Messages in this thread | | | From | Christian Brauner <> | Date | Fri, 27 Apr 2018 10:41:58 +0200 | Subject | Re: [PATCH net-next 1/2 v2] netns: restrict uevents |
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On Thu, Apr 26, 2018 at 07:35:47PM -0500, Eric W. Biederman wrote: > Christian Brauner <christian.brauner@canonical.com> writes: > > > On Thu, Apr 26, 2018 at 12:10:30PM -0500, Eric W. Biederman wrote: > >> Christian Brauner <christian.brauner@canonical.com> writes: > >> > >> > On Thu, Apr 26, 2018 at 11:47:19AM -0500, Eric W. Biederman wrote: > >> >> Christian Brauner <christian.brauner@canonical.com> writes: > >> >> > >> >> > On Tue, Apr 24, 2018 at 06:00:35PM -0500, Eric W. Biederman wrote: > >> >> >> Christian Brauner <christian.brauner@canonical.com> writes: > >> >> >> > >> >> >> > On Wed, Apr 25, 2018, 00:41 Eric W. Biederman <ebiederm@xmission.com> 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; > > } > > I believe the above is what we already have. > > > 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); > > } > > The above is most definitely wrong as we store kuids and kgids in > "NETLINK_CB(skb).creds". > > I am pretty certain what we want is: > kuid_t root_uid = make_kuid(net->user_ns, 0); > kgid_g root_gid = make_kgid(net->user_ns, 0);
Thanks! I looked at user_namespace.c which contained map_id_down() which is the function that I wanted and remembered from a prior patchset of mine but they weren't exported. I didn't spot make_k{g,u}id() which are wrapping those. These are the droids^H^H^H^H^H^Hfunctions I was looking for!
> if (!uid_valid(root_uid)) > root_uid = GLOBAL_ROOT_UID; > if (!gid_valid(root_gid)) > root_gid = GLOBAL_ROOT_GID; > NETLINK_CB(skb).creds.uid = root_uid; > NETLINK_CB(skb).creds.gid = root_gid; > > Let's be careful and only fix this for the networking uevents please. > We want the other onces to just go away.
This is already handled by the
if (!net) handle_untagged_uevents() else handle_taggged_uevents()
The else branch will only every contain network devices as to my knowledge no other kernel devices are currently tagged.
Thanks! Christian
> > The networking uevents we have to fix or they will be gone completely. > > Eric
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