On Fri, Jun 30, 2023 at 01:15:47AM +0200, Toke Høiland-Jørgensen wrote: > Andrii Nakryiko <andrii@xxxxxxxxxx> writes: > > > This patch set introduces new BPF object, BPF token, which allows to delegate > > a subset of BPF functionality from privileged system-wide daemon (e.g., > > systemd or any other container manager) to a *trusted* unprivileged > > application. Trust is the key here. This functionality is not about allowing > > unconditional unprivileged BPF usage. Establishing trust, though, is > > completely up to the discretion of respective privileged application that > > would create a BPF token, as different production setups can and do achieve it > > through a combination of different means (signing, LSM, code reviews, etc), > > and it's undesirable and infeasible for kernel to enforce any particular way > > of validating trustworthiness of particular process. > > > > The main motivation for BPF token is a desire to enable containerized > > BPF applications to be used together with user namespaces. This is currently > > impossible, as CAP_BPF, required for BPF subsystem usage, cannot be namespaced > > or sandboxed, as a general rule. E.g., tracing BPF programs, thanks to BPF > > helpers like bpf_probe_read_kernel() and bpf_probe_read_user() can safely read > > arbitrary memory, and it's impossible to ensure that they only read memory of > > processes belonging to any given namespace. This means that it's impossible to > > have namespace-aware CAP_BPF capability, and as such another mechanism to > > allow safe usage of BPF functionality is necessary. BPF token and delegation > > of it to a trusted unprivileged applications is such mechanism. Kernel makes > > no assumption about what "trusted" constitutes in any particular case, and > > it's up to specific privileged applications and their surrounding > > infrastructure to decide that. What kernel provides is a set of APIs to create > > and tune BPF token, and pass it around to privileged BPF commands that are > > creating new BPF objects like BPF programs, BPF maps, etc. > > So a colleague pointed out today that the Seccomp Notify functionality > would be a way to achieve your stated goal of allowing unprivileged > containers to (selectively) perform bpf() syscall operations. Christian > Brauner has a pretty nice writeup of the functionality here: > https://people.kernel.org/brauner/the-seccomp-notifier-new-frontiers-in-unprivileged-container-development I'm amazed you read this. :) The seccomp notifier comes with a lot of caveats. I think it would be impractical if not infeasible to handle bpf() delegation. > > In fact he even mentions allowing unprivileged access to bpf() as a > possible use case (in the second-to-last paragraph). Yeah, I tried to work around a userspace regression with the introduction of the cgroup v2 devices controller.
