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

["Mickaël Salaün" <mic@digikod.net>]

On Wed, Jul 01, 2026 at 07:32:57PM -0400, Paul Moore wrote:
> On Wed, Jul 1, 2026 at 5:28 PM Mickaël Salaün <mic@digikod.net> wrote:
> > On Wed, Jul 01, 2026 at 04:02:36PM -0400, Paul Moore wrote:
> > > On Wed, Jul 1, 2026 at 3:55 PM Justin Suess <utilityemal77@gmail.com> wrote:
> > > > 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.
> > >
> > > The landlock_initialized flag is not really a LSM framework API, that
> > > is still Landlock specific which is something we try hard to avoid.
> > >
> > > > 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);
> > >
> > > That implies a set of LSM kfunc APIs which Alexei has been deadset
> > > against (see other ongoing threads).
> > >
> > > > 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).
> > >
> > > I'm not opposed to the LSM supporting a set of kfuncs, see my comments
> > > in other threads, but we should treat these kfuncs just as we treat
> > > other LSM hooks today because that is what they are: LSM hooks that
> > > happened to be called from within a BPF program.
> >
> > What an LSM hook is or should be is the crux of the misunderstanding.  I
> > explained my point of view here:
> > https://lore.kernel.org/all/20260701.jei4Paej3zen@digikod.net/
> >
> >   LSM hooks make sense because they are designed for a specific subsystem
> >   (the caller) and their goal is to return an access decision or to keep
> >   up-to-date related states, which means that their API is designed for
> >   the caller, with its own types and specificities, not the other way
> >   around.  This case is different, the kfunc is strongly typed and tied to
> >   the Landlock (subsystem) semantic with an API defined by and for
> >   Landlock.  I don't think a multiplexer would be a good idea.
> >
> > I'd try to explain better: in a nutshell, an LSM hook exposes a subset
> > of the context of the caller, for any access control system to be able
> > to make a decision.
> 
> That is true for some LSM hooks, but not all of them.  LSM hooks are
> really just another name for the functions that compose parts of the
> LSM framework API; it isn't always strictly about access control in
> the kernel.

That's why I wrote "in a nutshell".  Concrete examples and the rationale
for such hooks would help.

> We leverage the "hooks" for the LSM syscalls, we've
> discussed "hooks" for implementing a common LSM namespace API, and
> there have also been early efforts at LSM policy loading via "hooks".

All that is doable, my question is: why a kfunc multiplexer?  What are
the pros and cons?  I only see disadvantages for now.  Please, convince
us.

> 
> > It makes sense to have such dispatcher because the
> > callees must adapt to the caller's context, and then the API is tailored
> > to the caller, so even with several consumers, the API would ultimately
> > be the same.  In the case of this kfunc, the callee is one specific
> > subsystem that happens to be Landlock.  The caller asks a specific
> > subsystem to do something specific to this subsystem, not to ask all
> > potential access control systems to give a generic verdict to grant an
> > access or not.
> 
> > For this kfunc, the caller passes arguments which are
> > specific to the callee subsystem (e.g. a Landlock ruleset), not the
> > other way around.  Every LSM has its own configuration, and it doesn't
> > make sense to somehow wrap these configurations with a common layer/API.
> 
> Once again, there have already been discussions about trying to build
> a common API for that.  I'd rather have us pick that up for an
> in-kernel/kfunc users than treat Landlock as an exception.  We're
> trying to get rid of the exceptions in the LSM space.

I understand and I agree about the argument for dispatchers (e.g. access
control and state hooks) but not for multiplexers (i.e. which would be a
kfunc in this case).

How would look such multiplexer kfunc?  Something like this?

  union lsm_restrict_arg {
    struct {
	    const struct landlock_rulest *ruleset,
	    int flags,
    } landlock;
    ...?
  };

 int bpf_lsm_restrict_binprm(struct lsm_id target_lsm,
                             union lsm_restrict_arg multiplexed_generic_argument);

I don't see the point of such multiplexer.  It adds complexity for no
gain, except maybe to sneak in new features by only extending the
argument types?  What would be the value for eBPF users?

> 
> > Why not start with something simple that fits a use case now?  If and
> > when another LSM will need a kfunc, then we'll have something concrete
> > to talk about.
> 
> I think Casey's reply answers that question rather well.

This was mainly about (access control/security) hooks, not multiplexer
interfaces.