linux-kernel - Re: [PATCH bpf-next v4 5/8] bpf: lsm: Implement attach, detach and execution

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Date:   Fri, 21 Feb 2020 13:02:40 +0100
From:   KP Singh <kpsingh@...omium.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     KP Singh <kpsingh@...omium.org>,
        open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        Linux Security Module list 
        <linux-security-module@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH bpf-next v4 5/8] bpf: lsm: Implement attach, detach and
 execution

On 20-Feb 18:17, Alexei Starovoitov wrote:
> On Thu, Feb 20, 2020 at 06:52:47PM +0100, KP Singh wrote:
> > +
> > +   /* This is the first program to be attached to the LSM hook, the hook
> > +    * needs to be enabled.
> > +    */
> > +   if (prog->type == BPF_PROG_TYPE_LSM && tr->progs_cnt[kind] == 1)
> > +           err = bpf_lsm_set_enabled(prog->aux->attach_func_name, true);
> >  out:
> >     mutex_unlock(&tr->mutex);
> >     return err;
> > @@ -336,7 +348,11 @@ int bpf_trampoline_unlink_prog(struct bpf_prog *prog)
> >     }
> >     hlist_del(&prog->aux->tramp_hlist);
> >     tr->progs_cnt[kind]--;
> > -   err = bpf_trampoline_update(prog->aux->trampoline);
> > +   err = bpf_trampoline_update(prog);
> > +
> > +   /* There are no more LSM programs, the hook should be disabled */
> > +   if (prog->type == BPF_PROG_TYPE_LSM && tr->progs_cnt[kind] == 0)
> > +           err = bpf_lsm_set_enabled(prog->aux->attach_func_name, false);
>
> Overall looks good, but I don't think above logic works.
> Consider lsm being attached, then fexit, then lsm detached, then fexit detached.
> Both are kind==fexit and static_key stays enabled.

You're right. I was weary of introducing a new kind (something like
BPF_TRAMP_LSM) since they are just fexit trampolines. For now, I
added nr_lsm_progs as a member in struct bpf_trampoline and refactored
the increment and decrement logic into inline helper functions e.g.

static inline void bpf_trampoline_dec_progs(struct bpf_prog *prog,
                                            enum bpf_tramp_prog_type kind)
{
        struct bpf_trampoline *tr = prog->aux->trampoline;

        if (prog->type == BPF_PROG_TYPE_LSM)
                tr->nr_lsm_progs--;

        tr->progs_cnt[kind]--;
}

and doing the check as:

  if (prog->type == BPF_PROG_TYPE_LSM && tr->nr_lsm_progs == 0)
        err = bpf_lsm_set_enabled(prog->aux->attach_func_name, false);

This should work, If you're okay with it, I will update it in the next
revision of the patch-set.

- KP