Re: [PATCH bpf-next v1 00/13] MAC and Audit policy using eBPF (KRSI)

[KP Singh <kpsingh@chromium.org>]

Hi Casey,

Thanks for taking a look!

On Fri, Dec 20, 2019 at 6:17 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
>
> On 12/20/2019 7:41 AM, KP Singh wrote:
> > 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.
>
> This effectively exposes the LSM hooks as external APIs.
> It would mean that we can't change or delete them. That
> would be bad.

Perhaps this should have been clearer, we *do not* want to make LSM hooks
a stable API and expect the eBPF programs to adapt when such changes occur.

Based on our comparison with the previous approach, this still ends up
being a better trade-off (w.r.t. maintenance) when compared to adding
specific helpers or verifier logic for  each new hook or field that
needs to be exposed.

- KP

>
>
> >  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
> >
>