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SubjectRe: [PATCH bpf-next v6 0/8] MAC and Audit policy using eBPF (KRSI)
On Wed, Mar 25, 2020 at 04:26:21PM +0100, KP Singh wrote:
> From: KP Singh <kpsingh@google.com>
>
> # v5 -> v6
>
> https://lwn.net/Articles/815826/

Random question: why the switch to lwn.net from lore URLs? The lore
URLs have been suggested to be the canonical way to refer to kernel
development discussion threads.

-Kees

>
> * Updated LSM_HOOK macro to define a default value and cleaned up the
> BPF LSM hook declarations.
> * Added Yonghong's Acks and Kees' Reviewed-by tags.
> * Simplification of the selftest code.
> * Rebase and fixes suggested by Andrii and Yonghong and some other minor
> fixes noticed in internal review.
>
> # v4 -> v5
>
> https://lwn.net/Articles/813057/
>
> * Removed static keys and special casing of BPF calls from the LSM
> framework.
> * Initialized the BPF callbacks (nops) as proper LSM hooks.
> * Updated to using the newly introduced BPF_TRAMP_MODIFY_RETURN
> trampolines in https://lkml.org/lkml/2020/3/4/877
> * Addressed Andrii's feedback and rebased.
>
> # v3 -> v4
>
> * Moved away from allocating a separate security_hook_heads and adding a
> new special case for arch_prepare_bpf_trampoline to using BPF fexit
> trampolines called from the right place in the LSM hook and toggled by
> static keys based on the discussion in:
>
> https://lore.kernel.org/bpf/CAG48ez25mW+_oCxgCtbiGMX07g_ph79UOJa07h=o_6B6+Q-u5g@mail.gmail.com/
>
> * Since the code does not deal with security_hook_heads anymore, it goes
> from "being a BPF LSM" to "BPF program attachment to LSM hooks".
> * Added a new test case which ensures that the BPF programs' return value
> is reflected by the LSM hook.
>
> # v2 -> v3 does not change the overall design and has some minor fixes:
>
> * LSM_ORDER_LAST is introduced to represent the behaviour of the BPF LSM
> * Fixed the inadvertent clobbering of the LSM Hook error codes
> * Added GPL license requirement to the commit log
> * The lsm_hook_idx is now the more conventional 0-based index
> * Some changes were split into a separate patch ("Load btf_vmlinux only
> once per object")
>
> https://lore.kernel.org/bpf/20200117212825.11755-1-kpsingh@chromium.org/
>
> * Addressed Andrii's feedback on the BTF implementation
> * Documentation update for using generated vmlinux.h to simplify
> programs
> * Rebase
>
> # Changes since v1
>
> https://lore.kernel.org/bpf/20191220154208.15895-1-kpsingh@chromium.org
>
> * Eliminate the requirement to maintain LSM hooks separately in
> security/bpf/hooks.h Use BPF trampolines to dynamically allocate
> security hooks
> * Drop the use of securityfs as bpftool provides the required
> introspection capabilities. Update the tests to use the bpf_skeleton
> and global variables
> * Use O_CLOEXEC anonymous fds to represent BPF attachment in line with
> the other BPF programs with the possibility to use bpf program pinning
> in the future to provide "permanent attachment".
> * Drop the logic based on prog names for handling re-attachment.
> * Drop bpf_lsm_event_output from this series and send it as a separate
> patch.
>
> # Motivation
>
> Google does analysis of rich runtime security data 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 allows BPF programs to be attached to LSM hooks 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 patchset introduces a new eBPF (https://docs.cilium.io/en/v1.6/bpf/)
> program type BPF_PROG_TYPE_LSM which can only be attached to LSM hooks.
> Loading and attachment of BPF programs requires CAP_SYS_ADMIN.
>
> The new LSM registers nop functions (bpf_lsm_<hook_name>) as LSM hook
