> 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. 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.
