On Aug 14, 2019, at 4:33 PM, Alexei Starovoitov [off-list ref] wrote:

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On Wed, Aug 14, 2019 at 03:30:51PM -0700, Andy Lutomirski wrote:

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On Aug 14, 2019, at 3:05 PM, Alexei Starovoitov [off-list ref] 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.

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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.

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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.

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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.

Cgroup bpf attach/detach, with the current API, gives very strong control over the whole system, and it will just get stronger as bpf gains features. Making it CAP_BPF means that you will never have the ability to make CAP_BPF safe to give to anything other than an extremely highly trusted process.  Unsafe pointers are similar.  The rest could plausibly be hardened in the future, although the by_id stuff may be tricky too.

Do new programs really need the by_id calls?  It could make sense to leave those unchanged and to have new programs use persistent maps instead.