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

[Stefano Garzarella <sgarzare@redhat.com>]

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

>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(-)
>
>diff --git a/Documentation/admin-guide/sysctl/net.rst b/Documentation/admin-guide/sysctl/net.rst
>index e586e17fc7a5..1e9c0d2be7b8 100644
>--- a/Documentation/admin-guide/sysctl/net.rst
>+++ b/Documentation/admin-guide/sysctl/net.rst
>@@ -515,6 +515,24 @@ their hosts. The behavior of VSOCK sockets in a network namespace is determined
> by the namespace's mode (``global`` or ``local``), which controls how CIDs
> (Context IDs) are allocated and how sockets interact across namespaces.
>
>+In a guest, the vsock device owned by the guest-to-host (G2H) transport belongs
>+to one network namespace at a time. The ``IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS``
>+ioctl on ``/dev/vsock`` moves it to the namespace of the calling process, which
>+requires ``CAP_NET_ADMIN`` in the initial user namespace. The namespace's mode
>+decides who may then use the device:
>+
>+- ``global`` - every ``global`` mode namespace may use it.
>+- ``local`` - only that namespace may use it, which reserves the connection to
>+  the host for it alone.
>+
>+The device starts out in the initial namespace, so until the ioctl is issued
>+nothing has moved and no mode has changed.
>+
>+Connections made before the move, from a namespace that can no longer reach the
>+device, are reset. The device returns to the initial namespace when the
>+namespace it was moved to is deleted, so assigning it to the initial namespace
>+is how an assignment is undone.
>+
> ns_mode
> -------
>
>diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
>index 87fdec60ba45..64c4b205a11b 100644
>--- a/include/net/af_vsock.h
>+++ b/include/net/af_vsock.h
>@@ -190,6 +190,9 @@ struct vsock_transport {
>
> 	/* Zero-copy. */
> 	bool (*msgzerocopy_allow)(void);
>+
>+	/* True if the transport honours IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS. */

"if the G2H transport ..."

>+	bool (*netns_assign_allow)(void);

What about having just a bool (and maybe we should do the same for
msgzerocopy_allow())?

