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@xxxxxxxxx> 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@xxxxxxxxx> 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.
