Subject: Re: [PATCH net-next 1/2 v2] netns: restrict uevents

From: Christian Brauner <[email-masked]>
Date: Thu, 26 Apr 2018 23:27:45 +0200

On Thu, Apr 26, 2018 at 12:10:30PM -0500, Eric W. Biederman wrote:
> Christian Brauner <[email-masked]> writes:
> 
> > On Thu, Apr 26, 2018 at 11:47:19AM -0500, Eric W. Biederman wrote:
> >> Christian Brauner <[email-masked]> writes:
> >> 
> >> > On Tue, Apr 24, 2018 at 06:00:35PM -0500, Eric W. Biederman wrote:
> >> >> Christian Brauner <[email-masked]> writes:
> >> >> 
> >> >> > On Wed, Apr 25, 2018, 00:41 Eric W. Biederman <[email-masked]> wrote:
> >> >> >
> >> >> >  Bah. This code is obviously correct and probably wrong.
> >> >> >
> >> >> >  How do we deliver uevents for network devices that are outside of the
> >> >> >  initial user namespace? The kernel still needs to deliver those.
> >> >> >
> >> >> >  The logic to figure out which network namespace a device needs to be
> >> >> >  delivered to is is present in kobj_bcast_filter. That logic will almost
> >> >> >  certainly need to be turned inside out. Sign not as easy as I would
> >> >> >  have hoped.
> >> >> >
> >> >> > My first patch that we discussed put additional filtering logic into kobj_bcast_filter for that very reason. But I can move that logic
> >> >> > out and come up with a new patch.
> >> >> 
> >> >> I may have mis-understood.
> >> >> 
> >> >> I heard and am still hearing additional filtering to reduce the places
> >> >> the packet is delievered.
> >> >> 
> >> >> I am saying something needs to change to increase the number of places
> >> >> the packet is delivered.
> >> >> 
> >> >> For the special class of devices that kobj_bcast_filter would apply to
> >> >> those need to be delivered to netowrk namespaces  that are no longer on
> >> >> uevent_sock_list.
> >> >> 
> >> >> So the code fundamentally needs to split into two paths.  Ordinary
> >> >> devices that use uevent_sock_list.  Network devices that are just
> >> >> delivered in their own network namespace.
> >> >> 
> >> >> netlink_broadcast_filtered gets to go away completely.
> >> >
> >> > The split *might* make sense but I think you're wrong about removing the
> >> > kobj_bcast_filter. The current filter doesn't operate on the uevent
> >> > socket in uevent_sock_list itself it rather operates on the sockets in
> >> > mc_list. And if socket in mc_list can have a different network namespace
> >> > then the uevent_socket itself then your way won't work. That's why my
> >> > original patch added additional filtering in there. The way I see it we
> >> > need something like:
> >> 
> >> We already filter the sockets in the mc_list by network namespace.
> >
> > Oh really? That's good to know. I haven't found where in the code this
> > actually happens. I thought that when netlink_bind() is called anyone
> > could register themselves in mc_list.
> 
> The code in af_netlink.c does:
> > static void do_one_broadcast(struct sock *sk,
> > 				    struct netlink_broadcast_data *p)
> > {
> > 	struct netlink_sock *nlk = nlk_sk(sk);
> > 	int val;
> > 
> > 	if (p->exclude_sk == sk)
> > 		return;
> > 
> > 	if (nlk->portid == p->portid || p->group - 1 >= nlk->ngroups ||
> > 	    !test_bit(p->group - 1, nlk->groups))
> > 		return;
> > 
> > 	if (!net_eq(sock_net(sk), p->net)) {
>             ^^^^^^^^^^^^ Here
> > 		if (!(nlk->flags & NETLINK_F_LISTEN_ALL_NSID))
> > 			return;
>                 ^^^^^^^^^^^ Here
> > 
> > 		if (!peernet_has_id(sock_net(sk), p->net))
> > 			return;
> > 
> > 		if (!file_ns_capable(sk->sk_socket->file, p->net->user_ns,
> > 				     CAP_NET_BROADCAST))
> > 			return;
> > 	}
> 
> Which if you are not a magic NETLINK_F_LISTEN_ALL_NSID socket filters
> you out if you are the wrong network namespace.
> 
> 
> >> When a packet is transmitted with netlink_broadcast it is only
> >> transmitted within a single network namespace.
> >> 
> >> Even in the case of a NETLINK_F_LISTEN_ALL_NSID socket the skb is tagged
> >> with it's source network namespace so no confusion will result, and the
> >> permission checks have been done to make it safe. So you can safely
> >> ignore that case.  Please ignore that case.  It only needs to be
> >> considered if refactoring af_netlink.c
> >> 
> >> When I added netlink_broadcast_filtered I imagined that we would need
