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143 lines
4.5 KiB
143 lines
4.5 KiB
.. 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 Documentation/bpf/btf.rst do not need to include kernel |
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headers for accessing information from the attached eBPF program's context. |
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They can simply declare the structures in the eBPF program and only specify |
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the fields 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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Documentation/bpf/btf.rst information. Since the BPF verifier is aware of the |
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types, it also validates all the accesses made to the various types in the |
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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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