Re: [PATCH bpf-next v7 8/8] bpf: lsm: Add Documentation

[Andrii Nakryiko]

On Thu, Mar 26, 2020 at 1:56 PM KP Singh <kpsingh@xxxxxxxxxxxx> wrote:
>
> Thanks for the reviews!
>
> On 26-MÃr 12:31, Andrii Nakryiko wrote:
> > 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 This needs another pass and re-reading, has a bunch of outdated info :)
>
> Indeed :)
>
> >
> > >  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?
>
> I changed it to simply be:
>
> .. 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 dump file <path-to-btf-vmlinux> format c > vmlinux.h
> > > +
> >
> > bpftool btf *dump* file
>
> Done.
>
> >
> > > +.. 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?
>
> Fixed.
>
> >
> > > +       }
> > > +
> > > +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.
>
>
> Simplified and modernized this section as:
>
>
> 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.
>
> </end>
>
> If this looks okay, I will send a v8 with this updated and other
> fixes.
>

Sounds good.

> - KP
>
> >
> > > +
> > > +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?
>
> Correct, updated. Thanks!
>
> >
> > > +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 ;)
>
> Oops, I incorrectly picked it up from link->destroy(link); and wrote
> something stupid.
>
> >
> > bpf_link__destroy(link);
>
> Updated in the snippet posted above.
>
> - KP
>
> >
> > > +
> > > +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
> > >