Re: [PATCH 0/2] Introduce security_create_user_ns()

[Casey Schaufler]

On 6/27/2022 3:27 PM, Paul Moore wrote:
> On Mon, Jun 27, 2022 at 6:15 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> > On 6/27/22 11:56 PM, Paul Moore wrote:
> > > On Mon, Jun 27, 2022 at 8:11 AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> > > > On Thu, Jun 23, 2022 at 11:21:37PM -0400, Paul Moore wrote:
> > > ...
> > >
> > > > > This is one of the reasons why I usually like to see at least one LSM
> > > > > implementation to go along with every new/modified hook.  The
> > > > > implementation forces you to think about what information is necessary
> > > > > to perform a basic access control decision; sometimes it isn't always
> > > > > obvious until you have to write the access control :)
> > > > I spoke to Frederick at length during LSS and as I've been given to
> > > > understand there's a eBPF program that would immediately use this new
> > > > hook. Now I don't want to get into the whole "Is the eBPF LSM hook
> > > > infrastructure an LSM" but I think we can let this count as a legitimate
> > > > first user of this hook/code.
> > > Yes, for the most part I don't really worry about the "is a BPF LSM a
> > > LSM?" question, it's generally not important for most discussions.
> > > However, there is an issue unique to the BPF LSMs which I think is
> > > relevant here: there is no hook implementation code living under
> > > security/.  While I talked about a hook implementation being helpful
> > > to verify the hook prototype, it is also helpful in providing an
> > > in-tree example for other LSMs; unfortunately we don't get that same
> > > example value when the initial hook implementation is a BPF LSM.
> > I would argue that such a patch series must come together with a BPF
> > selftest which then i) contains an in-tree usage example, ii) adds BPF
> > CI test coverage. Shipping with a BPF selftest at least would be the
> > usual expectation.
> I'm not going to disagree with that, I generally require matching
> tests for new SELinux kernel code, but I was careful to mention code
> under 'security/' and not necessarily just a test implementation :)  I
> don't want to get into a big discussion about it, but I think having a
> working implementation somewhere under 'security/' is more
> discoverable for most LSM folks.

I agree. It would be unfortunate if we added a hook explicitly for eBPF
only to discover that the proposed user needs something different. The
LSM community should have a chance to review the code before committing
to all the maintenance required in supporting it.

Is there a reference on how to write an eBPF security module?
There should be something out there warning the eBPF programmer of the
implications of providing a secid_to_secctx hook for starters.