bpf: lsm: Add Documentation

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 <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Brendan Jackman <[e-mail]>
Reviewed-by: Florent Revest <[e-mail]>
Reviewed-by: Thomas Garnier <[e-mail]>
Reviewed-by: James Morris <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

Documentation/bpf/bpf_lsm.rst

···
+.. 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

Documentation/bpf/index.rst

···
    prog_cgroup_sockopt
    prog_cgroup_sysctl
    prog_flow_dissector
+   bpf_lsm
 
 
 Testing and debugging BPF