linux-kernel - Re: [PATCH v3 0/4] Introducing Hornet LSM

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Message-ID: <CACYkzJ43yJRMS+e-7LefYts_v2ZH7_FCbw_LfG3mYfu6q6VD0g@mail.gmail.com>
Date: Tue, 20 May 2025 01:42:06 +0200
From: KP Singh <kpsingh@...nel.org>
To: Zvi Effron <zeffron@...tgames.com>
Cc: Paul Moore <paul@...l-moore.com>, Alexei Starovoitov <alexei.starovoitov@...il.com>, 
	Blaise Boscaccy <bboscaccy@...ux.microsoft.com>, 
	James Bottomley <James.Bottomley@...senpartnership.com>, bpf <bpf@...r.kernel.org>, 
	code@...icks.com, Jonathan Corbet <corbet@....net>, "David S. Miller" <davem@...emloft.net>, 
	David Howells <dhowells@...hat.com>, Günther Noack <gnoack@...gle.com>, 
	Herbert Xu <herbert@...dor.apana.org.au>, Jarkko Sakkinen <jarkko@...nel.org>, 
	James Morris <jmorris@...ei.org>, Jan Stancek <jstancek@...hat.com>, 
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	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Nicolas Schier <nicolas@...sle.eu>, nkapron@...gle.com, 
	Roberto Sassu <roberto.sassu@...wei.com>, "Serge E . Hallyn" <serge@...lyn.com>, 
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	Cong Wang <xiyou.wangcong@...il.com>, kysrinivasan@...il.com, 
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3 0/4] Introducing Hornet LSM

> > > > > > No. New hook is not needed.

[...]

> > > > >
> > > > > It would be good for you to explain how the existing LSM hook is sufficient
> > > > > to authorize the loading of a BPF program using the signature validation
> > > > > state determined in the BPF verifier.
> > > >
> > > > I already explained:
> > > > .. a job of trivial LSM:
> > > > if (prog_attr doesn't have signature &&
> > > >    (task == .. || task is under certain cgroup || whatever))
> > > >   disallow.
> > >
> > > I read that earlier reply as an example that covers a sample use case,
> > > I didn't realize you were asserting that was the only approach you
> > > were considering.  Perhaps that was the source of confusion earlier,
> > > we may disagree, but I don't intentionally "twist" words; not only is
> > > that rude, it's just stupid in public, archived discussions.
> > >
> > > As I mentioned previously, we really need to see an explicit yes/no
> > > flag from the BPF verifier to indicate that the signature on the BPF
> > > program has been validated.  It really should be as simple as adding a
> > > bool to bpf_prog_aux which the BPF verifier sets to true upon
> > > successful signature validation, and then an LSM can use this flag as
> > > input to an access control decision in a hook placed after the
> > > verifier.  Are you objecting to the addition of a flag in the
> > > bpf_prog_aux struct (or some other struct tightly coupled to the BPF
> > > program), the LSM hook after the verifier, or both?  It would also be
> > > helpful if you can elaborate on the technical reasons behind these
> > > objections.
> >
> > Neither the aux field, nor the hook are required because:
> >
> > * If the signature is passed, it will be enforced, there are no
> > "runtime aspects" that need to be configurable here.
> > * What the LSM can specify a policy for is when a signature is not
> > passed, for this, it does not need an aux field or a signature or the
> > new hook, existing hooks are sufficient.
> >
>
> What about wanting to create a policy that requires signatures under certain
> situations and allowing the lack of a signature under others? How is that
> implemented with the existing hooks?
> As I understand it, all the existing hooks know (would know) is that _if_ there
> is a signature _then_ it will be enforced. There is no way to know _whether_
> there is a signature.
>

The signature is passed in bpf_attr and if there is a signature the
LSM's job is done.

   https://elixir.bootlin.com/linux/v6.14.7/source/kernel/bpf/syscall.c#L5771

 It will be enforced.


- KP

> An example policy I can think of is that most users (with CAP_BPF) must submit
> signed programs but some users are exempted. Would that policy be able to be
> made with the current hooks?
>
> > - KP
> >
> > >
> > > --
> > > paul-moore.com
> >