On Tue, Aug 13, 2019 at 2:58 PM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > On Tue, Aug 06, 2019 at 10:24:25PM -0700, Andy Lutomirski wrote: > > > > > > Inside containers and inside nested containers we need to start processes > > > that will use bpf. All of the processes are trusted. > > > > Trusted by whom? In a non-nested container, the container manager > > *might* be trusted by the outside world. In a *nested* container, > > unless the inner container management is controlled from outside the > > outer container, it's not trusted. I don't know much about how > > Facebook's containers work, but the LXC/LXD/Podman world is moving > > very strongly toward user namespaces and maximally-untrusted > > containers, and I think bpf() should work in that context. > > agree that containers (namespaces) reduce amount of trust necessary > for apps to run, but the end goal is not security though. > Linux has become a single user system. > If user can ssh into the host they can become root. > If arbitrary code can run on the host it will be break out of any sandbox. I would argue that this is a reasonable assumption to make if you're designing a system using Linux, but it's not a valid assumption to make as kernel developers. Otherwise we should just give everyone CAP_SYS_ADMIN and call it a day. There really is a difference between root and non-root. > Containers are not providing the level of security that is enough > to run arbitrary code. VMs can do it better, but cpu bugs don't make it easy. > Containers are used to make production systems safer. > Some people call it more 'secure', but it's clearly not secure for > arbitrary code and that is what kernel.unprivileged_bpf_disabled allows. > When we say 'unprivileged bpf' we really mean arbitrary malicious bpf program. > It's been a constant source of pain. The constant blinding, randomization, > verifier speculative analysis, all spectre v1, v2, v4 mitigations > are simply not worth it. It's a lot of complex kernel code without users. Seccomp really will want eBPF some day, and it should work without privilege. Maybe it should be a restricted subset of eBPF, and Spectre will always be an issue until dramatically better hardware shows up, but I think people will want the ability for regular programs to load eBPF seccomp programs. > Hence I prefer this /dev/bpf mechanism to be as simple a possible. > The applications that will use it are going to be just as trusted as systemd. I still don't understand your systemd example. systemd --users is not trusted systemwide in any respect. The main PID 1 systemd is root. No matter how you dice it, granting a user systemd instance extra bpf access is tantamount to granting the user extra bpf access in general. It sounds to me like you're thinking of eBPF as a feature a bit like unprivileged user namespaces: *in principle*, it's supposed to be safe to give any unprivileged process the ability to use it, and you consider security flaws in it to be bugs worth fixing. But you think it's a large attack surface and that most unprivileged programs shouldn't be allowed to use it. Is that reasonable? > > > > To solve your concern of bypassing all capable checks... > > > How about we do /dev/bpf/full_verifier first? > > > It will replace capable() checks in the verifier only. > > > > I'm not convinced that "in the verifier" is the right distinction. > > Telling administrators that some setting lets certain users bypass > > bpf() verifier checks doesn't have a clear enough meaning. > > linux is a single user system. there are no administrators any more. > No doubt, folks will disagree, but that game is over. > At least on bpf side it's done. > > > I propose, > > instead, that the current capable() checks be divided into three > > categories: > > I don't see a use case for these categories. > All bpf programs extend the kernel in some way. > The kernel vs user is one category. > Conceptually CAP_BPF is enough. It would be similar to CAP_NET_ADMIN. > When application has CAP_NET_ADMIN it covers all of networking knobs. > There is no use case that would warrant fine grain CAP_ROUTE_ADMIN, > CAP_ETHTOOL_ADMIN, CAP_ETH0_ADMIN, etc. > Similarly CAP_BPF as the only knob is enough. > The only disadvantage of CAP_BPF is that it's not possible to > pass it from one systemd-like daemon to another systemd-like daemon. > Hence /dev/bpf idea and passing file descriptor. > > > This type of thing actually fits quite nicely into an idea I've been > > thinking about for a while called "implicit rights". In very brief > > summary, there would be objects called /dev/rights/xyz, where xyz is > > the same of a "right". If there is a readable object of the right > > type at the literal path "/dev/rights/xyz", then you have right xyz. > > There's a bit more flexibility on top of this. BPF could use > > /dev/rights/bpf/maptypes/lpm and > > /dev/rights/bpf/verifier/bounded_loops, for example. Other non-BPF > > use cases include a biggie: > > /dev/rights/namespace/create_unprivileged_userns. > > /dev/rights/bind_port/80 would be nice, too. > > The concept of "implicit rights" is very nice and I'm sure it will > be a good fit somewhere, but I don't see why use it in bpf space. > There is no use case for fine grain partition of bpf features. >
