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| -rw-r--r-- | Documentation/bpf/bpf_lsm.rst | 142 |
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diff --git a/Documentation/bpf/bpf_lsm.rst b/Documentation/bpf/bpf_lsm.rst new file mode 100644 index 000000000..1c0a75a51 --- /dev/null +++ b/Documentation/bpf/bpf_lsm.rst @@ -0,0 +1,142 @@ +.. 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 |