[PATCH V33 24/30] bpf: Restrict bpf when kernel lockdown is in confidentiality mode

[Andy Lutomirski]

On Fri, Jun 28, 2019 at 11:47 AM Matthew Garrett <mjg59 at google.com> wrote:
>
> On Thu, Jun 27, 2019 at 4:27 PM Andy Lutomirski <luto at kernel.org> wrote:
> > They're really quite similar in my mind.  Certainly some things in the
> > "integrity" category give absolutely trivial control over the kernel
> > (e.g. modules) while others make it quite challenging (ioperm), but
> > the end result is very similar.  And quite a few "confidentiality"
> > things genuinely do allow all kernel memory to be read.
> >
> > I agree that finer-grained distinctions could be useful. My concern is
> > that it's a tradeoff, and the other end of the tradeoff is an ABI
> > stability issue.  If someone decides down the road that some feature
> > that is currently "integrity" can be split into a narrow "integrity"
> > feature and a "confidentiality" feature then, if the user policy knows
> > about the individual features, there's a risk of breaking people's
> > systems.  If we keep the fine-grained control, do we have a clear
> > compatibility story?
>
> My preference right now is to retain the fine-grained aspect of things
> in the internal API, simply because it'll be more annoying to add it
> back later if we want to. I don't want to expose it via the Lockdown
> user facing API for the reasons you've described, but it's not
> impossible that another LSM would find a way to do this reasonably.
> Does it seem reasonable to punt this discussion out to the point where
> another LSM tries to do something with this information, based on the
> implementation they're attempting?

I think I can get behind this, as long as it's clear to LSM authors
that this list is only a little bit stable.  I can certainly see the
use for the fine-grained info being available for auditing.