> >> code that worked across network namespaces that worked for different
> >> namespaces.   So it looked like we would need the level of granularity
> >> that you can get with netlink_broadcast_filtered.  It turns out we don't
> >> and that it was a case of over design.  As the only split we care about
> >> is per network namespace there is no need for
> >> netlink_broadcast_filtered.
> >> 
> >> > init_user_ns_broadcast_filtered(uevent_sock_list, kobj_bcast_filter);
> >> > user_ns_broadcast_filtered(uevent_sock_list,kobj_bcast_filter);
> >> >
> >> > The question that remains is whether we can rely on the network
> >> > namespace information we can gather from the kobject_ns_type_operations
> >> > to decide where we want to broadcast that event to. So something
> >> > *like*:
> >> 
> >> We can.  We already do.  That is what kobj_bcast_filter implements.
> >> 
> >> > 	ops = kobj_ns_ops(kobj);
> >> > 	if (!ops && kobj->kset) {
> >> > 		struct kobject *ksobj = &kobj->kset->kobj;
> >> > 		if (ksobj->parent != NULL)
> >> > 			ops = kobj_ns_ops(ksobj->parent);
> >> > 	}
> >> >
> >> > 	if (ops && ops->netlink_ns && kobj->ktype->namespace)
> >> > 		if (ops->type == KOBJ_NS_TYPE_NET)
> >> > 			net = kobj->ktype->namespace(kobj);
> >> 
> >> Please note the only entry in the enumeration in the kobj_ns_type
> >> enumeration other than KOBJ_NS_TYPE_NONE is KOBJ_NS_TYPE_NET.  So the
> >> check for ops->type in this case is redundant.
> >
> > Yes, I know the reason for doing it explicitly is to block the case
> > where kobjects get tagged with other namespaces. So we'd need to be
> > vigilant should that ever happen but fine.
> 
> It is fine to keep the check.
> 
> I was intending to point out that it is much more likely that we remove
> the enumeration and remove some of the extra abstraction, than another
> namespace is implemented there.
> 
> >> That is something else that could be simplifed.  At the time it was the
> >> necessary to get the sysfs changes merged.
> >> 
> >> > 	if (!net || net->user_ns == &init_user_ns)
> >> > 		ret = init_user_ns_broadcast(env, action_string, devpath);
> >> > 	else
> >> > 		ret = user_ns_broadcast(net->uevent_sock->sk, env,
> >> > 					action_string, devpath);
> >> 
> >> Almost.
> >> 
> >> 	if (!net)
> >>         	kobject_uevent_net_broadcast(kobj, env, action_string,
> >>         					dev_path);
> >> 	else
> >>         	netlink_broadcast(net->uevent_sock->sk, skb, 0, 1, GFP_KERNEL);
> >> 
> >> 
> >> I am handwaving to get the skb in the netlink_broadcast case but that
> >> should be enough for you to see what I am thinking.
> >
> > I have added a helper alloc_uevent_skb() that can be used in both cases.
> >
> > static struct sk_buff *alloc_uevent_skb(struct kobj_uevent_env *env,
> > 					const char *action_string,
> > 					const char *devpath)
> > {
> > 	struct sk_buff *skb = NULL;
> > 	char *scratch;
> > 	size_t len;
> >
> > 	/* allocate message with maximum possible size */
> > 	len = strlen(action_string) + strlen(devpath) + 2;
> > 	skb = alloc_skb(len + env->buflen, GFP_KERNEL);
> > 	if (!skb)
> > 		return NULL;
> >
> > 	/* add header */
> > 	scratch = skb_put(skb, len);
> > 	sprintf(scratch, "%s@%s", action_string, devpath);
> >
> > 	skb_put_data(skb, env->buf, env->buflen);
> >
> > 	NETLINK_CB(skb).dst_group = 1;
> >
> > 	return skb;
> > }
> >
> >> 
> >> My only concern with the above is that we almost certainly need to fix
> >> the credentials on the skb so that userspace does not drop the packet

I guess we simply want:
	if (user_ns != &init_user_ns) {
		NETLINK_CB(skb).creds.uid = (kuid_t)0;
		NETLINK_CB(skb).creds.gid = kgid_t)0;
	}

instead of the more complicated and - imho wrong:

	if (user_ns != &init_user_ns) {
		/* fix credentials for udev running in user namespace */
		kuid_t uid = NETLINK_CB(skb).creds.uid;
		kgid_t gid = NETLINK_CB(skb).creds.gid;
		NETLINK_CB(skb).creds.uid = from_kuid_munged(user_ns, uid);
		NETLINK_CB(skb).creds.gid = from_kgid_munged(user_ns, gid);
	}

Christian

> >> sent to a network namespace because it has the credentials that will
> >> cause userspace to drop the packet today.
> >> 
> >> But it should be straight forward to look at net->user_ns, to fix the
> >> credentials.
> >
> > Yes, afaict, the only thing that needs to be updated is the uid.
> 
> I suspect there may also be a gid.
> 
> Eric