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

From: KP Singh <kpsingh@chromium.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: KP Singh <kpsingh@chromium.org>,
	open list <linux-kernel@vger.kernel.org>,
	bpf <bpf@vger.kernel.org>,
	Linux Security Module list 
	<linux-security-module@vger.kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	James Morris <jmorris@namei.org>,
	Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>,
	"David S. Miller" <davem@davemloft.net>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH bpf-next v4 5/8] bpf: lsm: Implement attach, detach and execution
Date: Fri, 21 Feb 2020 13:02:40 +0100	[thread overview]
Message-ID: <CACYkzJ6E7FDE0xnnZPCmxgC+vEw1o4qcu9szV1DMeDeukbnFxQ@mail.gmail.com> (raw)
In-Reply-To: <20200221021755.3z7ifyyeh6seo3zs@ast-mbp>

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