[PATCH bpf-next v1 13/13] bpf: lsm: Add Documentation

[KP Singh <kpsingh@chromium.org>]

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