Re: [PATCH v2 bpf-next 00/18] BPF token

Toke Høiland-Jørgensen
2023-06-14T12:12:29Z

Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Mon, Jun 12, 2023 at 3:49 AM Toke Høiland-Jørgensen <toke@kernel.org> wrote:
>>
>> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
>>
>> > On Fri, Jun 9, 2023 at 2:21 PM Toke Høiland-Jørgensen <toke@kernel.org> wrote:
>> >>
>> >> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
>> >>
>> >> > On Fri, Jun 9, 2023 at 4:17 AM Toke Høiland-Jørgensen <toke@kernel.org> 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.
>> >> >>
>> >> >> I am not convinced that this token-based approach is a good way to solve
>> >> >> this: having the delegation mechanism be one where you can basically
>> >> >> only grant a perpetual delegation with no way to retract it, no way to
>> >> >> check what exactly it's being used for, and that is transitive (can be
>> >> >> passed on to others with no restrictions) seems like a recipe for
>> >> >> disaster. I believe this was basically the point Casey was making as
>> >> >> well in response to v1.
>> >> >
>> >> > Most of this can be added, if we really need to. Ability to revoke BPF
>> >> > token is easy to implement (though of course it will apply only for
>> >> > subsequent operations). We can allocate ID for BPF token just like we
>> >> > do for BPF prog/map/link and let tools iterate and fetch information
>> >> > about it. As for controlling who's passing what and where, I don't
>> >> > think the situation is different for any other FD-based mechanism. You
>> >> > might as well create a BPF map/prog/link, pass it through SCM_RIGHTS
>> >> > or BPF FS, and that application can keep doing the same to other
>> >> > processes.
>> >>
>> >> No, but every other fd-based mechanism is limited in scope. E.g., if you
>> >> pass a map fd that's one specific map that can be passed around, with a
>> >> token it's all operations (of a specific type) which is way broader.
>> >
>> > It's not black and white. Once you have a BPF program FD, you can
>> > attach it many times, for example, and cause regressions. Sure, here
>> > we are talking about creating multiple BPF maps or loading multiple
>> > BPF programs, so it's wider in scope, but still, it's not that
>> > fundamentally different.
>>
>> Right, but the difference is that a single BPF program is a known
>> entity, so even if the application you pass the fd to can attach it
>> multiple times, it can't make it do new things (e.g., bpf_probe_read()
>> stuff it is not supposed to). Whereas with bpf_token you have no such
>> guarantee.
>
> Sure, I'm not claiming BPF token is just like passing BPF program FD
> around. My point is that anything in the kernel that is representable
> by FD can be passed around to an unintended process through
> SCM_RIGHTS. And if you want to have tighter control over who's passing
> what, you'd probably need LSM. But it's not a requirement.
>
> With BPF token it is important to trust the application you are
> passing BPF token to. This is not a mechanism to just freely pass
> around the ability to do BPF. You do it only to applications you
> control.

Trust is not binary, though. "Do I trust this application to perform
this specific action" is different from "do I trust this application to
perform any action in the future". A security mechanism should grant the
minimum required privileges required to perform the operation; this
token thing encourages (defaults to) broader grants, which is worrysome.

> With user namespaces, if we could grant CAP_BPF and co to use BPF,
> we'd do that. But we can't. BPF token at least gives us this
> opportunity.

If the use case is to punch holes in the user namespace isolation I feel
like that is better solved at the user namespace level than the BPF
subsystem level...

-Toke

(Ran out of time and I'm about to leave for PTO, so dropping the RPC
discussion for now)