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

We already do this in practice in userspace. It doesn't make much
sense to perform this delivery. So we might as well make this optimization.

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>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.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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