Hi Casey, Thanks for taking a look! On Fri, Dec 20, 2019 at 6:17 PM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote: > > On 12/20/2019 7:41 AM, KP Singh wrote: > > From: KP Singh <kpsingh@xxxxxxxxxx> > > > > This patch series is a continuation of the KRSI RFC > > (https://lore.kernel.org/bpf/20190910115527.5235-1-kpsingh@xxxxxxxxxxxx/) > > > > # 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@xxxxxxxxxxxx/), > > 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@xxxxxxxxxxxx/). > > > > # 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 > > >
