Re: [PATCH 00/12] Signed BPF programs

["Toke Høiland-Jørgensen" <toke@kernel.org>]

KP Singh <kpsingh@kernel.org> writes:

> On Mon, Jun 9, 2025 at 10:20 AM Toke Høiland-Jørgensen <toke@kernel.org> wrote:
>>
>>
>> > Given that many use-cases (e.g. Cilium) generate trusted BPF programs,
>> > trusted loaders are an inevitability and a requirement for signing support, a
>> > entrusting loader programs will be a fundamental requirement for an security
>> > policy.
>>
>> So I've been following this discussion a bit on the sidelines, and have
>> a question related to this:
>>
>> From your description a loader would have embedded hashes for a concrete
>> BPF program, which doesn't really work for dynamically generated
>> programs. So how would a "trusted loader" work for dynamically generated
>> programs?
>
> The trusted loader for dynamically generated programs would be the
> binary that loads the BPF program. So a security policy will need to
> allow certain trusted binaries (signed with a different key) to load
> unsigned BPF programs for cilium.

OK, so this refers to a policy along the line of: "Only allow signed BPF
program except for this particular userspace binary that is allowed to
load anything"?

> For a stronger policy, the generators can use a derived key and
> identity (e.g from the Kubernetes / machine / TLS certificate) and
> then sign their programs using this certificate. The LSM policy then
> allows verification with a trusted build key and for certain binaries,
> with the delegated credentials.

And this means "add a separate trusted key on the kernel side that the
userspace binary signs things with before passing it to the kernel"?

In which case, how does that tie into the original statement I quoted at
the top of this email? The "trusted loaders are an inevitability" bit? I
was assuming that the "trusted loaders" in that sentence referred to the
light-skeleton loader program, but from your reply I'm not thinking
maybe it just means "some userspace binaries need to be exempt from any
signing requirement"? Or am I missing something?

-Toke