Re: [RFC PATCH 00/20] BPF interface for applying Landlock rulesets

[Ihor Solodrai]

On 4/7/26 1:01 PM, Justin Suess wrote:
> Hello,
> 
> This series lets sleepable BPF LSM programs apply an existing,
> userspace-created Landlock ruleset to a program during exec.
> 
> The goal is not to move Landlock policy definition into BPF, nor to create a
> second policy engine.  Instead, BPF is used only to select when an already
> valid Landlock ruleset should be applied, based on runtime exec context.
> 
> Background
> ===
> 
> Landlock is primarily a syscall-driven, unprivileged-first LSM.  That model
> works well when the application being sandboxed can create and enforce its own
> rulesets, or when a trusted launcher can impose restrictions directly before
> running a trusted target.
> 
> That becomes harder when the target program is not under first-party control,
> for example:
> 
> 1. third-party binaries,
> 2. unmodified container images,
> 3. programs reached through shells, wrappers, or service managers, and
> 4. user-supplied or otherwise untrusted code.
> 
> In these cases, an external supervisor may want to apply a Landlock ruleset to
> the final executed program, while leaving unrelated parents or helper
> processes alone.
> 
> Why external sandboxing is awkward today
> ===
> 
> There are two recurring problems.
> 
> First, userspace cannot reliably predict every file a target may need across
> different systems, packaging layouts, and runtime conditions.  Shared
> libraries, configuration files, interpreters, and helper binaries often depend
> on details that are only known at runtime.
> 
> Second, Landlock inheritance is intentionally one-way.  Once a task is
> restricted, descendants inherit that domain and may only become more
> restricted.  This is exactly what Landlock should do, but it makes external
> sandboxing awkward when the program of interest is buried inside a larger exec
> chain.  Applying restrictions too early can affect unrelated intermediates;
> applying them too late misses the target entirely.
> 
> This series addresses that target-selection problem.
> 
> Overview
> ===
> 
> This series adds a small BPF-to-Landlock bridge:
> 
> 1. userspace creates a normal Landlock ruleset through the existing ABI;
> 2. userspace inserts that ruleset FD into a new
> 	BPF_MAP_TYPE_LANDLOCK_RULESET map;
> 3. a sleepable BPF LSM program attached to an exec-time hook looks up the
> 	ruleset; and
> 4. the program calls a kfunc to apply that ruleset to the new program's
> 	credentials before exec completes.
> 
> The important point is that BPF does not create, inspect, or mutate Landlock
> policy here.  It only decides whether to apply a ruleset that was already
> created and validated through Landlock's existing userspace API.
> 
> Interface
> ===
> 
> The series adds:
> 
> 1. bpf_landlock_restrict_binprm(), which applies a referenced ruleset to
> 	struct linux_binprm credentials;
> 2. bpf_landlock_put_ruleset(), which releases a referenced ruleset; and
> 3. BPF_MAP_TYPE_LANDLOCK_RULESET, a specialized map type for holding
> 	references to Landlock rulesets originating from userspace file
> 	descriptors.
> 4. A new field in the linux_binprm struct to enable application of
>    task_set_no_new_privs once execution is beyond the point of no return.
> 
> The kfuncs are restricted to sleepable BPF LSM programs attached to
> bprm_creds_for_exec and bprm_creds_from_file, which are the points where the
> new program's credentials may still be updated safely.
> 
> This series also adds LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS.  On the BPF path,
> this is staged through the exec context and committed only after exec reaches
> point-of-no-return.  This avoids side effects on failed executions while
> ensuring that the resulting task cannot gain more privileges through later exec
> transitions. This is done through the set_nnp_on_point_of_no_return field.
> 
> This has a little subtlety: LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS in the BPF
> path will not stop the current execution from escalating at all; only subsequent
> ones. This is intentional to allow landlock policies to be applied through a
> setuid transition for instance, without affecting the current escalation.
> 
> Semantics
> ===
> 
> This proposal is intended to preserve Landlock semantics as much as practical
> for an exec-time BPF attachment model:
> 
> 1. only pre-existing Landlock rulesets may be applied;
> 2. BPF cannot construct, inspect, or modify rulesets;
> 3. enforcement still happens before the new program begins execution;
> 4. normal Landlock inheritance, layering, and future composition remain
> 	unchanged; and
> 5. this does not bypass Landlock's privilege checks for applying Landlock
>     rulesets.
> 
> In other words, BPF acts as an external selector for when to apply Landlock,
> not as a replacement for Landlock's enforcement engine.
> 
> All behavior, future access rights, and previous access rights are designed
> to automatically be supported from either BPF or existing syscall contexts.
> 
> The main semantic difference is LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS on the BPF
> path: it guarantees that the resulting task is pinned with no_new_privs before
> it can perform later exec transitions, but it does not retroactively suppress
> privilege gain for the current exec transition itself.
> 
> The other exception to semantics is the LANDLOCK_RESTRICT_SELF_TSYNC flag.
> (see Points of Feedback section)
> 
> Patch layout
> ===
> 
> Patches 1-5 prepare the Landlock side by moving shared ruleset logic out of
> syscalls.c, adding a no_new_privs flag for non-syscall callers, exposing
> linux_binprm->set_nnp_on_point_of_no_return as an interface to set no_new_privs
> on the point of no return, and making deferred ruleset destruction RCU-safe.
> 
> Patches 6-10 add the BPF-facing pieces: the Landlock kfuncs, the new map type,
> syscall handling for that map, and verifier support.
> 
> Patches 11-15 add selftests and the small bpftool update needed for the new
> map type.
> 
> Patches 16-20 add docs and bump the ABI version and update MAINTAINERS.
> 
> Feedback is especially welcome on the overall interface shape, the choice of
> hooks, and the map semantics.
> 
> Testing
> ===
> 
> This patch series has two portions of tests.
> 
> One lives in the traditional Landlock selftests, for the new
> LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS flag.
> 
> The other suite lives under the BPF selftests, and this tests the Landlock
> kfuncs and the new BPF_MAP_TYPE_LANDLOCK_RULESET.
> 
> This patch series was run through BPF CI, the results of which are here. [1]
> 
> All mentioned tests are passing, as well as the BPF CI.
> 
> [1] : https://github.com/kernel-patches/bpf/pull/11562