> };
>
> /**** CORE ****/
>@@ -235,6 +238,10 @@ void vsock_for_each_connected_socket(const struct vsock_transport *transport,
> int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk);
> bool vsock_find_cid(unsigned int cid);
> void vsock_linger(struct sock *sk);
>+struct net *vsock_g2h_net_get(void);
>+bool vsock_g2h_net_reachable(struct net *net);
>+bool vsock_g2h_reachable_sk(struct vsock_sock *vsk);
>+bool vsock_maybe_set_connected(struct vsock_sock *vsk);
>
> /**** TAP ****/
>
>diff --git a/include/uapi/linux/vm_sockets.h b/include/uapi/linux/vm_sockets.h
>index e05280e41522..894b0d65b458 100644
>--- a/include/uapi/linux/vm_sockets.h
>+++ b/include/uapi/linux/vm_sockets.h
>@@ -195,6 +195,12 @@ struct sockaddr_vm {
>
> #define IOCTL_VM_SOCKETS_GET_LOCAL_CID		_IO(7, 0xb9)
>
>+/* Assign the guest's vsock device to the network namespace of the calling
>+ * process. Requires CAP_NET_ADMIN in the initial user namespace. To undo an
>+ * assignment, assign the device to the initial network namespace.
>+ */
>+#define IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS	_IO(7, 0xba)
>+
> /* MSG_ZEROCOPY notifications are encoded in the standard error format,
>  * sock_extended_err. See Documentation/networking/msg_zerocopy.rst in
>  * kernel source tree for more details.
>diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
>index 29cde17e08f3..ad11f0f56eb8 100644
>--- a/net/vmw_vsock/af_vsock.c
>+++ b/net/vmw_vsock/af_vsock.c
>@@ -130,6 +130,24 @@
>  *       a different transport that *does* support local mode. For
>  *       example, virtio-vsock may not support local mode, but the socket
>  *       may still accept a connection from vhost-vsock which does.
>+ *
>+ * - A guest has a single vsock device, owned by the guest->host transport.
>+ *   IOCTL_VM_SOCKETS_ASSIGN_G2H_NETNS on /dev/vsock assigns it to the
>+ *   namespace of the caller. It starts out in init_net. The mode rules then
>+ *   decide who may use it, and which namespace packets from the host are
>+ *   delivered to:
>+ *
>+ *   - assigned to a global mode namespace - every global mode namespace may
>+ *     use it. Until the ioctl is issued nothing has moved and no mode has
>+ *     changed, so the default is the behaviour that predates it.
>+ *   - assigned to a local mode namespace - only that namespace may use it.
>+ *     This is how a nested VM is isolated from the rest of the guest.
>+ *
>+ *   Connections made before an assignment, from a namespace that can no
>+ *   longer reach the device, are reset.
>+ *
>+ *   No reference is taken on the assigned namespace. As is done for netdevs,
>+ *   the device is moved back to init_net when that namespace is destroyed.
>  */
>
> #include <linux/compat.h>
>@@ -208,6 +226,11 @@ static const struct vsock_transport *transport_dgram;
> static const struct vsock_transport *transport_local;
> static DEFINE_MUTEX(vsock_register_mutex);
>
>+/* Network namespace of the g2h device. Protected by
>+ * vsock_register_mutex/RCU.
>+ */
>+static struct net __rcu *vsock_g2h_net = RCU_INITIALIZER(&init_net);
>+
> /**** UTILS ****/
>
> /* Each bound VSocket is stored in the bind hash table and each connected
>@@ -548,6 +571,17 @@ static void vsock_deassign_transport(struct vsock_sock *vsk)
> 	vsk->transport = NULL;
> }
>
>+/* Return true if the loaded g2h transport honours namespace assignment. One
>+ * that does not keeps the reachability rules it had before the ioctl existed.
>+ *
>+ * Must be called with vsock_register_mutex held.
>+ */
>+static bool vsock_g2h_netns_assignable(void)
>+{
>+	return transport_g2h && transport_g2h->netns_assign_allow &&
>+	       transport_g2h->netns_assign_allow();
>+}
>+
> /* Assign a transport to a socket and call the .init transport callback.
>  *
>  * Note: for connection oriented socket this must be called when vsk->remote_addr
>@@ -622,6 +656,13 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
> 		goto err;
> 	}
>
>+	if (new_transport && new_transport == transport_g2h &&
>+	    vsock_g2h_netns_assignable() &&
>+	    !vsock_g2h_net_reachable(sock_net(sk))) {
>+		ret = -ENETUNREACH;
>+		goto err;
>+	}
>+
> 	/* We increase the module refcnt to prevent the transport unloading
> 	 * while there are open sockets assigned to it.
> 	 */
>@@ -710,6 +751,140 @@ bool vsock_find_cid(unsigned int cid)
> }
> EXPORT_SYMBOL_GPL(vsock_find_cid);
>
>+/* Return the g2h devices' namespace with a reference held, or NULL if that
>+ * namespace is being destroyed.
>+ */
>+struct net *vsock_g2h_net_get(void)
>+{
>+	struct net *assigned;
>+	struct net *net;
>+
>+	rcu_read_lock();
>+	assigned = rcu_dereference(vsock_g2h_net);
>+	net = maybe_get_net(assigned);
>+
>+	/* !net means the net is about to be destroyed, at which point the g2h
>+	 * device will move to the init_net.  If the init_net and the dying net
>+	 * are both global mode, we use the init_net as a fallback to avoid
>+	 * disrupting global-mode flows. The per-net destructor hook will
>+	 * eventually move the g2h device to the init_net anyway.
>+	 */
>+	if (!net && vsock_net_check_mode(&init_net, assigned))
>+		net = get_net(&init_net);
>+	rcu_read_unlock();
>+
>+	return net;
>+}
>+EXPORT_SYMBOL_GPL(vsock_g2h_net_get);
>+
>+bool vsock_g2h_net_reachable(struct net *net)
>+{
>+	bool reachable;
>+
>+	rcu_read_lock();
>+	reachable = vsock_net_check_mode(net, rcu_dereference(vsock_g2h_net));
>+	rcu_read_unlock();
>+
>+	return reachable;
>+}
>+EXPORT_SYMBOL_GPL(vsock_g2h_net_reachable);
>+
>+bool vsock_g2h_reachable_sk(struct vsock_sock *vsk)
>+{
>+	const struct vsock_transport *t = vsk->transport;
>+
>+	if (!t || !t->netns_assign_allow || !t->netns_assign_allow())
>+		return true;
>+
>+	return vsock_g2h_net_reachable(sock_net(sk_vsock(vsk)));
>+}
>+EXPORT_SYMBOL_GPL(vsock_g2h_reachable_sk);

This seems used only in af_vsock.c

>+
>+/* Move @vsk to TCP_ESTABLISHED and into the connected table, unless the device
>+ * has moved to a namespace @vsk cannot reach. Returns false without doing
>+ * either in that case.
>+ *
>+ * vsock_g2h_net_assign() resets the sockets it finds in the same table under
>+ * the same lock. Either vsock_g2h_net_assign() sees the vsk in the table and
>+ * resets it, or it does not see the @vsk in the table and this function
>+ * refuses to add it. This avoids netns assignment racing with outstanding
>+ * connection responses and incoming connection requests.
>+ */
>+bool vsock_maybe_set_connected(struct vsock_sock *vsk)
>+{
>+	struct list_head *list = vsock_connected_sockets(&vsk->remote_addr,
>+							 &vsk->local_addr);
>+	bool reachable;
>+
>+	spin_lock_bh(&vsock_table_lock);
>+	reachable = vsock_g2h_reachable_sk(vsk);
>+	if (reachable) {
>+		sk_vsock(vsk)->sk_state = TCP_ESTABLISHED;
>+		__vsock_insert_connected(list, vsk);
>+	}
>+	spin_unlock_bh(&vsock_table_lock);
>+
>+	return reachable;
>+}
>+EXPORT_SYMBOL_GPL(vsock_maybe_set_connected);
>+
>+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?

>+}
>+
>+/* 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 ?

> 	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)

> }
> EXPORT_SYMBOL_GPL(vsock_core_unregister);
>
>
>-- 
>2.53.0-Meta
>