Message-ID: <20200325194238.GB22898@chromium.org>
Date:   Wed, 25 Mar 2020 20:42:38 +0100
From:   KP Singh <kpsingh@...omium.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>, Paul Turner <pjt@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH bpf-next v6 0/8] MAC and Audit policy using eBPF (KRSI)

On 25-Mär 12:24, Kees Cook wrote:
> On Wed, Mar 25, 2020 at 04:26:21PM +0100, KP Singh wrote:
> > From: KP Singh <kpsingh@...gle.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.

No real reason apart from the fact these were shoter:)

Duly noted for future patches and revisions. Thanks!

- KP

> 
> -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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