netdev - Re: [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf

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Message-ID: <201907021115.DCD56BBABB@keescook>
Date:   Tue, 2 Jul 2019 11:24:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Song Liu <songliubraving@...com>,
        "linux-security@...r.kernel.org" <linux-security@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Jann Horn <jannh@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 bpf-next 1/4] bpf: unprivileged BPF access via /dev/bpf

On Mon, Jul 01, 2019 at 06:59:13PM -0700, Andy Lutomirski wrote:
> I think I'm understanding your motivation.  You're not trying to make
> bpf() generically usable without privilege -- you're trying to create
> a way to allow certain users to access dangerous bpf functionality
> within some limits.
> 
> That's a perfectly fine goal, but I think you're reinventing the
> wheel, and the wheel you're reinventing is quite complicated and
> already exists.  I think you should teach bpftool to be secure when
> installed setuid root or with fscaps enabled and put your policy in
> bpftool.  If you want to harden this a little bit, it would seem
> entirely reasonable to add a new CAP_BPF_ADMIN and change some, but
> not all, of the capable() checks to check CAP_BPF_ADMIN instead of the
> capabilities that they currently check.

If finer grained controls are wanted, it does seem like the /dev/bpf
path makes the most sense. open, request abilities, use fd. The open can
be mediated by DAC and LSM. The request can be mediated by LSM. This
provides a way to add policy at the LSM level and at the tool level.
(i.e. For tool-level controls: leave LSM wide open, make /dev/bpf owned
by "bpfadmin" and bpftool becomes setuid "bpfadmin". For fine-grained
controls, leave /dev/bpf wide open and add policy to SELinux, etc.)

With only a new CAP, you don't get the fine-grained controls. (The
"request abilities" part is the key there.)

-- 
Kees Cook