Re: [PATCH net-next 2/6] vsock: add IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, Sep 15, 2026 at 11:50:10AM -0700, Bobby Eshleman wrote:
>On Tue, Sep 15, 2026 at 05:28:25PM +0200, Stefano Garzarella wrote:
>> On Wed, Sep 02, 2026 at 04:00:48PM -0700, Bobby Eshleman wrote:
>> > From: Bobby Eshleman <bobbyeshleman@meta.com>
>> >
>> > Namespaces let a host isolate a VM's vsock traffic to a specific
>> > namespace, but in a guest vsock traffic cannot be isolated to a
>> > namespace. The vsock device is hardcoded to global mode and can't be
>> > moved into a local-mode namespace.
>> >
>> > Introduce ioctl IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS on /dev/vsock that
>>
>> We already discussed about netlink, but I'm not sure how much work can
>> take, here just another alternative, what about adding
>> /proc/sys/net/vsock/g2h_owner sysctl that can be read/write where:
>> 0 - no owner
>> 1 - owner
>>
>> 0 -> 1 transition, move the device in the new nets
>> 1 -> 0 transition, reset back to init_ns
>
>That seems reasonable, but if multi-device support ever lands, this
>might be harder to adapt cleanly?

I see, but maybe we can have a dev0/ dev1/ subfolders in that case.

>
>>
>> > gives userspace a way to move the device to the calling pid's namespace.
>> > The call requires CAP_NET_ADMIN in the root user namespace. A privileged
>> > user wishing to "unassign" the device can move it to the init_netns,
>> > which is hardcoded to global mode (so no unassign call is necessary).
>> >
>> > A getter to read the current assignment back was considered, returning
>> > either the namespace's net_cookie or its nsfs inode number, but neither
>> > seemed useful enough to bake into the uAPI now. It can be added later if
>> > a user turns up that needs it.
>> >
>> > Add a transport hook to indicate support for guest namespacing, so that
>> > transports may opt in/out. A transport that opts out keeps the
>> > reachability rules it had before this ioctl existed.
>> >
>> > Sockets are reset when the underlying device moves to a different
>> > namespace, so as to prevent reachability from the previous and now
>> > disallowed namespace.
>> >
>> > Following the approach of netdevs, the device returns to init_net when
>> > its namespace is removed. Care is taken to not break flows when the
>> > device is inside a global namespace that is being torn down and alive
>> > sockets are in a different global namespace. In this scenario, the
>> > device's netns getter pre-emptively falls back to the init_net (always
>> > global) so that these flows are not disrupted. If init_netns ever
>> > supports local-mode in the future, this logic will have to be changed.
>> >
>> > Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
>> > Link: https://lore.kernel.org/all/20200427142518.uwssa6dtasrp3bfc@steredhat/
>> > Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
>> > ---
>> > Documentation/admin-guide/sysctl/net.rst |  18 +++
>> > include/net/af_vsock.h                   |   7 ++
>> > include/uapi/linux/vm_sockets.h          |   6 +
>> > net/vmw_vsock/af_vsock.c                 | 198 ++++++++++++++++++++++++++++++-
>> > 4 files changed, 228 insertions(+), 1 deletion(-)
>> >

[...]

>> > +
>> > +static void vsock_reset_unreachable_sock(struct sock *sk)
>> > +{
>> > +	if (vsock_g2h_net_reachable(sock_net(sk)))
>> > +		return;
>> > +
>> > +	sk->sk_state = TCP_CLOSE;
>> > +	sk->sk_err = ECONNRESET;
>> > +	sk_error_report(sk);
>>
>> Should we send the reset to the other peer too?
>>
>> Or avoid to set TCP_CLOSE, so the user will see the error and close it?
>
>Sending the reset sounds good. At least the peer won't be waiting around
>forever. Will work that into v2.

If it's too much of work, we can skip it, it's not a strong opinion on 
my side. It was more a question of what we should do ;-)

