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

[Alexei Starovoitov]

On Wed, Aug 14, 2019 at 04:59:18PM -0700, Andy Lutomirski wrote:
> 
> 
> > On Aug 14, 2019, at 4:33 PM, Alexei Starovoitov <alexei.starovoitov at gmail.com> wrote:
> > 
> >> 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.
> > 
> 
> 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. 

'never to less trusted process' ? why do you think so?
I don't see a problem adding /dev/bpf/foo in the future and make things
more granular. There is no such use case today. Hence I don't want to
spend time and design something without clear use case in mind.

> Do new programs really need the by_id calls? 

yes. Lorenz gave an example earlier. map-in-map returns map_id.
To operate on that map by_id is needed.