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Subject[PATCH bpf-next v1 13/13] bpf: lsm: Add Documentation
Date
From: KP Singh <kpsingh@google.com>

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@google.com>
---
Documentation/security/bpf.rst | 164 +++++++++++++++++++++++++++++++
Documentation/security/index.rst | 1 +
MAINTAINERS | 1 +
3 files changed, 166 insertions(+)
create mode 100644 Documentation/security/bpf.rst

diff --git a/Documentation/security/bpf.rst b/Documentation/security/bpf.rst
new file mode 100644
index 000000000000..898b7de148a0
--- /dev/null
+++ b/Documentation/security/bpf.rst
@@ -0,0 +1,164 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright 2019 Google LLC.
+
+==========================
+eBPF Linux Security Module
+==========================
+
+This LSM allows runtime instrumentation of the LSM hooks by privileged users to
+implement system-wide MAC (Mandatory Access Control) and Audit policies using
+eBPF. The LSM is priveleged and stackable and requires both ``CAP_SYS_ADMIN``
+and ``CAP_MAC_ADMIN`` for the loading of BPF programs and modification of MAC
+policies respectively.
+
+eBPF Programs
+==============
+
+`eBPF (extended BPF) <https://cilium.readthedocs.io/en/latest/bpf>`_ is a
+virtual machine-like construct in the Linux Kernel allowing the execution of
+verifiable, just-in-time compiled byte code at various points in the Kernel.
+
+The eBPF LSM adds a new type, ``BPF_PROG_TYPE_LSM``, of eBPF programs which
+have the following characteristics:
+
+ * Multiple eBPF programs can be attached to the same LSM hook.
+ * LSM hooks can return an ``-EPERM`` to indicate the decision of the
+ MAC policy being enforced or simply be used for auditing.
+ * Allowing the eBPF programs to be attached to all the LSM hooks by
+ making :doc:`/bpf/btf` type information available for all LSM hooks
+ and allowing the BPF verifier to perform runtime relocations and
+ validation on the 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);
+
+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;
+ };
+
+ struct vm_area_struct {
+ unsigned long start_brk, brk, start_stack;
+ unsigned long vm_start, vm_end;
+ struct mm_struct *vm_mm;
+ };
+
+
+.. 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.
+
+The eBPF programs can be declared using macros similar to the ``BPF_TRACE_<N>``
+macros defined in `tools/testing/selftests/bpf/bpf_trace_helpers.h`_. In this
+example:
+
+ * The LSM hook takes 3 args so we use ``BPF_TRACE_3``.
+ * ``"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
+
+ BPF_TRACE_3("lsm/file_mprotect", mprotect_audit,
+ struct vm_area_struct *, vma,
+ unsigned long, reqprot, unsigned long, prot)
+ {
+ int is_heap = 0;
+
+ __builtin_preserve_access_index(({
+ 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
+ */
+ }
+
+The ``__builtin_preserve_access_index`` is a clang primitive 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
+-------
+
+eBPP 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);
+
+Attachment to LSM Hooks
+-----------------------
+
+The LSM creates a file in securityfs for each LSM hook to which eBPF programs
+can be attached using :manpage:`bpf(2)` syscall's ``BPF_PROG_ATTACH`` operation
+or more simply by 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 attachment can be verified by:
+
+.. code-block:: console
+
+ # cat /sys/kernel/security/bpf/file_mprotect
+ mprotect_audit
+
+If, when a program is attached, another program by the same name is already attached to the hook, that program is replaced.
+
+
+.. note:: This requires that the ``BPF_F_ALLOW_OVERRIDE`` flag be passed to
+ the :manpage:`bpf(2)` syscall. If not, an ``-EEXIST`` error is returned instead.
+
+For conveniently versioning updating programs, program names are only compared up to the first ``"__"``. Thus if a program ``mprotect_audit__v1`` is attached and then ``mprotect_audit__v2`` is attached to the same hook, the latter will *replace* the former.
+
+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();
+
+Examples
+--------
+
+An example eBPF program can be found in
+`tools/testing/selftests/bpf/progs/lsm_mprotect_audit.c`_ and the corresponding
+userspace code in
+`tools/testing/selftests/bpf/prog_tests/lsm_mprotect_audit.c`_
+
+.. Links
+.. _tools/testing/selftests/bpf/bpf_trace_helpers.h:
+ https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/selftests/bpf/bpf_trace_helpers.h
+.. _tools/testing/selftests/bpf/progs/lsm_mprotect_audit.c:
+ https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/progs/lsm_mprotect_audit.c
+.. _tools/testing/selftests/bpf/prog_tests/lsm_mprotect_audit.c:
+ https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/prog_tests/lsm_mprotect_audit.c
diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst
index fc503dd689a7..844463df4547 100644
--- a/Documentation/security/index.rst
+++ b/Documentation/security/index.rst
@@ -5,6 +5,7 @@ Security Documentation
.. toctree::
:maxdepth: 1

+ bpf
credentials
IMA-templates
keys/index
diff --git a/MAINTAINERS b/MAINTAINERS
index 652c93292ae9..6f34c24519ca 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3184,6 +3184,7 @@ F: security/bpf/
F: include/linux/bpf_lsm.h
F: tools/testing/selftests/bpf/progs/lsm_mprotect_audit.c
F: tools/testing/selftests/bpf/prog_tests/lsm_mprotect_audit.c
+F: Documentation/security/bpf.rst

BROADCOM B44 10/100 ETHERNET DRIVER
M: Michael Chan <michael.chan@broadcom.com>
--
2.20.1
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