Re: [PATCH 07/10] isolation: Replace drop_caps() with a version that actually does something

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 13 Oct 2022 06:01:19 +0200
Stefano Brivio <sbrivio@redhat.com> wrote:

> On Tue, 11 Oct 2022 16:40:15 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > @@ -251,7 +275,19 @@ int isolate_prefork(struct ctx *c)
> >  		return -errno;
> >  	}
> >  
> > -	drop_caps();	/* Relative to the new user namespace this time. */
> > +	/* Drop capabilites in our new userns */
> > +	if (c->mode == MODE_PASTA) {
> > +		/* Keep CAP_SYS_ADMIN, so that we can setns() to the
> > +		 * netns when we need to act upon it
> > +		 */
> > +		ns_caps |= 1UL << CAP_SYS_ADMIN;
> > +		/* Keep CAP_NET_BIND_SERVICE, so we can splice
> > +		 * outbound connections to low port numbers
> > +		 */
> > +		ns_caps |= 1UL << CAP_NET_BIND_SERVICE;
> > +	}
> > +
> > +	drop_caps_ep_except(ns_caps);  
> 
> Hmm, I didn't really look into this yet, but there seems to be an issue
> with filesystem-bound network namespaces now. Running something like:
> 
>   pasta --config-net --netns /run/user/1000/netns/netns-6466ff4b-1efc-2b58-685b-cbc12feb9ccc
> 
> (from Podman), this happens:
> 
> [...]
>
> [pid 1763223] setns(7, CLONE_NEWNET)    = -1 EPERM (Operation not permitted)

Ah, "of course". Podman calls us with UID 0 in the user namespace it
just created, so if we drop CAP_SYS_ADMIN in isolate_initial() we can't
join the network namespace, and if we drop CAP_NET_ADMIN we can't
configure it.

So for that case (and only for that, I suppose), we need something like
(tested):

diff --git a/isolation.c b/isolation.c
index 1769180..fee6dbd 100644
--- a/isolation.c
+++ b/isolation.c
@@ -190,7 +190,7 @@ void isolate_initial(void)
         * namespace if we have it, so that we can forward low ports
         * into the guest/namespace
         */
-       drop_caps_ep_except((1UL << CAP_NET_BIND_SERVICE));
+       drop_caps_ep_except(BIT(CAP_SYS_ADMIN) | BIT(CAP_NET_ADMIN));
 }

...which is a bit pointless. Better than *any* capability, but not by
far.

So, if we make this totally independent from configuration, we need
those two capabilities.

We could add a "postconf" stage and cover a tiny bit more of conf.c.

Or we could have a special path in isolate_initial() for the case we
know we're not in the init namespace.

I'm not sure. If you have a specific preference/strong opinion I would
actually be happier. :)

-- 
@@ -190,7 +190,7 @@ void isolate_initial(void)
         * namespace if we have it, so that we can forward low ports
         * into the guest/namespace
         */
-       drop_caps_ep_except((1UL << CAP_NET_BIND_SERVICE));
+       drop_caps_ep_except(BIT(CAP_SYS_ADMIN) | BIT(CAP_NET_ADMIN));
 }

...which is a bit pointless. Better than *any* capability, but not by
far.

So, if we make this totally independent from configuration, we need
those two capabilities.

We could add a "postconf" stage and cover a tiny bit more of conf.c.

Or we could have a special path in isolate_initial() for the case we
know we're not in the init namespace.

I'm not sure. If you have a specific preference/strong opinion I would
actually be happier. :)

-- 
Stefano