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

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*:

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);

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);

Christian

> The logic of figuring out the network namespace though becomes trickier.
>
> Now it may make sense to have all of that as an additional patch on top
> of this one or perhaps a precursor patch that addresses the problem. We
> will unfortunately drop those uevents today because their uids are not
> valid. But they are not delivered anywhere else so to allow them to be
> received we need to fix them.
>
> Eric
>
> >
> > Christian Brauner <christian.brauner@ubuntu.com> writes:
> > > commit 07e98962fa77 ("kobject: Send hotplug events in all network namespaces")
> > >
> > > enabled sending hotplug events into all network namespaces back in 2010.
> > > Over time the set of uevents that get sent into all network namespaces has
> > > shrunk a little. We have now reached the point where hotplug events for all
> > > devices that carry a namespace tag are filtered according to that
> > > namespace. Specifically, they are filtered whenever the namespace tag of
> > > the kobject does not match the namespace tag of the netlink socket. One
> > > example are network devices. Uevents for network devices only show up in
> > > the network namespaces these devices are moved to or created in.
> > >
> > > However, any uevent for a kobject that does not have a namespace tag
> > > associated with it will not be filtered and we will broadcast it into all
> > > network namespaces. This behavior stopped making sense when user namespaces
> > > were introduced.
> > >
> > > This patch restricts uevents to the initial user namespace for a couple of
> > > reasons that have been extensively discusses on the mailing list [1].
> > > - Thundering herd:
> > > Broadcasting uevents into all network namespaces introduces significant
> > > overhead.
> > > All processes that listen to uevents running in non-initial user
> > > namespaces will end up responding to uevents that will be meaningless to
> > > them. Mainly, because non-initial user namespaces cannot easily manage
> > > devices unless they have a privileged host-process helping them out. This
> > > means that there will be a thundering herd of activity when there
> > > shouldn't be any.
> > > - Uevents from non-root users are already filtered in userspace:
> > > Uevents are filtered by userspace in a user namespace because the
> > > received uid != 0. Instead the uid associated with the event will be
> > > 65534 == "nobody" because the global root uid is not mapped.
> > > This means we can safely and without introducing regressions modify the
> > > kernel to not send uevents into all network namespaces whose owning user
> > > namespace is not the initial user namespace because we know that
> > > userspace will ignore the message because of the uid anyway. I have
> > > a) verified that is is true for every udev implementation out there b)
> > > that this behavior has been present in all udev implementations from the
> > > very beginning.
> > > - Removing needless overhead/Increasing performance:
> > > Currently, the uevent socket for each network namespace is added to the
> > > global variable uevent_sock_list. The list itself needs to be protected
> > > by a mutex. So everytime a uevent is generated the mutex is taken on the
> > > list. The mutex is held *from the creation of the uevent (memory
> > > allocation, string creation etc. until all uevent sockets have been
> > > handled*. This is aggravated by the fact that for each uevent socket that
> > > has listeners the mc_list must be walked as well which means we're
> > > talking O(n^2) here. Given that a standard Linux workload usually has
> > > quite a lot of network namespaces and - in the face of containers - a lot
> > > of user namespaces this quickly becomes a performance problem (see
> > > "Thundering herd" above). By just recording uevent sockets of network
> > > namespaces that are owned by the initial user namespace we significantly
> > > increase performance in this codepath.
> > > - Injecting uevents:
> > > There's a valid argument that containers might be interested in receiving
> > > device events especially if they are delegated to them by a privileged
> > > userspace process. One prime example are SR-IOV enabled devices that are
> > > explicitly designed to be handed of to other users such as VMs or
> > > containers.
> > > This use-case can now be correctly handled since
> > > commit 692ec06d7c92 ("netns: send uevent messages"). This commit
> > > introduced the ability to send uevents from userspace. As such we can let
> > > a sufficiently privileged (CAP_SYS_ADMIN in the owning user namespace of
> > > the network namespace of the netlink socket) userspace process make a
> > > decision what uevents should be sent. This removes the need to blindly
> > > broadcast uevents into all user namespaces and provides a performant and
> > > safe solution to this problem.
> > > - Filtering logic:
> > > This patch filters by *owning user namespace of the network namespace a
> > > given task resides in* and not by user namespace of the task per se. This
> > > means if the user namespace of a given task is unshared but the network
> > > namespace is kept and is owned by the initial user namespace a listener
> > > that is opening the uevent socket in that network namespace can still
> > > listen to uevents.
> > >
> > > [1]: https://lkml.org/lkml/2018/4/4/739
> > > Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> > > ---
> > > Changelog v1->v2:
> > > * patch unchanged
> > > Changelog v0->v1:
> > > * patch unchanged
> > > ---
> > > lib/kobject_uevent.c | 18 ++++++++++++------
> > > 1 file changed, 12 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
> > > index 15ea216a67ce..f5f5038787ac 100644
> > > --- a/lib/kobject_uevent.c
> > > +++ b/lib/kobject_uevent.c
> > > @@ -703,9 +703,13 @@ static int uevent_net_init(struct net *net)
> > >
> > > net->uevent_sock = ue_sk;
> > >
> > > - mutex_lock(&uevent_sock_mutex);
> > > - list_add_tail(&ue_sk->list, &uevent_sock_list);
> > > - mutex_unlock(&uevent_sock_mutex);
> > > + /* Restrict uevents to initial user namespace. */
> > > + if (sock_net(ue_sk->sk)->user_ns == &init_user_ns) {
> > > + mutex_lock(&uevent_sock_mutex);
> > > + list_add_tail(&ue_sk->list, &uevent_sock_list);
> > > + mutex_unlock(&uevent_sock_mutex);
> > > + }
> > > +
> > > return 0;
> > > }
> > >
> > > @@ -713,9 +717,11 @@ static void uevent_net_exit(struct net *net)
> > > {
> > > struct uevent_sock *ue_sk = net->uevent_sock;
> > >
> > > - mutex_lock(&uevent_sock_mutex);
> > > - list_del(&ue_sk->list);
> > > - mutex_unlock(&uevent_sock_mutex);
> > > + if (sock_net(ue_sk->sk)->user_ns == &init_user_ns) {
> > > + mutex_lock(&uevent_sock_mutex);
> > > + list_del(&ue_sk->list);
> > > + mutex_unlock(&uevent_sock_mutex);
> > > + }
> > >
> > > netlink_kernel_release(ue_sk->sk);
> > > kfree(ue_sk);

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