netdev - Re: New skb extension for use by LSMs (skb "security blob")?

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Date:   Thu, 22 Aug 2019 12:32:18 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Florian Westphal <fw@...len.de>
Cc:     netdev@...r.kernel.org, linux-security-module@...r.kernel.org,
        selinux@...r.kernel.org
Subject: Re: New skb extension for use by LSMs (skb "security blob")?

On Thu, Aug 22, 2019 at 3:03 AM Florian Westphal <fw@...len.de> wrote:
> Paul Moore <paul@...l-moore.com> wrote:
> > Hello netdev,
> >
> > 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.
>
> Is that "blob" in addition to skb->secmark, or a replacement?

That's a good question.  While I thought about that, I wasn't sure if
that was worth bringing up as previous attempts to trade the secmark
field for a void pointer met with failure.  Last time I played with it
I was able to take the additional 32-bits from holes in the skb, and
possibly even improve some of the cacheline groupings (but that is
always going to be a dependent on use case I think), but that wasn't
enough.

I think we could consider freeing up the secmark in the main skb, and
move it to a skb extension, but this would potentially increase the
chances that we would need to add an extension to a skb.  I don't have
any hard numbers, but based on discussions and questions I suspect
Secmark is more widely used than NetLabel and/or labeled IPsec;
although I'm confident it is still a minor percentage of the overall
Linux installed base.

For me the big question is what would it take for us to get a security
blob associated with the skb?  Would moving the secmark into the skb
extension be enough?  Something else?  Or is this simply never going
to happen?  I want to remain optimistic, but I've been trying for this
off-and-on for over a decade and keep running into a brick wall ;)

-- 
paul moore
www.paul-moore.com