

   prog_cgroup_sysctl

   prog_flow_dissector

   bpf_lsm

Testing and debugging BPF