Thanks for the reviews!

On 26-Mär 12:31, Andrii Nakryiko wrote:

On Thu, Mar 26, 2020 at 7:29 AM KP Singh [off-list ref] wrote:

quoted

From: KP Singh <redacted>

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 <redacted>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Florent Revest <redacted>
Reviewed-by: Thomas Garnier <redacted>
---

This needs another pass and re-reading, has a bunch of outdated info :)

Indeed :)

quoted

 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.

quoted

+
+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.

quoted

+.. 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.

quoted

+       }
+
+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.

- KP

quoted

+
+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!

quoted

+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

quoted

+
+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