linux-kernel - Re: [PATCH bpf-next v3 5/9] bpf: Allow LSM programs to use bpf spin locks

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Date:   Wed, 4 Nov 2020 23:44:19 +0100
From:   KP Singh <kpsingh@...omium.org>
To:     Martin KaFai Lau <kafai@...com>
Cc:     open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Song Liu <songliubraving@...com>, Paul Turner <pjt@...gle.com>,
        Jann Horn <jannh@...gle.com>, Hao Luo <haoluo@...gle.com>
Subject: Re: [PATCH bpf-next v3 5/9] bpf: Allow LSM programs to use bpf spin locks

On Wed, Nov 4, 2020 at 11:35 PM Martin KaFai Lau <kafai@...com> wrote:
>
> On Wed, Nov 04, 2020 at 05:44:49PM +0100, KP Singh wrote:
> > From: KP Singh <kpsingh@...gle.com>
> >
> > Usage of spin locks was not allowed for tracing programs due to
> > insufficient preemption checks. The verifier does not currently prevent
> > LSM programs from using spin locks, but the helpers are not exposed
> > via bpf_lsm_func_proto.
> This could be the first patch but don't feel strongly about it.
>
> >
> > Based on the discussion in [1], non-sleepable LSM programs should be
> > able to use bpf_spin_{lock, unlock}.
> >
> > Sleepable LSM programs can be preempted which means that allowng spin
> > locks will need more work (disabling preemption and the verifier
> > ensuring that no sleepable helpers are called when a spin lock is held).
> >
> > [1]: https://lore.kernel.org/bpf/20201103153132.2717326-1-kpsingh@chromium.org/T/#md601a053229287659071600d3483523f752cd2fb
> >
> > Signed-off-by: KP Singh <kpsingh@...gle.com>
> > ---
> >  kernel/bpf/bpf_lsm.c  |  4 ++++
> >  kernel/bpf/verifier.c | 17 +++++++++++++++++
> >  2 files changed, 21 insertions(+)
> >
> > diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> > index 61f8cc52fd5b..93383df2140b 100644
> > --- a/kernel/bpf/bpf_lsm.c
> > +++ b/kernel/bpf/bpf_lsm.c
> > @@ -63,6 +63,10 @@ bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >               return &bpf_task_storage_get_proto;
> >       case BPF_FUNC_task_storage_delete:
> >               return &bpf_task_storage_delete_proto;
> > +     case BPF_FUNC_spin_lock:
> > +             return &bpf_spin_lock_proto;
> > +     case BPF_FUNC_spin_unlock:
> > +             return &bpf_spin_unlock_proto;
> >       default:
> >               return tracing_prog_func_proto(func_id, prog);
> >       }
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 314018e8fc12..7c6c246077cf 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -9739,6 +9739,23 @@ static int check_map_prog_compatibility(struct bpf_verifier_env *env,
> >               return -EINVAL;
> >       }
> >
> > +     if (map_value_has_spin_lock(map)) {
> > +             if (prog_type == BPF_PROG_TYPE_SOCKET_FILTER) {
> > +                     verbose(env, "socket filter progs cannot use bpf_spin_lock yet\n");
> > +                     return -EINVAL;
> > +             }
> > +
> > +             if (is_tracing_prog_type(prog_type)) {
> > +                     verbose(env, "tracing progs cannot use bpf_spin_lock yet\n");
> > +                     return -EINVAL;
> > +             }
> It is good to have a more specific verifier log.  However,
> these are duplicated checks (a few lines above in the same function).
> They should at least be removed.
>

Thanks, I fixed this up and will move this as the first patch.

> > +
> > +             if (prog->aux->sleepable) {
> > +                     verbose(env, "sleepable progs cannot use bpf_spin_lock yet\n");
> > +                     return -EINVAL;
> > +             }
> > +     }
> > +
> >       if ((bpf_prog_is_dev_bound(prog->aux) || bpf_map_is_dev_bound(map)) &&
> >           !bpf_offload_prog_map_match(prog, map)) {
> >               verbose(env, "offload device mismatch between prog and map\n");
> > --
> > 2.29.1.341.ge80a0c044ae-goog
> >