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

From: [email-masked] (Eric W. Biederman)
Date: Thu, 26 Apr 2018 12:10:30 -0500

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
>> 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