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

[Jordan Glover]

On Thursday, August 15, 2019 5:28 PM, Alexei Starovoitov <alexei.starovoitov at gmail.com> wrote:

> On Thu, Aug 15, 2019 at 11:24:54AM +0000, Jordan Glover wrote:
>
> > On Wednesday, August 14, 2019 10:05 PM, Alexei Starovoitov alexei.starovoitov at 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.
>
> systemd itself is trusted. It's the same binary whether it runs as pid=1
> or as pid=123. One of the use cases is to make IPAddressDeny= work with --user.
> Subset of that feature already works with AmbientCapabilities=CAP_NET_ADMIN.
> CAP_BPF is a natural step in the same direction.

The point was that systemd will run any arbitrary command you'll throw at it and you want
to automatically attach CAP_BPF to it. AmbientCapabilities is not valid option for
systemd --user instance (otherwise it would be nuts).

I think we may have misunderstanding here. Did you mean systemd "system" service with
"User=xxx" option instead of "systemd --user" service? It would make sense then.