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

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Message-ID: <196e1f03128.28a7.85c95baa4474aabc7814e68940a78392@paul-moore.com>
Date: Sun, 18 May 2025 01:48:57 -0400
From: Paul Moore <paul@...l-moore.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
CC: KP Singh <kpsingh@...nel.org>, 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>, 
	Justin Stitt <justinstitt@...gle.com>, <keyrings@...r.kernel.org>, 
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>, 
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, 
	Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, 
	LSM List <linux-security-module@...r.kernel.org>, 
	clang-built-linux <llvm@...ts.linux.dev>, Masahiro Yamada <masahiroy@...nel.org>, 
	Mickaël Salaün <mic@...ikod.net>, 
	Bill Wendling <morbo@...gle.com>, Nathan Chancellor <nathan@...nel.org>, Neal Gompa <neal@...pa.dev>, 
	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>, Shuah Khan <shuah@...nel.org>, Matteo Croce <teknoraver@...a.com>, 
	Cong Wang <xiyou.wangcong@...il.com>, <kysrinivasan@...il.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3 0/4] Introducing Hornet LSM

On May 17, 2025 12:13:50 PM Alexei Starovoitov 
<alexei.starovoitov@...il.com> wrote:
> On Sat, May 17, 2025 at 8:03 AM Paul Moore <paul@...l-moore.com> wrote:
>> On Fri, May 16, 2025 at 7:49 PM Alexei Starovoitov
>> <alexei.starovoitov@...il.com> wrote:
>>> On Fri, May 16, 2025 at 12:49 PM Paul Moore <paul@...l-moore.com> wrote:
>>>>
>>>> I think we need some clarification on a few of these details, it would
>>>> be good if you could answer the questions below about the
>>>> authorization aspects of your design?
>>>>
>>>> * Is the signature validation code in the BPF verifier *always* going
>>>> to be enforced when a signature is passed in from userspace?  In other
>>>> words, in your design is there going to be either a kernel build time
>>>> or runtime configuration knob that could selectively enable (or
>>>> disable) signature verification in the BPF verifier?
>>>
>>> If there is a signature in union bpf_attr and it's incorrect
>>> the prog_load command will be rejected.
>>> No point in adding a knob to control that.
>>
>> I agree that when a signature is provided and that signature check
>> fails, the BPF load should be rejected.  I'm simply trying to
>> understand how you envision your design handling all of the cases, not
>> just this one, as well as what build and runtime options you expect
>> for controlling various aspects of this behavior.
>>
>>>> * In the case where the signature validation code in the BPF verifier
>>>> is active, what happens when a signature is *not* passed in from
>>>> userspace?  Will the BPF verifier allow the program load to take
>>>> place?  Will the load operation be blocked?  Will the load operation
>>>> be subject to a more granular policy, and if so, how do you plan to
>>>> incorporate that policy decision into the BPF program load path?
>>>
>>> If there is no signature the existing loading semantics will remain intact.
>>> We can discuss whether to add a sysctl or cgroup knob to disallow
>>> loading when signature is not present ...
>>
>> As mentioned earlier this week, if the BPF verifier is performing the
>> signature verification as KP described, we will need a LSM hook after
>> the verifier to serve as an access control point.  Of course that
>> doesn't preclude the addition of some type of sysctl/cgroup/whatever
>> based access control, but the LSM hook would be needed regardless.
>
> 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.

--
paul-moore.com

>