> 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? > > 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.
