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

[Casey Schaufler]

On 6/28/2022 8:14 AM, Frederick Lawler wrote:
> On 6/27/22 6:18 PM, Casey Schaufler wrote:
> > 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's a documentation page that briefly touches on a BPF LSM implementation [1].

That's a brief touch, alright. I'll grant that the LSM interface isn't
especially well documented for C developers, but we have done tutorials
and have multiple examples. I worry that without an in-tree example for
eBPF we might well be setting developers up for spectacular failure.

> > There should be something out there warning the eBPF programmer of the
> > implications of providing a secid_to_secctx hook for starters.
>
> Links:
> 1. https://docs.kernel.org/bpf/prog_lsm.html?highlight=bpf+lsm#