netdev - Re: [RFC PATCH bpf-next 0/2] unprivileged BPF_PROG_TEST

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Date:   Thu, 19 Dec 2019 11:26:47 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Edwin Peer <epeer@...iper.net>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Y Song <ys114321@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [RFC PATCH bpf-next 0/2] unprivileged BPF_PROG_TEST_RUN

On Thu, Dec 19, 2019 at 05:05:42PM +0000, Edwin Peer wrote:
> On 12/19/19, 07:47, "Daniel Borkmann" <daniel@...earbox.net> wrote:
> 
> >  What about CAP_BPF?
> 
> What is the status of this? It might solve some of the problems, but it is still puts testing
> BPF outside reach of normal users.

why?
I think CAP_BPF is solving exactly what you're trying to achieve.
Use CAP_BPF to load _any_ program type and use prog_test_run to run it.
While discussing CAP_BPF during plumbers conf we realized that the kernel doesn't need
to check CAP_BPF for prog_test_run. It's user supplied data. No security risk. Though
the kernel needs to make sure that dangerous helpers are not used for prog_test_run.
Whether bpf_clone_redirect() is such helper is still tbd. Unpriv user can flood netdevs
without any bpf.

> > IIRC, there are also other issues e.g. you could abuse the test interface as a packet
> >  generator (bpf_clone_redirect) which is not something fully unpriv should be doing.
> 
> Good point. I suspect solutions exist - I'm trying to ascertain if they are worth pursuing
> or if the idea of unprivileged testing is a complete non-starter to begin with.
> 
> Are there other helpers of concern that come immediately to mind? A first stab might
> add these to the list in the verifier that require privilege. This has the drawback that
> programs that actually need this kind of functionality are beyond the test framework.

So far majority of programs require root-only verifier features. The programs are
getting more complex and benefit the most from testing. Relaxing test_run for unpriv
progs is imo very narrow use case. I'd rather use CAP_BPF.