[PATCH v5 4/5] bpf: Only enable BPF LSM hooks when an LSM program is attached

[KP Singh]

On Fri, Oct 6, 2023 at 11:05 AM Jiri Olsa <olsajiri at gmail.com> wrote:
>
> On Fri, Oct 06, 2023 at 09:27:57AM +0200, Jiri Olsa wrote:
>
> SNIP
>
> > >  static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
> > > struct bpf_trampoline *tr)
> > >  {
> > >         enum bpf_tramp_prog_type kind;
> > >         struct bpf_tramp_link *link_exiting;
> > > -       int err = 0, num_lsm_progs = 0;
> > > +       int err = 0;
> > >         int cnt = 0, i;
> > >
> > >         kind = bpf_attach_type_to_tramp(link->link.prog);
> > > @@ -547,15 +566,14 @@ static int __bpf_trampoline_link_prog(struct
> > > bpf_tramp_link *link, struct bpf_tr
> > >                 /* prog already linked */
> > >                 return -EBUSY;
> > >
> > > -               if (link_exiting->link.prog->type == BPF_PROG_TYPE_LSM)
> > > -                       num_lsm_progs++;
> > >         }
> > >
> > > -       if (!num_lsm_progs && link->link.prog->type == BPF_PROG_TYPE_LSM)
> > > -               bpf_lsm_toggle_hook(tr->func.addr, true);
> > > -
> > >         hlist_add_head(&link->tramp_hlist, &tr->progs_hlist[kind]);
> > >         tr->progs_cnt[kind]++;
> > > +
> > > +       if (link->link.prog->type == BPF_PROG_TYPE_LSM)
> > > +               bpf_trampoline_toggle_lsm(tr, kind);
> >
> > how about keeping BPF_PROG_TYPE_LSM progs type count of attached programs
> > in bpf_trampoline and toggle lsm on first coming in and last going out?
>
> hm we actually allow other tracing program types to attach to bpf_lsm_*
> functions, so I wonder we should toggle the lsm hook for each program
> type (for bpf_lsm_* trampolines) because they'd expect the hook is called

Tracing is about tracing, attaching a tracing program to bpf_lsm_ that
changes the actual trace here is not something I would recommend.
Infact, I have used tracing programs to figure out whether bpf_lsm_*
is being called to debug stuff. Tracing users can always attach to
security_* if they like.

- KP

>
> but I'm not sure it's a valid use case to have like normal fentry program
> attached to bpf_lsm_XXX function
>
> jirka
>
> >
> > also the trampoline attach is actually made in bpf_trampoline_update,
> > so I wonder it'd make more sense to put it in there, but it's already
> > complicated, so it actually might be easier in here
> >
> > jirka
> >
> > > +
> > >         err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> > >         if (err) {
> > >                 hlist_del_init(&link->tramp_hlist);
> > > @@ -578,7 +596,6 @@ static int __bpf_trampoline_unlink_prog(struct
> > > bpf_tramp_link *link, struct bpf_
> > >  {
> > >         struct bpf_tramp_link *link_exiting;
> > >         enum bpf_tramp_prog_type kind;
> > > -       bool lsm_link_found = false;
> > >         int err, num_lsm_progs = 0;
> > >
> > >         kind = bpf_attach_type_to_tramp(link->link.prog);
> > > @@ -595,18 +612,14 @@ static int __bpf_trampoline_unlink_prog(struct
> > > bpf_tramp_link *link, struct bpf_
> > >                                      tramp_hlist) {
> > >                         if (link_exiting->link.prog->type == BPF_PROG_TYPE_LSM)
> > >                                 num_lsm_progs++;
> > > -
> > > -                       if (link_exiting->link.prog == link->link.prog)
> > > -                               lsm_link_found = true;
> > >                 }
> > >         }
> > >
> > >         hlist_del_init(&link->tramp_hlist);
> > >         tr->progs_cnt[kind]--;
> > >
> > > -       if (lsm_link_found && num_lsm_progs == 1)
> > > -               bpf_lsm_toggle_hook(tr->func.addr, false);
> > > -
> > > +       if (link->link.prog->type == BPF_PROG_TYPE_LSM)
> > > +               bpf_trampoline_toggle_lsm(tr, kind);
> > >         return bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> > >  }
> > >
> > >
> > > - KP
>