Re: [PATCH bpf-next v3 07/15] lsm: Add the bpf_lsm_policy_apply_bprm kfunc

[Justin Suess <utilityemal77@gmail.com>]

On Fri, Sep 11, 2026 at 08:38:16PM -0700, Alexei Starovoitov wrote:
> On Wed Sep 9, 2026 at 12:37 PM PDT, Justin Suess wrote:
> > Add the kfunc applying a policy object to an execution:
> >
> >   bpf_lsm_policy_apply_bprm(object, bprm, flags)        KF_SLEEPABLE
> >
> > It asks the LSM owning @object, through the bprm_apply_policy_object
> > hook, to restrict the credentials prepared in @bprm, so that the
> > executed task starts confined by the policy.  The meaning of @flags
> > and the composition with restrictions the credentials already carry
> > are the owning LSM's; an LSM without execution policy support makes
> > the call fail with -EOPNOTSUPP.
> >
> > The kfunc runs the hook in a root memcg charging scope: the policy
> > restricts the execution on behalf of the BPF program, not of the
> > mediated task, so what the owning LSM allocates to compute it, e.g.
> > Landlock's merged domain, is not charged to the task the program
> > supervises.
> >
> > The filter makes the kfunc exclusive to the sleepable LSM programs
> > attached to the bprm_creds_for_exec() or bprm_creds_from_file()
> > hooks, the only contexts where the bprm's credentials are prepared
> > but not yet committed.  The verifier's argument typing does not draw
> > this boundary on its own: a trusted struct linux_binprm pointer is
> > also available at the other bprm hooks -- bprm_check_security(),
> > which runs per binfmt while an interpreter may still rewrite the
> > execution, and bprm_committing_creds()/bprm_committed_creds(), which
> > run at or past the point of no return, where the prepared
> > credentials are frozen or installed -- and to tp_btf programs via
> > the sched_prepare_exec and sched_process_exec tracepoints, which
> > resolve kfuncs from the same registration bucket as LSM programs.
> > The attach-point filter, not the argument type, is the authorization
> > boundary.
> >
> > No filter case is needed for BPF_LSM_CGROUP programs: since
> > commit 5b038319be44 ("bpf: Reject sleepable BPF_LSM_CGROUP programs
> > at load time") they cannot be sleepable, so KF_SLEEPABLE already
> > excludes them.
> >
> > Cc: Paul Moore <paul@paul-moore.com>
> > Cc: KP Singh <kpsingh@kernel.org>
> > Signed-off-by: Justin Suess <utilityemal77@gmail.com>
> > ---
> >
> > Notes:
> >     v2->v3:
> >         - Drop the BPF_LSM_CGROUP case from the kfunc filter: since
> >           commit 5b038319be44 ("bpf: Reject sleepable BPF_LSM_CGROUP
> >           programs at load time") such programs cannot be sleepable, so
> >           KF_SLEEPABLE already excludes them from the apply kfunc.
> >         - Document, in the commit message and the filter comment, why the
> >           attach-point filter rather than the verifier's argument typing
> >           is the authorization boundary.
> >
> >  security/bpf_lsm_kfuncs.c | 68 +++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 68 insertions(+)
> >
> > diff --git a/security/bpf_lsm_kfuncs.c b/security/bpf_lsm_kfuncs.c
> > index 43a4bf57fd31..857e9a316d1c 100644
> > --- a/security/bpf_lsm_kfuncs.c
> > +++ b/security/bpf_lsm_kfuncs.c
> > @@ -2,16 +2,25 @@
> >  
> >  /* BPF kfuncs exposing LSM policy objects. */
> >  
> > +#include <linux/binfmts.h>
> >  #include <linux/bpf.h>
> >  #include <linux/btf.h>
> >  #include <linux/btf_ids.h>
> >  #include <linux/cfi.h>
> >  #include <linux/init.h>
> >  #include <linux/lsm_hooks.h>
> > +#include <linux/memcontrol.h>
> > +#include <linux/sched/mm.h>
> >  #include <linux/security.h>
> >  
> >  #include "lsm.h"
> >  
> > +/* The sleepable LSM hooks bpf_lsm_policy_apply_bprm() may be called from. */
> > +BTF_SET_START(bpf_lsm_policy_bprm_hooks)
> > +BTF_ID(func, bpf_lsm_bprm_creds_for_exec)
> > +BTF_ID(func, bpf_lsm_bprm_creds_from_file)
> > +BTF_SET_END(bpf_lsm_policy_bprm_hooks)
> > +
> >  __bpf_kfunc_start_defs();
> >  
> >  /**
> > @@ -44,6 +53,49 @@ bpf_lsm_policy_acquire(struct lsm_policy_object *object)
> >  	return NULL;
> >  }
> >  
> > +/**
> > + * bpf_lsm_policy_apply_bprm - Apply a policy object to exec credentials
> > + * @object: policy object to apply
> > + * @bprm: execution context providing the prepared credentials to
> > + *        restrict
> > + * @flags: flags defined by the LSM owning @object
> > + *
> > + * Ask the LSM owning @object to restrict the credentials prepared in
> > + * @bprm with it, so that the executed task starts confined by the
> > + * policy.  How the policy composes with restrictions the credentials
> > + * already carry, and the meaning of @flags, are defined by the owning
> > + * LSM.  @object is only borrowed: the caller keeps its reference.
> > + * The hook runs in a root memcg charging scope: policy the LSM
> > + * computes on behalf of the program is not charged to the mediated
> > + * task.
> > + *
> > + * Return: 0 on success, -EOPNOTSUPP if the LSM owning @object does
> > + * not support applying policy to an execution, -EINVAL on unsupported
> > + * @flags, other negative values on LSM-specific failures.
> > + */
> > +__bpf_kfunc int bpf_lsm_policy_apply_bprm(struct lsm_policy_object *object,
> > +					  struct linux_binprm *bprm, u32 flags)
> > +{
> > +	struct lsm_static_call *scall;
> > +	struct mem_cgroup *old_memcg;
> > +	int err;
> > +
> > +	lsm_for_each_hook(scall, bprm_apply_policy_object) {
> > +		if (scall->hl->lsmid->id != object->lsmid)
> > +			continue;
> > +		/*
> > +		 * The hook runs on behalf of the BPF program, not of the
> > +		 * mediated task: charge its allocations to the root memcg.
> > +		 */
> > +		old_memcg = set_active_memcg(root_mem_cgroup);
> > +		err = scall->hl->hook.bprm_apply_policy_object(bprm, object,
> > +							       flags);
> > +		set_active_memcg(old_memcg);
> > +		return err;
> > +	}
> > +	return -EOPNOTSUPP;
> 
> Nack.

It was hard to tell why you Nack'd this patch in particular from the
context.

If you wouldn't mind, providing some guidance as to why the code is
unacceptable in the current form would give a direction to work
towards...

(This is for the kfunc placement in security/ I assume?)

Thanks,
Justin