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Subject[PATCH bpf-next v1 00/13] MAC and Audit policy using eBPF (KRSI)
Date
From: KP Singh <kpsingh@google.com>

This patch series is a continuation of the KRSI RFC
(https://lore.kernel.org/bpf/20190910115527.5235-1-kpsingh@chromium.org/)

# Motivation

Google does rich analysis of runtime security data collected from
internal Linux deployments (corporate devices and servers) to detect and
thwart threats in real-time. Currently, this is done in custom kernel
modules but we would like to replace this with something that's upstream
and useful to others.

The current kernel infrastructure for providing telemetry (Audit, Perf
etc.) is disjoint from access enforcement (i.e. LSMs). Augmenting the
information provided by audit requires kernel changes to audit, its
policy language and user-space components. Furthermore, building a MAC
policy based on the newly added telemetry data requires changes to
various LSMs and their respective policy languages.

This patchset proposes a new stackable and privileged LSM which allows
the LSM hooks to be implemented using eBPF. This facilitates a unified
and dynamic (not requiring re-compilation of the kernel) audit and MAC
policy.

# Why an LSM?

Linux Security Modules target security behaviours rather than the
kernel's API. For example, it's easy to miss out a newly added system
call for executing processes (eg. execve, execveat etc.) but the LSM
framework ensures that all process executions trigger the relevant hooks
irrespective of how the process was executed.

Allowing users to implement LSM hooks at runtime also benefits the LSM
eco-system by enabling a quick feedback loop from the security community
about the kind of behaviours that the LSM Framework should be targeting.

# How does it work?

The LSM introduces a new eBPF (https://docs.cilium.io/en/v1.6/bpf/)
program type, BPF_PROG_TYPE_LSM, which can only be attached to a LSM
hook. All LSM hooks are exposed as files in securityfs. Attachment
requires CAP_SYS_ADMIN for loading eBPF programs and CAP_MAC_ADMIN for
modifying MAC policies.

The eBPF programs are passed the same arguments as the LSM hooks and
executed in the body of the hook. If any of the eBPF programs returns an
error (like ENOPERM), the behaviour represented by the hook is denied.

Audit logs can be written using a format chosen by the eBPF program to
the perf events buffer and can be further processed in user-space.

# Limitations of RFC v1

In the previous design
(https://lore.kernel.org/bpf/20190910115527.5235-1-kpsingh@chromium.org/),
the BPF programs received a context which could be queried to retrieve
specific pieces of information using specific helpers.

For example, a program that attaches to the file_mprotect LSM hook and
queries the VMA region could have had the following context:

// Special context for the hook.
struct bpf_mprotect_ctx {
struct vm_area_struct *vma;
};

and accessed the fields using a hypothetical helper
"bpf_mprotect_vma_get_start:

SEC("lsm/file_mprotect")
int mprotect_audit(bpf_mprotect_ctx *ctx)
{
unsigned long vm_start = bpf_mprotect_vma_get_start(ctx);
return 0;
}

or directly read them from the context by updating the verifier to allow
accessing the fields:

int mprotect_audit(bpf_mprotect_ctx *ctx)
{
unsigned long vm_start = ctx->vma->vm_start;
return 0;
}

As we prototyped policies based on this design, we realized that this
approach is not general enough. Adding helpers or verifier code for all
usages would imply a high maintenance cost while severely restricting
the instrumentation capabilities which is the key value add of our
eBPF-based LSM.

Feedback from the BPF maintainers at Linux Plumbers also pushed us
towards the following, more general, approach.

# BTF Based Design

The current design uses BTF
(https://facebookmicrosites.github.io/bpf/blog/2018/11/14/btf-enhancement.html,
https://lwn.net/Articles/803258/) which allows verifiable read-only
structure accesses by field names rather than fixed offsets. This allows
accessing the hook parameters using a dynamically created context which
provides a certain degree of ABI stability:

/* Clang builtin to handle field accesses. */
#define _(P) (__builtin_preserve_access_index(P))

// Only declare the structure and fields intended to be used
// in the program
struct vm_area_struct {
unsigned long vm_start;
};

// Declare the eBPF program mprotect_audit which attaches to
// to the file_mprotect LSM hook and accepts three arguments.
BPF_TRACE_3("lsm/file_mprotect", mprotect_audit,
struct vm_area_struct *, vma,
unsigned long, reqprot, unsigned long, prot
{
unsigned long vm_start = _(vma->vm_start);
return 0;
}

By relocating field offsets, BTF makes a large portion of kernel data
structures readily accessible across kernel versions without requiring a
large corpus of BPF helper functions and requiring recompilation with
every kernel version. The limitations of BTF compatibility are described
in BPF Co-Re (http://vger.kernel.org/bpfconf2019_talks/bpf-core.pdf,
i.e. field renames, #defines and changes to the signature of LSM hooks).

This design imposes that the MAC policy (eBPF programs) be updated when
the inspected kernel structures change outside of BTF compatibility
guarantees. In practice, this is only required when a structure field
used by a current policy is removed (or renamed) or when the used LSM
hooks change. We expect the maintenance cost of these changes to be
acceptable as compared to the previous design
(https://lore.kernel.org/bpf/20190910115527.5235-1-kpsingh@chromium.org/).

# Distinction from Landlock

We believe there exist two distinct use-cases with distinct set of users:

* Unprivileged processes voluntarily relinquishing privileges with the
primary users being software developers.

* Flexible privileged (CAP_MAC_ADMIN, CAP_SYS_ADMIN) MAC and Audit with
the primary users being system policy admins.

These use-cases imply different APIs and trade-offs:

* The unprivileged use case requires defining more stable and custom APIs
(through opaque contexts and precise helpers).

