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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Wed, 17 Jul 2019 14:40:42 -0700
Message-ID: <CAADnVQJY_=yeY0C3k1ZKpRFu5oNbB4zhQf5tQnLr=Mi8i6cgeQ@mail.gmail.com>
Subject: Re: [PATCH RFC 0/4] Add support to directly attach BPF program to ftrace
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Android Kernel Team <kernel-team@android.com>,
        Network Development <netdev@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
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On Wed, Jul 17, 2019 at 6:01 AM Joel Fernandes <joel@joelfernandes.org> wrote:

I trimmed cc. some emails were bouncing.

> > I think allowing one tracepoint and disallowing another is pointless
> > from security point of view. Tracing bpf program can do bpf_probe_read
> > of anything.
>
> I think the assumption here is the user controls the program instructions at
> runtime, but that's not the case. The BPF program we are loading is not
> dynamically generated, it is built at build time and it is loaded from a
> secure verified partition, so even though it can do bpf_probe_read, it is
> still not something that the user can change.

so you're saying that by having a set of signed bpf programs which
instructions are known to be non-malicious and allowed set of tracepoints
to attach via selinux whitelist, such setup will be safe?
Have you considered how mix and match will behave?

> And, we are planning to make it
> even more secure by making it kernel verify the program at load time as well
> (you were on some discussions about that a few months ago).

It sounds like api decisions for this sticky raw_tp feature are
driven by security choices which are not actually secure.
I'm suggesting to avoid bringing up point of security as a reason for
this api design, since it's making the opposite effect.