> callbacks. Their purpose is to provide a definite point where BPF
> programs can be attached as BPF_TRAMP_MODIFY_RETURN trampoline programs
> for hooks that return an int, and BPF_TRAMP_FEXIT trampoline programs
> for void LSM hooks.
>
> Audit logs can be written using a format chosen by the eBPF program to
> the perf events buffer or to global eBPF variables or maps and can be
> further processed in user-space.
>
> # 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:
>
>
> // Only declare the structure and fields intended to be used
> // in the program
> struct vm_area_struct {
> unsigned long vm_start;
> } __attribute__((preserve_access_index));
>
> // Declare the eBPF program mprotect_audit which attaches to
> // to the file_mprotect LSM hook and accepts three arguments.
> SEC("lsm/file_mprotect")
> int BPF_PROG(mprotect_audit, struct vm_area_struct *vma,
> unsigned long reqprot, unsigned long prot, int ret)
> {
> 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 BTF type information is also used by the BPF
> verifier to validate memory accesses within the BPF program and also
> prevents arbitrary writes to the kernel memory.
>
> 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 design presented in the RFC.
>
> (https://lore.kernel.org/bpf/20190910115527.5235-1-kpsingh@chromium.org/).
>
> # 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 (not-ready for review) 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 (8):
> bpf: Introduce BPF_PROG_TYPE_LSM
> security: Refactor declaration of LSM hooks
> bpf: lsm: provide attachment points for BPF LSM programs
> bpf: lsm: Implement attach, detach and execution
> bpf: lsm: Initialize the BPF LSM hooks
> tools/libbpf: Add support for BPF_PROG_TYPE_LSM
> bpf: lsm: Add selftests for BPF_PROG_TYPE_LSM
> bpf: lsm: Add Documentation
>
> Documentation/bpf/bpf_lsm.rst | 150 +++++
> Documentation/bpf/index.rst | 1 +
> MAINTAINERS | 1 +
> include/linux/bpf.h | 3 +
> include/linux/bpf_lsm.h | 32 +
> include/linux/bpf_types.h | 4 +
> include/linux/lsm_hook_defs.h | 378 +++++++++++
> include/linux/lsm_hooks.h | 627 +-----------------
> include/uapi/linux/bpf.h | 2 +
> init/Kconfig | 10 +
> kernel/bpf/Makefile | 1 +
> kernel/bpf/bpf_lsm.c | 60 ++
> kernel/bpf/btf.c | 9 +-
> kernel/bpf/syscall.c | 56 +-
> kernel/bpf/trampoline.c | 17 +-
> kernel/bpf/verifier.c | 19 +-
> kernel/trace/bpf_trace.c | 12 +-
> security/Kconfig | 10 +-
> security/Makefile | 2 +
> security/bpf/Makefile | 5 +
> security/bpf/hooks.c | 26 +
> security/security.c | 432 ++++++------
> tools/include/uapi/linux/bpf.h | 2 +
> tools/lib/bpf/bpf.c | 3 +-
> tools/lib/bpf/libbpf.c | 39 +-
> tools/lib/bpf/libbpf.h | 4 +
> tools/lib/bpf/libbpf.map | 3 +
> tools/lib/bpf/libbpf_probes.c | 1 +
> tools/testing/selftests/bpf/config | 2 +
> tools/testing/selftests/bpf/lsm_helpers.h | 19 +
> .../selftests/bpf/prog_tests/lsm_test.c | 112 ++++
> .../selftests/bpf/progs/lsm_int_hook.c | 54 ++
> .../selftests/bpf/progs/lsm_void_hook.c | 41 ++
> 33 files changed, 1277 insertions(+), 860 deletions(-)
> create mode 100644 Documentation/bpf/bpf_lsm.rst
> create mode 100644 include/linux/bpf_lsm.h
> create mode 100644 include/linux/lsm_hook_defs.h
> create mode 100644 kernel/bpf/bpf_lsm.c
> create mode 100644 security/bpf/Makefile
> create mode 100644 security/bpf/hooks.c
> create mode 100644 tools/testing/selftests/bpf/lsm_helpers.h
> create mode 100644 tools/testing/selftests/bpf/prog_tests/lsm_test.c
> create mode 100644 tools/testing/selftests/bpf/progs/lsm_int_hook.c
> create mode 100644 tools/testing/selftests/bpf/progs/lsm_void_hook.c
>
> --
> 2.20.1
>

--
Kees Cook

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