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

[Alexei Starovoitov]

On Wed, Aug 14, 2019 at 03:30:51PM -0700, Andy Lutomirski wrote:
> 
> 
> > On Aug 14, 2019, at 3:05 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.
> 
> The new pseudo-capability in this patch set is absurdly broad. I’ve proposed some finer-grained divisions in this thread. Do you have comments on them?

Initially I agreed that it's probably too broad, but then realized
that they're perfect as-is. There is no need to partition further.

> > 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.
> 
> What for?  If there’s a CAP_BPF, then why do you want /dev/bpf? Especially if you define it to do the same thing.

Indeed, ambient capabilities should work for all cases.

> No, I’m not.  I have no objection at all if you try to come up with a clear definition of what the capability checks do and what it means to grant a new permission to a task.  Changing *all* of the capable checks is needlessly broad.

There are not that many bits left. I prefer to consume single CAP_BPF bit.
All capable(CAP_SYS_ADMIN) checks in kernel/bpf/ will become CAP_BPF.
This is no-brainer.

The only question is whether few cases of CAP_NET_ADMIN in kernel/bpf/
should be extended to CAP_BPF or not.
imo devmap and xskmap can stay CAP_NET_ADMIN,
but cgroup bpf attach/detach should be either CAP_NET_ADMIN or CAP_BPF.
Initially cgroup-bpf hooks were limited to networking.
It's no longer the case. Requiring NET_ADMIN there make little sense now.