On Thu, Mar 26, 2020 at 7:29 AM KP Singh <kpsingh@xxxxxxxxxxxx> wrote:
>
> From: KP Singh <kpsingh@xxxxxxxxxx>
>
> 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 <kpsingh@xxxxxxxxxx>
> Reviewed-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
> Reviewed-by: Florent Revest <revest@xxxxxxxxxx>
> Reviewed-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
> ---
This needs another pass and re-reading, has a bunch of outdated info :)
> Documentation/bpf/bpf_lsm.rst | 150 ++++++++++++++++++++++++++++++++++
> Documentation/bpf/index.rst | 1 +
> 2 files changed, 151 insertions(+)
> create mode 100644 Documentation/bpf/bpf_lsm.rst
>
> diff --git a/Documentation/bpf/bpf_lsm.rst b/Documentation/bpf/bpf_lsm.rst
> new file mode 100644
> index 000000000000..2a2c3b4a74d4
> --- /dev/null
> +++ b/Documentation/bpf/bpf_lsm.rst
> @@ -0,0 +1,150 @@
> +.. 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. Since these program end up modifying the MAC policies of
> +the system, they require both ``CAP_MAC_ADMIN`` and also require
> +``CAP_SYS_ADMIN`` for the loading of BPF programs.
> +
> +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:: Only the size and the names of the fields must match the type in the
> + kernel and the order of the fields is irrelevant.
type should match/be compatible as well?
> +
> +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 dump file <path-to-btf-vmlinux> format c > vmlinux.h
> +
bpftool btf *dump* file
> +.. 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
> + */
return missing?
> + }
> +
> +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 or more simply by using the the libbpf helper
> +``bpf_prog_load_xattr``:
> +
> +
> +.. code-block:: c
> +
> + struct bpf_prog_load_attr attr = {
> + .file = "./prog.o",
> + };
> + struct bpf_object *prog_obj;
> + struct bpf_program *prog;
> + int prog_fd;
> +
> + bpf_prog_load_xattr(&attr, &prog_obj, &prog_fd);
Can you please update this to not use deprecated/legacy APIs. Please
suggest bpf_object__open/bpf_object__load and/or BPF skeleton as an
example.
> +
> +Attachment to LSM Hooks
> +-----------------------
> +
> +The LSM allows attachment of eBPF programs as LSM hooks using :manpage:`bpf(2)`
> +syscall's ``BPF_PROG_ATTACH`` operation or more simply by
BPF_PROG_ATTACH is incorrect, it's RAW_TRACEPOINT_OPEN, isn't it?
> +using the libbpf helper ``bpf_program__attach_lsm``. In the code shown below
> +``prog`` is the eBPF program loaded using ``BPF_PROG_LOAD``:
> +
> +.. code-block:: c
> +
> + struct bpf_link *link;
> +
> + link = bpf_program__attach_lsm(prog);
> +
> +The program can be detached from the LSM hook by *destroying* the ``link``
> +link returned by ``bpf_program__attach_lsm``:
> +
> +.. code-block:: c
> +
> + link->destroy();
that's not how it works in C ;)
bpf_link__destroy(link);
> +
> +Examples
> +--------
> +
> +An example eBPF programs 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/progs/lsm_void_hook.c:
> + https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/progs/lsm_void_hook.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
> diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst
> index 7be43c5f2dcf..f99677f3572f 100644
> --- a/Documentation/bpf/index.rst
> +++ b/Documentation/bpf/index.rst
> @@ -45,6 +45,7 @@ Program types
> prog_cgroup_sockopt
> prog_cgroup_sysctl
> prog_flow_dissector
> + bpf_lsm
>
>
> Testing and debugging BPF
> --
> 2.20.1
>
