Re: [PATCH] RFC: Remove unusable --netns-only option

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Jul 19, 2022 at 10:39:25PM +0200, Stefano Brivio wrote:
> On Tue, 19 Jul 2022 16:23:10 +1000
> David Gibson <david(a)gibson.dropbear.id.au> wrote:
> 
> > The intended semantics of --netns-only are pretty unclear to me.  It's
> > intended for pasta, but it's not clear whether its saying the spawned shell
> > should only enter the target netns, or that the passt/pasta packet
> > forwarding process should only sandbox itself in a network namespace, not
> > a user namespace.
> 
> The latter. I think this is marginally more clear in the man page, but needs
> indeed a better explanation.

Definitely.  At present it also appears to affect the spawned shell as
well, it a rather counter-intuitive way.

> > In any case, as far as I can tell there's not actually any case in which
> > the --netns-only option will work.  If nothing else, we will always fail
> > in sandbox(), because it attempts a number of operations which require
> > CAP_SYS_ADMIN in our current user namespace.  We drop all capabilities in
> > our initial user namespace when we start, so the only way we can have
> > CAP_SYS_ADMIN at this point is if we've joined a new user namespace, which
> > we won't do with --netns-only.
> > 
> > For pasta joining an existing namespace (the apparently intended use case), we'll actually fail before
> > we'll fail before we get to that point: in conf_ns_check() we'll attempt
> > to join the target network namespace.  This also requires CAP_SYS_ADMIN in
> > both our current user namespace and the user namespace which owns the
> > target network namespace.  Again, since we've dropped capabilities in our
> > original namespace this will never be the case.
> 
> ...however, we can also have UID 0 in a non-init user namespace, and
> that will work.

Hrm..  I thought being UID 0 just meant we started with all the
capabilities, so once we've explicitly dropped them we still won't be
able to do this.  That seemed to be what happened when I tried running
it as root.

> This is what happens in the Podman integration case. Unfortunately the
> demo is broken at the moment (I had to rebase the patch with a bit of
> care, I'll publish the updated one soon).

Can you explain a bit more about what the podman use case is, and why
it requires the netns only logic?

> > For pasta creating its own network namespace we'll fail for a similar
> > reason in yet another place.  This time we'll fail in nl_sock_init() again
> > because we attempt to enter the new network ns via NS_CALL without having
> > regained CAP_SYS_ADMIN by joining a new user namespace.  Because this
> > happens after spawning the shell, it results in a weird failure mode, where
> > the pasta spawned shell is running, but pasta isn't actually handling
> > packets.  Exiting the shell will lead to a hang until the process is
> > explicitly killed.
> 
> Ouch, I didn't think of this.
> 
> Anyway, let me get back to you in a couple of days on the whole issue.
> The usage is there, albeit poorly documented, with a broken demo, and
> no handling of (kind of) corner cases.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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