Re: [RFC PATCH 06/20] bpf: lsm: Add Landlock kfuncs

[Justin Suess <utilityemal77@gmail.com>]

On Wed, Jul 01, 2026 at 09:49:07PM +0200, Mickaël Salaün wrote:
> On Wed, Jul 01, 2026 at 02:38:08PM -0400, Paul Moore wrote:
> > On Wed, Jul 1, 2026 at 2:34 PM Mickaël Salaün <mic@digikod.net> wrote:
> > > On Wed, Jul 01, 2026 at 09:28:22AM -0400, Paul Moore wrote:
> > > > On Wed, Jul 1, 2026 at 8:52 AM Justin Suess <utilityemal77@gmail.com> wrote:
> > > > > On Wed, Jul 01, 2026 at 08:12:34AM -0400, Paul Moore wrote:
> > > > > > On Wed, Jul 1, 2026 at 6:59 AM Mickaël Salaün <mic@digikod.net> wrote:
> > > > > > > On Tue, Apr 07, 2026 at 04:01:28PM -0400, Justin Suess wrote:
> > > > > > > > Create 2 kfuncs exposing control over Landlock functionality to BPF
> > > > > > > > callers. Export an opaque struct bpf_landlock_ruleset preventing callers
> > > > > > > > from accessing unstable internal Landlock fields.
> > > > > >
> > > > > > Generally speaking we don't want to provide APIs, either in-kernel or
> > > > > > at the userspace/kernel boundary, that are specific to a single LSM,
> > > > > > see the LSM syscalls or the security_current_getlsmprop_subj()
> > > > > > function as examples.
> > >
> > > This patch series is not about the LSM framework, only about Landlock
> > > and its specific model and use case.  Landlock using some of the LSM API
> > > is not relevant here.
> > 
> > Based on a quick look the patchset enables BPF programs to call
> > directly into Landlock.  For the same reason we discourage other parts
> > of the kernel to call directly into individual LSMs, we want to
> > discourage BPF programs from calling directly into individual LSMs.
> 
> We're OK for a dedicated kfunc to call directly into Landlock (with a
> tailored interface).  Landlock is designed around its syscall interfaces
> (well documented, tailored, tested), and this would be a new user of
> almost the same UAPI.

Paul, Mickaël,

I think there's a cleaner way to resolve this.

First, walking back my earlier email: I was wrong saying that we need to call
into security/security.c to check whether Landlock is enabled. Landlock's
init only runs when it's in the active lsm= list, so I can just test
landlock_initialized directly. There's no per-invocation reason to route
through the LSM framework for that.

Rather than routing each kfunc *invocation* through a security/security.c
wrapper, I think the right place for the framework to be involved is
*registration*: have the LSM framework own registration of an LSM's
kfunc sets, e.g.

    int security_register_lsm_kfunc_set(u64 lsm_id, enum bpf_prog_type type,
                                        const struct btf_kfunc_id_set *kset);

Each LSM calls this once to register its sets. Because registration goes
through the framework, the framework gets to decide whether to actually
register them so you could, for example, run an LSM while explicitly
opting its BPF kfuncs out. (something that should be done at the LSM
framework level).

This gives the framework control over kfunc enablement without an
pointless indirection on every call, and without making the kfunc
interface any more complex.

So this satisfies both sides of this argument:

Mickaël, this fits your suggestion to move them to security/landlock/bpf.c 
and call directly into a Landlock function without needless abstraction.
We just register the landlock kfunc set with
security_register_lsm_kfunc_set, and that's it.

Paul, this way the LSM framework would have visibility into the
registration and enablement of the kfuncs that concern it.

Does this strike a reasonable balance?

Justin