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

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

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On Thu, Oct 13, 2022 at 06:37:05PM +0200, Stefano Brivio wrote:
> On Thu, 13 Oct 2022 15:08:02 +0200
> Stefano Brivio <sbrivio@redhat.com> wrote:
> 
> > 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. :)
> 
> Further on, if we are started as root, we'll fail to drop to 'nobody'
> or any other user, if we lose CAP_SETUID and CAP_SETGID here. I have
> tested this version of isolate_initial():
> 
> 	drop_caps_ep_except(BIT(CAP_SYS_ADMIN) | BIT(CAP_NET_ADMIN) |
> 			    BIT(CAP_SETGID)    | BIT(CAP_SETUID)    |
> 			    BIT(CAP_NET_BIND_SERVICE));
> 
> for any use case I can reasonably think of. Yes, it's a lot -- we
> should make it really clear that those are not the capabilities we
> actually use at "run time".

Ah, right.  I didn't think through the --netns-only case.

I think what we need to do in the short term at least is:

 * Add all those caps and a comment in isolate_initial() (or even just
   remove the drop_caps there)
 * In isolate user drop the caps we can at that point (SETGID and
   SETUID) - whether or not we've joined a new userns
 * Remove the rest of the "runtime" caps in isolate_prefork() like we
   do now.

I'll rework accordingly.

-- 
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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