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

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Message-ID: <20200221021755.3z7ifyyeh6seo3zs@ast-mbp>
Date:   Thu, 20 Feb 2020 18:17:57 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     KP Singh <kpsingh@...omium.org>
Cc:     linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        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 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.