On Thu, Aug 15, 2019 at 11:24:54AM +0000, Jordan Glover wrote:

On Wednesday, August 14, 2019 10:05 PM, Alexei Starovoitov [off-list ref] wrote:

quoted

On Wed, Aug 14, 2019 at 10:51:23AM -0700, Andy Lutomirski wrote:

quoted

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.

quoted

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.