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

[Jordan Glover <Golden_Miller83@protonmail.ch>]

On Wednesday, August 14, 2019 10:05 PM, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> On Wed, Aug 14, 2019 at 10:51:23AM -0700, Andy Lutomirski wrote:
>
> > If eBPF is genuinely not usable by programs that are not fully trusted
> > by the admin, then no kernel changes at all are needed. Programs that
> > want to reduce their own privileges can easily fork() a privileged
> > subprocess or run a little helper to which they delegate BPF
> > operations. This is far more flexible than anything that will ever be
> > in the kernel because it allows the helper to verify that the rest of
> > the program is doing exactly what it's supposed to and restrict eBPF
> > operations to exactly the subset that is needed. So a container
> > manager or network manager that drops some provilege could have a
> > little bpf-helper that manages its BPF XDP, firewalling, etc
> > configuration. The two processes would talk over a socketpair.
>
> there were three projects that tried to delegate bpf operations.
> All of them failed.
> bpf operational workflow is much more complex than you're imagining.
> fork() also doesn't work for all cases.
> I gave this example before: consider multiple systemd-like deamons
> that need to do bpf operations that want to pass this 'bpf capability'
> to other deamons written by other teams. Some of them will start
> non-root, but still need to do bpf. They will be rpm installed
> and live upgraded while running.
> We considered to make systemd such centralized bpf delegation
> authority too. It didn't work. bpf in kernel grows quickly.
> libbpf part grows independently. llvm keeps evolving.
> All of them are being changed while system overall has to stay
> operational. Centralized approach breaks apart.
>
> > The interesting cases you're talking about really do involved
> > unprivileged or less privileged eBPF, though. Let's see:
> > systemd --user: systemd --user is not privileged at all. There's no
> > issue of reducing privilege, since systemd --user doesn't have any
> > privilege to begin with. But systemd supports some eBPF features, and
> > presumably it would like to support them in the systemd --user case.
> > This is unprivileged eBPF.
>
> Let's disambiguate the terminology.
> This /dev/bpf patch set started as describing the feature as 'unprivileged bpf'.
> I think that was a mistake.
> Let's call systemd-like deamon usage of bpf 'less privileged bpf'.
> This is not unprivileged.
> 'unprivileged bpf' is what sysctl kernel.unprivileged_bpf_disabled controls.
>
> There is a huge difference between the two.
> I'm against extending 'unprivileged bpf' even a bit more than what it is
> today for many reasons mentioned earlier.
> The /dev/bpf is about 'less privileged'.
> Less privileged than root. We need to split part of full root capability
> into bpf capability. So that most of the root can be dropped.
> This is very similar to what cap_net_admin does.
> cap_net_amdin can bring down eth0 which is just as bad as crashing the box.
> cap_net_admin is very much privileged. Just 'less privileged' than root.
> Same thing for cap_bpf.
>
> May be we should do both cap_bpf and /dev/bpf to make it clear that
> this is the same thing. Two interfaces to achieve the same result.
>

systemd --user processes aren't "less privileged". The are COMPLETELY unprivileged.
Granting them cap_bpf is the same as granting it to every other unprivileged user
process. Also unprivileged user process can start systemd --user process with any
command they like.

Jordan