On Thu, Aug 15, 2019 at 11:24:54AM +0000, Jordan Glover wrote: > On Wednesday, August 14, 2019 10:05 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. > > > > 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. > > > > systemd --user processes aren't "less privileged". The are COMPLETELY unprivileged. > Granting them cap_bpf is the same as granting it to every other unprivileged user > process. Also unprivileged user process can start systemd --user process with any > command they like. systemd itself is trusted. It's the same binary whether it runs as pid=1 or as pid=123. One of the use cases is to make IPAddressDeny= work with --user. Subset of that feature already works with AmbientCapabilities=CAP_NET_ADMIN. CAP_BPF is a natural step in the same direction.