* Privileged Audit and MAC requires deeper introspection of the kernel
data structures to maximise the flexibility that can be achieved without
kernel modification.

Landlock has demonstrated filesystem sandboxes and now Ptrace access
control in its patches which are excellent use cases for an unprivileged
process voluntarily relinquishing privileges.

However, Landlock has expanded its original goal, "towards unprivileged
sandboxing", to being a "low-level framework to build
access-control/audit systems" (https://landlock.io). We feel that the
design and implementation are still driven by the constraints and
trade-offs of the former use-case, and do not provide a satisfactory
solution to the latter.

We also believe that our approach, direct access to common kernel data
structures as with BTF, is inappropriate for unprivileged processes and
probably not a good option for Landlock.

In conclusion, we feel that the design for a privileged LSM and
unprivileged LSM are mutually exclusive and that one cannot be built
"on-top-of" the other. Doing so would limit the capabilities of what can
be done for an LSM that provides flexible audit and MAC capabilities or
provide in-appropriate access to kernel internals to an unprivileged
process.

Furthermore, the Landlock design supports its historical use-case only
when unprivileged eBPF is allowed. This is something that warrants
discussion before an unprivileged LSM that uses eBPF is upstreamed.

# Why not tracepoints or kprobes?

In order to do MAC with tracepoints or kprobes, we would need to
override the return value of the security hook. This is not possible
with tracepoints or call-site kprobes.

Attaching to the return boundary (kretprobe) implies that BPF programs
would always get called after all the other LSM hooks are called and
clobber the pre-existing LSM semantics.

Enforcing MAC policy with an actual LSM helps leverage the verified
semantics of the framework.

# Usage Examples

A simple example and some documentation is included in the patchset.

In order to better illustrate the capabilities of the framework some
more advanced prototype code has also been published separately:

* Logging execution events (including environment variables and arguments):
https://github.com/sinkap/linux-krsi/blob/patch/v1/examples/samples/bpf/lsm_audit_env.c
* Detecting deletion of running executables:
https://github.com/sinkap/linux-krsi/blob/patch/v1/examples/samples/bpf/lsm_detect_exec_unlink.c
* Detection of writes to /proc/<pid>/mem:
https://github.com/sinkap/linux-krsi/blob/patch/v1/examples/samples/bpf/lsm_audit_env.c

We have updated Google's internal telemetry infrastructure and have
started deploying this LSM on our Linux Workstations. This gives us more
confidence in the real-world applications of such a system.

KP Singh (13):
bpf: Refactor BPF_EVENT context macros to its own header.
bpf: lsm: Add a skeleton and config options
bpf: lsm: Introduce types for eBPF based LSM
bpf: lsm: Allow btf_id based attachment for LSM hooks
tools/libbpf: Add support in libbpf for BPF_PROG_TYPE_LSM
bpf: lsm: Init Hooks and create files in securityfs
bpf: lsm: Implement attach, detach and execution.
bpf: lsm: Show attached program names in hook read handler.
bpf: lsm: Add a helper function bpf_lsm_event_output
bpf: lsm: Handle attachment of the same program
tools/libbpf: Add bpf_program__attach_lsm
bpf: lsm: Add selftests for BPF_PROG_TYPE_LSM
bpf: lsm: Add Documentation

Documentation/security/bpf.rst | 164 +++
Documentation/security/index.rst | 1 +
MAINTAINERS | 11 +
include/linux/bpf_event.h | 78 ++
include/linux/bpf_lsm.h | 25 +
include/linux/bpf_types.h | 4 +
include/trace/bpf_probe.h | 30 +-
include/uapi/linux/bpf.h | 12 +-
kernel/bpf/syscall.c | 10 +
kernel/bpf/verifier.c | 84 +-
kernel/trace/bpf_trace.c | 24 +-
security/Kconfig | 11 +-
security/Makefile | 2 +
security/bpf/Kconfig | 25 +
security/bpf/Makefile | 7 +
security/bpf/include/bpf_lsm.h | 63 +
security/bpf/include/fs.h | 23 +
security/bpf/include/hooks.h | 1015 +++++++++++++++++
security/bpf/lsm.c | 160 +++
security/bpf/lsm_fs.c | 176 +++
security/bpf/ops.c | 224 ++++
tools/include/uapi/linux/bpf.h | 12 +-
tools/lib/bpf/bpf.c | 2 +-
tools/lib/bpf/bpf.h | 6 +
tools/lib/bpf/libbpf.c | 163 ++-
tools/lib/bpf/libbpf.h | 4 +
tools/lib/bpf/libbpf.map | 7 +
tools/lib/bpf/libbpf_probes.c | 1 +
.../bpf/prog_tests/lsm_mprotect_audit.c | 129 +++
.../selftests/bpf/progs/lsm_mprotect_audit.c | 58 +
30 files changed, 2451 insertions(+), 80 deletions(-)
create mode 100644 Documentation/security/bpf.rst
create mode 100644 include/linux/bpf_event.h
create mode 100644 include/linux/bpf_lsm.h
create mode 100644 security/bpf/Kconfig
create mode 100644 security/bpf/Makefile
create mode 100644 security/bpf/include/bpf_lsm.h
create mode 100644 security/bpf/include/fs.h
create mode 100644 security/bpf/include/hooks.h
create mode 100644 security/bpf/lsm.c
create mode 100644 security/bpf/lsm_fs.c
create mode 100644 security/bpf/ops.c
create mode 100644 tools/testing/selftests/bpf/prog_tests/lsm_mprotect_audit.c
create mode 100644 tools/testing/selftests/bpf/progs/lsm_mprotect_audit.c

--
2.20.1

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