Re: [PATCH 1/2] rtnetlink: gate MAC address with an LSM hook

[Paul Moore <paul@paul-moore.com>]

On Thu, Aug 29, 2019 at 3:45 AM Michal Kubecek <mkubecek@suse.cz> wrote:
> On Tue, Aug 27, 2019 at 04:47:04PM -0400, Paul Moore wrote:
> >
> > I'm also not a big fan of inserting the hook in rtnl_fill_ifinfo(); as
> > presented it is way too specific for a LSM hook for me to be happy.
> > However, I do agree that giving the LSMs some control over netlink
> > messages makes sense.  As others have pointed out, it's all a matter
> > of where to place the hook.
> >
> > If we only care about netlink messages which leverage nlattrs I
> > suppose one option that I haven't seen mentioned would be to place a
> > hook in nla_put().  While it is a bit of an odd place for a hook, it
> > would allow the LSM easy access to the skb and attribute type to make
> > decisions, and all of the callers should already be checking the
> > return code (although we would need to verify this).  One notable
> > drawback (not the only one) is that the hook is going to get hit
> > multiple times for each message.
>
> For most messages, "multiple times" would mean tens, for many even
> hundreds of calls. For each, you would have to check corresponding
> socket (and possibly also genetlink header) to see which netlink based
> protocol it is and often even parse existing part of the message to get
> the context (because the same numeric attribute type can mean something
> completely different if it appears in a nested attribute).
>
> Also, nla_put() (or rather __nla_put()) is not used for all attributes,
> one may also use nla_reserve() and then compose the attribute date in
> place.

I never said it was a great idea, just an idea ;)

Honestly I'm just trying to spur some discussion on this so we can
hopefully arrive at a solution which allows a LSM to control kernel
generated netlink messages that we can all accept.

-- 
paul moore
www.paul-moore.com