>
>>
>> > +}
>> > +
>> > +/* Move the g2h device to @net. Returns -ENODEV if no g2h transport is loaded
>> > + * and -EOPNOTSUPP if the loaded one cannot be moved.
>> > + */
>> > +static int vsock_g2h_net_assign(struct net *net)
>> > +{
>> > +	int ret = 0;
>> > +
>> > +	mutex_lock(&vsock_register_mutex);
>> > +	if (!transport_g2h) {
>> > +		ret = -ENODEV;
>> > +	} else if (!vsock_g2h_netns_assignable()) {
>> > +		ret = -EOPNOTSUPP;
>> > +	} else {
>> > +		/* See vsock_maybe_set_connected() comment about synchronizing
>> > +		 * with connecting sockets.
>> > +		 */
>> > +		rcu_assign_pointer(vsock_g2h_net, net);
>> > +		vsock_for_each_connected_socket(transport_g2h,
>> > +						vsock_reset_unreachable_sock);
>> > +	}
>> > +	mutex_unlock(&vsock_register_mutex);
>> > +
>> > +	return ret;
>> > +}
>> > +
>> > +/* Move the g2h device back to init_net if it lives in @net, which is about to
>> > + * be destroyed.
>> > + */
>> > +static void vsock_g2h_net_reset(struct net *net)
>> > +{
>> > +	bool reset = false;
>> > +
>> > +	/* Avoid taking the mutex if the namespaces don't match. */
>> > +	if (likely(rcu_access_pointer(vsock_g2h_net) != net))
>> > +		return;
>> > +
>> > +	mutex_lock(&vsock_register_mutex);
>> > +	if (rcu_access_pointer(vsock_g2h_net) == net) {
>> > +		rcu_assign_pointer(vsock_g2h_net, &init_net);
>> > +		reset = true;
>> > +	}
>> > +	mutex_unlock(&vsock_register_mutex);
>> > +
>> > +	if (reset)
>> > +		synchronize_rcu();
>> > +}
>> > +
>> > static struct sock *vsock_dequeue_accept(struct sock *listener)
>> > {
>> > 	struct vsock_sock *vlistener;
>> > @@ -2745,6 +2920,15 @@ static long vsock_dev_do_ioctl(struct file *filp,
>> > 			retval = -EFAULT;
>> > 		break;
>> >
>> > +	case IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS:
>> > +		if (!capable(CAP_NET_ADMIN)) {
>> > +			retval = -EPERM;
>> > +			break;
>> > +		}
>> > +
>> > +		retval = vsock_g2h_net_assign(current->nsproxy->net_ns);
>> > +		break;
>> > +
>> > 	default:
>> > 		retval = -ENOIOCTLCMD;
>> > 	}
>> > @@ -2978,6 +3162,7 @@ static __net_init int vsock_sysctl_init_net(struct net *net)
>> >
>> > static __net_exit void vsock_sysctl_exit_net(struct net *net)
>> > {
>> > +	vsock_g2h_net_reset(net);
>>
>> Why calling this in the sysctl_exit ?
>
>Should we add another pernet_operations? I agree the name of
>vsock_sysctl_ops does not really fit as a general "do all per-net
>operations here" location.

Aaaaa, it's not really related to sysctl.

Maybe we can rename it or add another pernet_operations.

>
>>
>> > 	vsock_sysctl_unregister(net);
>> > }
>> >
>> > @@ -3104,13 +3289,21 @@ EXPORT_SYMBOL_GPL(vsock_core_register);
>> >
>> > void vsock_core_unregister(const struct vsock_transport *t)
>> > {
>> > +	bool g2h_net_reset = false;
>> > +
>> > 	mutex_lock(&vsock_register_mutex);
>> >
>> > 	if (transport_h2g == t)
>> > 		transport_h2g = NULL;
>> >
>> > -	if (transport_g2h == t)
>> > +	if (transport_g2h == t) {
>> > 		transport_g2h = NULL;
>> > +		/* The device is gone, so is its namespace assignment. */
>> > +		if (rcu_access_pointer(vsock_g2h_net) != &init_net) {
>> > +			rcu_assign_pointer(vsock_g2h_net, &init_net);
>> > +			g2h_net_reset = true;
>> > +		}
>> > +	}
>> >
>> > 	if (transport_dgram == t)
>> > 		transport_dgram = NULL;
>> > @@ -3119,6 +3312,9 @@ void vsock_core_unregister(const struct vsock_transport *t)
>> > 		transport_local = NULL;
>> >
>> > 	mutex_unlock(&vsock_register_mutex);
>> > +
>> > +	if (g2h_net_reset)
>> > +		synchronize_rcu();
>>
>> Why we need this? (I'd add also a comment with the reason)
>
>We might actually be able to drop it... I do think we need to keep it in
>vsock_g2h_net_reset() because it makes sure that vsock_g2h_net_get()
>doesn't dereference a destroyed net.
>
>vsock_core_unregister() isn't on the teardown path, so even if we
>re-assign to init_net the previous net is still alive.

Agree.

>
>
>One thing I realized when answering your uAPI questions and looking at
>the code is that IOCTL_VM_SOCKETS_GET_LOCAL_CID is not namespace aware,
>but I think it probably should be? I remember for host ns we strived for
>truly strong isolation.

Can you elabore a bit more?

Thanks,
Stefano