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.
