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

From: [email-masked] (Eric W. Biederman)
Date: Thu, 26 Apr 2018 11:47:19 -0500

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.

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.

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.

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
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.

Eric