1.. SPDX-License-Identifier: GPL-2.0+
2.. Copyright (C) 2020 Google LLC.
3
4================
5LSM BPF Programs
6================
7
8These BPF programs allow runtime instrumentation of the LSM hooks by privileged
9users to implement system-wide MAC (Mandatory Access Control) and Audit
10policies using eBPF.
11
12Structure
13---------
14
15The example shows an eBPF program that can be attached to the ``file_mprotect``
16LSM hook:
17
18.. c:function:: int file_mprotect(struct vm_area_struct *vma, unsigned long reqprot, unsigned long prot);
19
20Other LSM hooks which can be instrumented can be found in
21``security/security.c``.
22
23eBPF programs that use Documentation/bpf/btf.rst do not need to include kernel
24headers for accessing information from the attached eBPF program's context.
25They can simply declare the structures in the eBPF program and only specify
26the fields that need to be accessed.
27
28.. code-block:: c
29
30	struct mm_struct {
31		unsigned long start_brk, brk, start_stack;
32	} __attribute__((preserve_access_index));
33
34	struct vm_area_struct {
35		unsigned long start_brk, brk, start_stack;
36		unsigned long vm_start, vm_end;
37		struct mm_struct *vm_mm;
38	} __attribute__((preserve_access_index));
39
40
41.. note:: The order of the fields is irrelevant.
42
43This can be further simplified (if one has access to the BTF information at
44build time) by generating the ``vmlinux.h`` with:
45
46.. code-block:: console
47
48	# bpftool btf dump file <path-to-btf-vmlinux> format c > vmlinux.h
49
50.. note:: ``path-to-btf-vmlinux`` can be ``/sys/kernel/btf/vmlinux`` if the
51	  build environment matches the environment the BPF programs are
52	  deployed in.
53
54The ``vmlinux.h`` can then simply be included in the BPF programs without
55requiring the definition of the types.
56
57The eBPF programs can be declared using the``BPF_PROG``
58macros defined in `tools/lib/bpf/bpf_tracing.h`_. In this
59example:
60
61	* ``"lsm/file_mprotect"`` indicates the LSM hook that the program must
62	  be attached to
63	* ``mprotect_audit`` is the name of the eBPF program
64
65.. code-block:: c
66
67	SEC("lsm/file_mprotect")
68	int BPF_PROG(mprotect_audit, struct vm_area_struct *vma,
69		     unsigned long reqprot, unsigned long prot, int ret)
70	{
71		/* ret is the return value from the previous BPF program
72		 * or 0 if it's the first hook.
73		 */
74		if (ret != 0)
75			return ret;
76
77		int is_heap;
78
79		is_heap = (vma->vm_start >= vma->vm_mm->start_brk &&
80			   vma->vm_end <= vma->vm_mm->brk);
81
82		/* Return an -EPERM or write information to the perf events buffer
83		 * for auditing
84		 */
85		if (is_heap)
86			return -EPERM;
87	}
88
89The ``__attribute__((preserve_access_index))`` is a clang feature that allows
90the BPF verifier to update the offsets for the access at runtime using the
91Documentation/bpf/btf.rst information. Since the BPF verifier is aware of the
92types, it also validates all the accesses made to the various types in the
93eBPF program.
94
95Loading
96-------
97
98eBPF programs can be loaded with the :manpage:`bpf(2)` syscall's
99``BPF_PROG_LOAD`` operation:
100
101.. code-block:: c
102
103	struct bpf_object *obj;
104
105	obj = bpf_object__open("./my_prog.o");
106	bpf_object__load(obj);
107
108This can be simplified by using a skeleton header generated by ``bpftool``:
109
110.. code-block:: console
111
112	# bpftool gen skeleton my_prog.o > my_prog.skel.h
113
114and the program can be loaded by including ``my_prog.skel.h`` and using
115the generated helper, ``my_prog__open_and_load``.
116
117Attachment to LSM Hooks
118-----------------------
119
120The LSM allows attachment of eBPF programs as LSM hooks using :manpage:`bpf(2)`
121syscall's ``BPF_RAW_TRACEPOINT_OPEN`` operation or more simply by
122using the libbpf helper ``bpf_program__attach_lsm``.
123
124The program can be detached from the LSM hook by *destroying* the ``link``
125link returned by ``bpf_program__attach_lsm`` using ``bpf_link__destroy``.
126
127One can also use the helpers generated in ``my_prog.skel.h`` i.e.
128``my_prog__attach`` for attachment and ``my_prog__destroy`` for cleaning up.
129
130Examples
131--------
132
133An example eBPF program can be found in
134`tools/testing/selftests/bpf/progs/lsm.c`_ and the corresponding
135userspace code in `tools/testing/selftests/bpf/prog_tests/test_lsm.c`_
136
137.. Links
138.. _tools/lib/bpf/bpf_tracing.h:
139   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/lib/bpf/bpf_tracing.h
140.. _tools/testing/selftests/bpf/progs/lsm.c:
141   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/progs/lsm.c
142.. _tools/testing/selftests/bpf/prog_tests/test_lsm.c:
143   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/prog_tests/test_lsm.c
144