Hello Justin.

I regret to disappoint you with a lame piece of feedback, but the
series hasn't been picked up by automated BPF CI pipeline properly:
https://github.com/kernel-patches/bpf/pull/11709

I suggest you rebase on top of bpf-next/master [1], and re-submit to
the mailing list with a bpf-next tag in subject:
   "[RFC PATCH bpf-next ...] bpf: ..."

I'm pretty sure AI bot will find something annoying to address.

Other than that, please be patient. It'll probably take a while for
maintainers and reviewers to digest this work before anyone can
meaningfully comment. Thanks!

[1] https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/

> 
> Points of Feedback
> ===
> 
> First, the new set_nnp_on_point_of_no_return field in struct linux_binprm.
> This field was needed to request that task_set_no_new_privs be set during an
> execution, but only after the execution has proceeded beyond the point of no
> return. I couldn't find a way to express this semantic without adding a new
> bitfield to struct linux_binprm and a conditional in fs/exec.c. Please see
> patch 2.
> 
> Feedback on the BPF testing harness, which was generated with AI assistance as
> disclosed in the commit footer, is welcomed. I have only limited familiarity
> with BPF testing practices. These tests were made with strong human supervision.
> See patches 14 and 15.
> 
> Feedback on the NO_NEW_PRIVS situation is also welcomed. Because task_set_no_new_privs()
> would otherwise leak state on failed executions or AT_EXECVE_CHECK, this series
> stages no_new_privs through the exec context and only commits it after
> point-of-no-return. This preserves failure behavior while still ensuring that
> the resulting task cannot elevate further through later exec transitions.
> When called from bprm_creds_from_file, this does not retroactively change the
> privilege outcome of the current exec transition itself.
> 
> See patch 2 and 3.
> 
> Next, the RCU in the landlock_ruleset. Existing BPF maps use RCU to make sure maps
> holding references stay valid. I altered the landlock ruleset to use rcu_work
> to make sure that the rcu is synchronized before putting on a ruleset, and
> acquire the rcu in the arraymap implementation. See patches 5-10.
> 
> Next, the semantics of the map. What operations should be supported from BPF
> and userspace and what data types should they return? I consider the struct
> bpf_landlock_ruleset to be opaque. Userspace can add items to the map via the
> fd, delete items by their index, and BPF can delete and lookup items by their
> index. Items cannot be updated, only swapped.
> 
> Finally, the handling of the LANDLOCK_RESTRICT_SELF_TSYNC flag. This flag has
> no meaning in a pre-execution context, as the credentials during the designated
> LSM hooks (bprm_creds_for_exec/creds_from_file) still represent the pre-execution
> task. Therefore, this flag is invalidated and attempting to use it with
> bpf_landlock_restrict_binprm will return -EINVAL. Otherwise, the flag would
> result in applying the landlock ruleset to the wrong target in addition to the
> intended one. (see patch 2). This behavior is validated with selftests.
> 
> Existing works / Credits
> ===
> 
> Mickaël Salaün created patchsets adding BPF tracepoints for landlock in [2] [3].
> 
> Mickaël also gave feedback on this feature and the idea in this GitHub thread. [4]
> 
> Günther Noack initially received and provided initial feedback on this idea as
> an early prototype.
> 
> Liz Rice, author of "Learning eBPF: Programming the Linux Kernel for Enhanced
> Observability, Networking, and Security" provided background and inspired me to
> experiment with BPF and the BPF LSM. [5]
> 
> [2] : https://lore.kernel.org/all/20250523165741.693976-1-mic@xxxxxxxxxxx/
> [3] : https://lore.kernel.org/linux-security-module/20260406143717.1815792-1-mic@xxxxxxxxxxx/
