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

[Jiri Olsa]

On Thu, Oct 05, 2023 at 06:07:28PM +0200, KP Singh wrote:

SNIP

> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index df9699bce372..4f31384b5637 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -511,11 +511,30 @@ static enum bpf_tramp_prog_type
> bpf_attach_type_to_tramp(struct bpf_prog *prog)
>         }
>  }
> 
> +static void bpf_trampoline_toggle_lsm(struct bpf_trampoline *tr,
> +                                     enum bpf_tramp_prog_type kind)
> +{
> +       struct bpf_tramp_link *link;
> +       volatile bool found = false;
> +
> +       /* Loop through the links and if any LSM program is attached, ensure
> +        * that the hook is enabled.
> +        */
> +       hlist_for_each_entry(link, &tr->progs_hlist[kind], tramp_hlist) {
> +               if (link->link.prog->type == BPF_PROG_TYPE_LSM) {
> +                       found  = true;
> +                       break;
> +               }
> +       }
> +
> +       bpf_lsm_toggle_hook(tr->func.addr, found);
> +}
> +
>  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?

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