New skb extension for use by LSMs (skb "security blob")?

[Paul Moore]

On Wed, Aug 21, 2019 at 6:50 PM David Miller <davem at davemloft.net> wrote:
> From: Paul Moore <paul at paul-moore.com>
> Date: Wed, 21 Aug 2019 18:00:09 -0400
>
> > I was just made aware of the skb extension work, and it looks very
> > appealing from a LSM perspective.  As some of you probably remember,
> > we (the LSM folks) have wanted a proper security blob in the skb for
> > quite some time, but netdev has been resistant to this idea thus far.
> >
> > If I were to propose a patchset to add a SKB_EXT_SECURITY skb
> > extension (a single extension ID to be shared among the different
> > LSMs), would that be something that netdev would consider merging, or
> > is there still a philosophical objection to things like this?
>
> Unlike it's main intended user (MPTCP), it sounds like LSM's would use
> this in a way such that it would be enabled on most systems all the
> time.
>
> That really defeats the whole purpose of making it dynamic. :-/

I would be okay with only adding a skb extension when we needed it,
which I'm currently thinking would only be when we had labeled
networking actually configured at runtime and not just built into the
kernel.  In SELinux we do something similar today when it comes to our
per-packet access controls; if labeled networking is not configured we
bail out of the LSM hooks early to improve performance (we would just
be comparing unlabeled_t to unlabeled_t anyway).  I think the other
LSMs would be okay with this usage as well.

While a number of distros due enable some form of LSM and the labeled
networking bits at build time, vary few (if any?) provide a default
configuration so I would expect no additional overhead in the common
case.

Would that be acceptable?

-- 
paul moore
www.paul-moore.com