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

[Andy Lutomirski]

> 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?

> 
> 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.

> 
>> Seccomp.  Seccomp already uses cBPF, which is a form of BPF although
>> it doesn't involve the bpf() syscall.  There are some seccomp
>> proposals in the works that will want some stuff from eBPF.  In
> 
> I'm afraid these proposals won't go anywhere.

Can you explain why?

> 
>> So it's a bit of a chicken-and-egg situation.  There aren't major
>> unprivileged eBPF users because the kernel support isn't there.
> 
> As I said before there are zero known use cases of 'unprivileged bpf'.
> 
> If I understand you correctly you're refusing to accept that
> 'less privileged bpf' is a valid use case while pushing for extending
> scope of 'unprivileged'.

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.