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

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Message-ID: <69E09581-78ED-45A9-BD2C-616A3620BCB0@fb.com>
Date:   Tue, 2 Jul 2019 23:48:28 +0000
From:   Song Liu <songliubraving@...com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     Kees Cook <keescook@...omium.org>,
        "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 Jul 3, 2019, at 5:32 AM, Andy Lutomirski <luto@...nel.org> wrote:
> 
> On Tue, Jul 2, 2019 at 2:04 PM Kees Cook <keescook@...omium.org> wrote:
>> 
>> 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.)
> 
> Sure you do: the effective set.  It has somewhat bizarre defaults, but
> I don't think that's a real problem.  Also, this wouldn't be like
> CAP_DAC_READ_SEARCH -- you can't accidentally use your BPF caps.
> 
> I think that a /dev capability-like object isn't totally nuts, but I
> think we should do it well, and this patch doesn't really achieve
> that.  But I don't think bpf wants fine-grained controls like this at
> all -- as I pointed upthread, a fine-grained solution really wants
> different treatment for the different capable() checks, and a bunch of
> them won't resemble capabilities or /dev/bpf at all.

Thanks everyone again for great inputs. We will discuss this again and 
respin the set. 

Best,
Song