> [4] : https://github.com/landlock-lsm/linux/issues/56
> [5] : https://wellesleybooks.com/book/9781098135126
> 
> Kind Regards,
> Justin Suess
> 
> Justin Suess (20):
>   landlock: Move operations from syscall into ruleset code
>   execve: Add set_nnp_on_point_of_no_return
>   landlock: Implement LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
>   selftests/landlock: Cover LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
>   landlock: Make ruleset deferred free RCU safe
>   bpf: lsm: Add Landlock kfuncs
>   bpf: arraymap: Implement Landlock ruleset map
>   bpf: Add Landlock ruleset map type
>   bpf: syscall: Handle Landlock ruleset maps
>   bpf: verifier: Add Landlock ruleset map support
>   selftests/bpf: Add Landlock kfunc declarations
>   selftests/landlock: Rename gettid wrapper for BPF reuse
>   selftests/bpf: Enable Landlock in selftests kernel.
>   selftests/bpf: Add Landlock kfunc test program
>   selftests/bpf: Add Landlock kfunc test runner
>   landlock: Bump ABI version
>   tools: bpftool: Add documentation for landlock_ruleset
>   landlock: Document LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
>   bpf: Document BPF_MAP_TYPE_LANDLOCK_RULESET
>   MAINTAINERS: update entry for the Landlock subsystem
> 
>  Documentation/bpf/map_landlock_ruleset.rst    | 181 +++++
>  Documentation/userspace-api/landlock.rst      |  22 +-
>  MAINTAINERS                                   |   4 +
>  fs/exec.c                                     |   8 +
>  include/linux/binfmts.h                       |   7 +-
>  include/linux/bpf_lsm.h                       |  15 +
>  include/linux/bpf_types.h                     |   1 +
>  include/linux/landlock.h                      |  92 +++
>  include/uapi/linux/bpf.h                      |   1 +
>  include/uapi/linux/landlock.h                 |  14 +
>  kernel/bpf/arraymap.c                         |  67 ++
>  kernel/bpf/bpf_lsm.c                          | 145 ++++
>  kernel/bpf/syscall.c                          |   4 +-
>  kernel/bpf/verifier.c                         |  15 +-
>  samples/landlock/sandboxer.c                  |   7 +-
>  security/landlock/limits.h                    |   2 +-
>  security/landlock/ruleset.c                   | 198 ++++-
>  security/landlock/ruleset.h                   |  25 +-
>  security/landlock/syscalls.c                  | 158 +---
>  .../bpf/bpftool/Documentation/bpftool-map.rst |   2 +-
>  tools/bpf/bpftool/map.c                       |   2 +-
>  tools/include/uapi/linux/bpf.h                |   1 +
>  tools/lib/bpf/libbpf.c                        |   1 +
>  tools/lib/bpf/libbpf_probes.c                 |   6 +
>  tools/testing/selftests/bpf/bpf_kfuncs.h      |  20 +
>  tools/testing/selftests/bpf/config            |   5 +
>  tools/testing/selftests/bpf/config.x86_64     |   1 -
>  .../bpf/prog_tests/landlock_kfuncs.c          | 733 ++++++++++++++++++
>  .../selftests/bpf/progs/landlock_kfuncs.c     |  92 +++
>  tools/testing/selftests/landlock/base_test.c  |  10 +-
>  tools/testing/selftests/landlock/common.h     |  28 +-
>  tools/testing/selftests/landlock/fs_test.c    | 103 +--
>  tools/testing/selftests/landlock/net_test.c   |  55 +-
>  .../testing/selftests/landlock/ptrace_test.c  |  14 +-
>  .../landlock/scoped_abstract_unix_test.c      |  51 +-
>  .../selftests/landlock/scoped_base_variants.h |  23 +
>  .../selftests/landlock/scoped_common.h        |   5 +-
>  .../selftests/landlock/scoped_signal_test.c   |  30 +-
>  tools/testing/selftests/landlock/wrappers.h   |   2 +-
>  39 files changed, 1877 insertions(+), 273 deletions(-)
>  create mode 100644 Documentation/bpf/map_landlock_ruleset.rst
>  create mode 100644 include/linux/landlock.h
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/landlock_kfuncs.c
>  create mode 100644 tools/testing/selftests/bpf/progs/landlock_kfuncs.c
> 
> 
> base-commit: 8c6a27e02bc55ab110d1828610048b19f903aaec