Subject: Re: [RFC PATCH net-next 00/11] netns: don't switch namespace while creating kernel sockets

From: [email-masked] (Eric W. Biederman)
Date: Thu, 07 May 2015 13:26:44 -0500

Cong Wang <[email-masked]> writes:

> On Thu, May 7, 2015 at 9:14 AM, Eric W. Biederman <[email-masked]> wrote:
>> Ying Xue <[email-masked]> writes:
>>
>>> When commit 23fe18669e7f ("[NETNS]: Fix race between put_net() and
>>> netlink_kernel_create().") attempted to fix the race between put_net()
>>> and kernel socket's creation, it adopted a complex solution: create
>>> netlink socket inside init_net namespace and then re-attach it to the
>>> desired one right after the socket is created; similarly, when close
>>> the socket, move back its namespace to init_net so that the socket can
>>> be destroyed in the context which is same as the socket creation.
>>>
>>> But the solution artificially makes the whole thing complex as its
>>> design is not only weird, but also it causes a bad consequence that
>>> when all kernel modules create kernel sockets, they have to follow
>>> the model of namespace switch. More importantly, with the way kernel
>>> sockets are created in init_net namespace, but they are released in
>>> another new ones. This inconsistent namespace brings some modules many
>>> inconvenience. For example, what tipc socket is inserted to rhashtable
>>> happens in socket's creation, and different namespace has different
>>> rhashtable for tipc socket. With the approach, a tipc kernel socket
>>> will be inserted into the rhashtable of init_net. But as releasing
>>> the socket happens in another one, it causes what the socket cannot
>>> be found from the rhashtable of the new namespace.
>>>
>>> Therefore, we propose a simpler solution to avoid the race: if we
>>> find there is still pending a cleanup work in __put_net(), we don't
>>> queue a new cleanup work to stop the cleanup process. The new proposal
>>> not only successfully solves the race, but also it can help us to
>>> avoid unnecessary namespace switches when creating kernel sockets.
>>> Moreover, it can guarantee that both creation and release of kernel
>>> sockets happen in the same namespace at all time.
>>>
>>> In the series, we first resolve the race with patch #1, and then
>>> prevent namespace switches from happening in all relevant kernel
>>> modules one by one from patch #2 to patch #9. Until now, as all
>>> dependencies on sk_change_net() are killed, we can delete the
>>> interface completely in patch #10. Lastly, we simplify the code of
>>> creating kernel sockets through changing the original behaviours
>>> of sock_create_kern() and sk_release_kernel(). If a kernel socket
>>> is created within a namespace which is different with init_net,
>>> we must put the reference counter of the namespace once the socket
>>> is successfully allocated in sk_alloc(), otherwise, the namespace
>>> is probably unable to be shut down finally. Therefore, we decrease
>>> namespace's reference counter once a kernel socket is created
>>> successfully by sock_create_kern() within a namespace which is
>>> different with init_net. Similarly, namespace's reference counter
>>> must be increased back before the socket is destroyed in
>>> sk_release_kernel().
>>>
>>> Welcome to any comments.
>>
>> I agree that commit 23fe18669e7f ("[NETNS]: Fix race between put_net()
>> and netlink_kernel_create()."  was a hack.
>>
>> However it is not appropriate to call get_net on a network namespace
>> whose count might be zero.  I believe all of your patches rely on that
>> currently.  Instead we need to build something like sk_release_kernel
>> that does not increase the network namespace reference count if you are
>> going to avoid changing the network namespace on a socket (a worthy
>> goal).
>>
>> The following change shows how it is possible to always know that your
>> network namespace has a non-zero reference count in the network
>> namespace initialization methods.  My implementation of
>> lock_network_namespaces is problematic in that it does not sleep
>> while network namespaces are unregistering.  But it is enough to show
>> how the locking and reference counting can be fixed.
>
> Why does this have to be so complicated? We can simply avoid
> calling ops_init() by skipping those in cleanup_list, no?

The problem is that there is a single list of methods to call and if you
simply skip calling the initialization methods for a struct net and add
yourself to the list cleanup_net will then call the cleanup methods
without calling the cleanup methods.

Simply limiting new network namespace registrations to a point when
network namespaces are not being registered or unregisted seems like
the simplest way to achieve this effect.

> diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
> index 78fc04a..c7cbd5a 100644
> --- a/net/core/net_namespace.c
> +++ b/net/core/net_namespace.c
> @@ -242,6 +242,7 @@ static __net_init int setup_net(struct net *net,
> struct user_namespace *user_ns)
>         net->dev_base_seq = 1;
>         net->user_ns = user_ns;
>         idr_init(&net->netns_ids);
> +       INIT_LIST_HEAD(&net->cleanup_list);
>
>         list_for_each_entry(ops, &pernet_list, list) {
>                 error = ops_init(ops, net);
> @@ -737,6 +738,8 @@ static int __register_pernet_operations(struct
> list_head *list,
>         list_add_tail(&ops->list, list);
>         if (ops->init || (ops->id && ops->size)) {
>                 for_each_net(net) {
> +                       if (!list_empty(net->cleanup_list)) // <---
> Need a big comment here;
> +                               continue;
>                         error = ops_init(ops, net);
>                         if (error)
>                                 goto out_undo;