143 lines
4.4 KiB
ReStructuredText
143 lines
4.4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+
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.. Copyright (C) 2020 Google LLC.
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================
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LSM BPF Programs
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================
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These BPF programs allow runtime instrumentation of the LSM hooks by privileged
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users to implement system-wide MAC (Mandatory Access Control) and Audit
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policies using eBPF.
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Structure
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---------
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The example shows an eBPF program that can be attached to the ``file_mprotect``
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LSM hook:
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.. c:function:: int file_mprotect(struct vm_area_struct *vma, unsigned long reqprot, unsigned long prot);
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Other LSM hooks which can be instrumented can be found in
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``include/linux/lsm_hooks.h``.
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eBPF programs that use :doc:`/bpf/btf` do not need to include kernel headers
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for accessing information from the attached eBPF program's context. They can
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simply declare the structures in the eBPF program and only specify the fields
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that need to be accessed.
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.. code-block:: c
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struct mm_struct {
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unsigned long start_brk, brk, start_stack;
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} __attribute__((preserve_access_index));
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struct vm_area_struct {
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unsigned long start_brk, brk, start_stack;
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unsigned long vm_start, vm_end;
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struct mm_struct *vm_mm;
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} __attribute__((preserve_access_index));
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.. note:: The order of the fields is irrelevant.
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This can be further simplified (if one has access to the BTF information at
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build time) by generating the ``vmlinux.h`` with:
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.. code-block:: console
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# bpftool btf dump file <path-to-btf-vmlinux> format c > vmlinux.h
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.. note:: ``path-to-btf-vmlinux`` can be ``/sys/kernel/btf/vmlinux`` if the
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build environment matches the environment the BPF programs are
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deployed in.
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The ``vmlinux.h`` can then simply be included in the BPF programs without
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requiring the definition of the types.
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The eBPF programs can be declared using the``BPF_PROG``
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macros defined in `tools/lib/bpf/bpf_tracing.h`_. In this
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example:
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* ``"lsm/file_mprotect"`` indicates the LSM hook that the program must
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be attached to
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* ``mprotect_audit`` is the name of the eBPF program
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.. code-block:: c
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SEC("lsm/file_mprotect")
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int BPF_PROG(mprotect_audit, struct vm_area_struct *vma,
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unsigned long reqprot, unsigned long prot, int ret)
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{
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/* ret is the return value from the previous BPF program
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* or 0 if it's the first hook.
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*/
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if (ret != 0)
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return ret;
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int is_heap;
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is_heap = (vma->vm_start >= vma->vm_mm->start_brk &&
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vma->vm_end <= vma->vm_mm->brk);
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/* Return an -EPERM or write information to the perf events buffer
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* for auditing
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*/
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if (is_heap)
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return -EPERM;
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}
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The ``__attribute__((preserve_access_index))`` is a clang feature that allows
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the BPF verifier to update the offsets for the access at runtime using the
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:doc:`/bpf/btf` information. Since the BPF verifier is aware of the types, it
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also validates all the accesses made to the various types in the eBPF program.
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Loading
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-------
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eBPF programs can be loaded with the :manpage:`bpf(2)` syscall's
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``BPF_PROG_LOAD`` operation:
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.. code-block:: c
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struct bpf_object *obj;
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obj = bpf_object__open("./my_prog.o");
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bpf_object__load(obj);
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This can be simplified by using a skeleton header generated by ``bpftool``:
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.. code-block:: console
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# bpftool gen skeleton my_prog.o > my_prog.skel.h
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and the program can be loaded by including ``my_prog.skel.h`` and using
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the generated helper, ``my_prog__open_and_load``.
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Attachment to LSM Hooks
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-----------------------
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The LSM allows attachment of eBPF programs as LSM hooks using :manpage:`bpf(2)`
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syscall's ``BPF_RAW_TRACEPOINT_OPEN`` operation or more simply by
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using the libbpf helper ``bpf_program__attach_lsm``.
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The program can be detached from the LSM hook by *destroying* the ``link``
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link returned by ``bpf_program__attach_lsm`` using ``bpf_link__destroy``.
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One can also use the helpers generated in ``my_prog.skel.h`` i.e.
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``my_prog__attach`` for attachment and ``my_prog__destroy`` for cleaning up.
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Examples
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--------
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An example eBPF program can be found in
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`tools/testing/selftests/bpf/progs/lsm.c`_ and the corresponding
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userspace code in `tools/testing/selftests/bpf/prog_tests/test_lsm.c`_
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.. Links
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.. _tools/lib/bpf/bpf_tracing.h:
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https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/lib/bpf/bpf_tracing.h
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.. _tools/testing/selftests/bpf/progs/lsm.c:
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https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/progs/lsm.c
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.. _tools/testing/selftests/bpf/prog_tests/test_lsm.c:
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https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/prog_tests/test_lsm.c
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