[PATCH v20 05/23] net: Prepare UDS for security module stacking

[Stephen Smalley]

On Tue, Sep 8, 2020 at 8:21 PM John Johansen
<john.johansen at canonical.com> wrote:
>
> On 9/8/20 4:37 PM, Casey Schaufler wrote:
> > On 9/8/2020 6:35 AM, Stephen Smalley wrote:
> >> On Mon, Sep 7, 2020 at 9:28 PM Stephen Smalley
> >> <stephen.smalley.work at gmail.com> wrote:
> >>> On Sat, Sep 5, 2020 at 3:07 PM John Johansen
> >>> <john.johansen at canonical.com> wrote:
> >>>> On 9/5/20 11:13 AM, Casey Schaufler wrote:
> >>>>> On 9/5/2020 6:25 AM, Paul Moore wrote:
> >>>>>> On Fri, Sep 4, 2020 at 7:58 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> >>>>>>> On 9/4/2020 2:53 PM, Paul Moore wrote:
> >>>>>>>> On Fri, Sep 4, 2020 at 5:35 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> >>>>>>>>> On 9/4/2020 1:08 PM, Paul Moore wrote:
> >>>>>> ...
> >>>>>>
> >>>>>>>> I understand the concerns you mention, they are all valid as far as
> >>>>>>>> I'm concerned, but I think we are going to get burned by this code as
> >>>>>>>> it currently stands.
> >>>>>>> Yes, I can see that. We're getting burned by the non-extensibility
> >>>>>>> of secids. It will take someone smarter than me to figure out how to
> >>>>>>> fit N secids into 32bits without danger of either failure or memory
> >>>>>>> allocation.
> >>>>>> Sooo what are the next steps here?  It sounds like there is some
> >>>>>> agreement that the currently proposed unix_skb_params approach is a
> >>>>>> problem, but it also sounds like you just want to merge it anyway?
> >>>>> There are real problems with all the approaches. This is by far the
> >>>>> least invasive of the lot. If this is acceptable for now I will commit
> >>>>> to including the dynamic allocation version in the full stacking
> >>>>> (e.g. Smack + SELinux) stage. If it isn't, well, this stage is going
> >>>>> to take even longer than it already has. Sigh.
> >>>>>
> >>>>>
> >>>>>> I was sorta hoping for something a bit better.
> >>>>> I will be looking at alternatives. I am very much open to suggestions.
> >>>>> I'm not even 100% convinced that Stephen's objections to my separate
> >>>>> allocation strategy outweigh its advantages. If you have an opinion on
> >>>>> that, I'd love to hear it.
> >>>>>
> >>>> fwiw I prefer the separate allocation strategy, but as you have already
> >>>> said it trading off one set of problems for another. I would rather see
> >>>> this move forward and one set of trade offs isn't significantly worse
> >>>> than the other to me so, either wfm.
> >>> I remain unclear that AppArmor needs this patch at all even when
> >>> support for SO_PEERSEC lands.
> >>> Contrary to the patch description, it is about supporting SCM_SECURITY
> >>> for datagram not SO_PEERSEC.  And I don't know of any actual users of
> >>> SCM_SECURITY even for SELinux, just SO_PEERSEC.
> >> I remembered that systemd once tried using SCM_SECURITY but that was a
> >> bug since systemd was using it with stream sockets and that wasn't
> >> supported by the kernel at the time,
> >> https://bugzilla.redhat.com/show_bug.cgi?id=1224211, so systemd
> >> switched over to using SO_PEERSEC.  Subsequently I did fix
> >> SCM_SECURITY to work with stream sockets via kernel commit
> >> 37a9a8df8ce9de6ea73349c9ac8bdf6ba4ec4f70 but SO_PEERSEC is still
> >> preferred.  Looking around, I see that there is still one usage of
> >> SCM_SECURITY in systemd-journald but it doesn't seem to be required
> >> (if provided, journald will pass the label along but nothing seems to
> >> depend on it AFAICT).  In any event, I don't believe this patch is
> >> needed to support stacking AppArmor.
> >
> > Stephen is, as is so often the case, correct. AppArmor has a stub
> > socket_getpeersec_dgram() that gets removed in patch 23. If I remove
>
> right but as I said before this is coming, I have been playing with
> it and have code. So the series doesn't need it today but sooner than
> later it will be needed

I don't understand why.  Is there a userspace component that relies on
SCM_SECURITY today for anything real (more than just blindly passing
it along and maybe writing to a log somewhere)?  And this doesn't
provide support for a composite SCM_SECURITY or SCM_CONTEXT, so it
doesn't really solve the stacking problem for it anyway.  What am I
missing?  Why do you care about this patch?