Re: [PATCH RESEND v3 bpf-next 00/14] BPF token

[Christian Brauner <brauner@kernel.org>]

On Fri, Jun 30, 2023 at 01:15:47AM +0200, Toke Høiland-Jørgensen wrote:
> Andrii Nakryiko <andrii@kernel.org> 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.