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

From: Frederick Lawler
Date: Tue Jun 28 2022 - 11:14:47 EST

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].

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#