Subject: Re: [PATCH v2] vsock: add G2H fallback for CIDs not owned by H2G transport

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 3 Mar 2026 06:59:15 -0500

On Mon, Mar 02, 2026 at 07:49:26PM +0000, Alexander Graf wrote:
> Vsock maintains a single CID number space which can be used to
> communicate to the host (G2H) or to a child-VM (H2G). The current logic
> assumes that G2H is only relevant for CID <= 2 because these target the
> hypervisor. However, in environments like Nitro Enclaves, an instance
> that hosts vhost_vsock powered VMs may still want to communicate to
> Enclaves that are reachable at higher CIDs through virtio-vsock-pci.
> 
> Vsock introduced VMADDR_FLAG_TO_HOST to allow user space applications
> to clearly express a desire to talk to the host instead of a guest via
> the passed target CID. However, users may not actually know which one
> they want to talk to and the application ecosystem has not picked up a
> way for users to specify that desire.
> 
> Instead, make it easy for users and introduce a G2H fallback mechanism:
> when user space attempts to connect to a CID and the H2G transport
> (vhost-vsock / VMCI) does not own it, automatically route the connection
> through the G2H transport. This provides a single unified CID address
> space where vhost-registered CIDs go to nested VMs and all other CIDs
> are routed to the hypervisor.

So as usual we have security/convenience tradeoff, if someone wanted
to block this, we might be opening a security hole.
I am not saying it's a big issue but I feel the security implications
should at least be documented better.

> To give user space at least a hint that the kernel applied this logic,
> automatically set the VMADDR_FLAG_TO_HOST on the remote address so it
> can determine the path taken via getpeername().
> 
> To force the system back into old behavior, provide a sysctl
> (net.vsock.g2h_fallback, defaults to 1).

Given it affects routing, this sysctl should be per netns?

> 
> Signed-off-by: Alexander Graf <[email-masked]>
> 
> ---
> 
> v1 -> v2:
> 
>   - Rebase on 7.0, include namespace support
>   - Add net.vsock.g2h_fallback sysctl
>   - Rework description
>   - Set VMADDR_FLAG_TO_HOST automatically
>   - Add VMCI support
>   - Update vsock_assign_transport() comment
> ---
>  Documentation/admin-guide/sysctl/net.rst | 22 ++++++++++++++++++++++
>  drivers/misc/vmw_vmci/vmci_context.c     |  1 +
>  drivers/vhost/vsock.c                    | 13 +++++++++++++
>  include/linux/vmw_vmci_api.h             |  1 +
>  include/net/af_vsock.h                   |  3 +++
>  net/vmw_vsock/af_vsock.c                 | 20 +++++++++++++++++++-
>  net/vmw_vsock/vmci_transport.c           |  6 ++++++
>  7 files changed, 65 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/sysctl/net.rst b/Documentation/admin-guide/sysctl/net.rst
> index 3b2ad61995d4..cc364baa9021 100644
> --- a/Documentation/admin-guide/sysctl/net.rst
> +++ b/Documentation/admin-guide/sysctl/net.rst
> @@ -602,3 +602,25 @@ it does not modify the current namespace or any existing children.
>  
>  A namespace with ``ns_mode`` set to ``local`` cannot change
>  ``child_ns_mode`` to ``global`` (returns ``-EPERM``).
> +
> +g2h_fallback
> +------------
> +
> +Controls whether connections to CIDs not owned by the host-to-guest (H2G)
> +transport automatically fall back to the guest-to-host (G2H) transport.
> +
> +When enabled, if a connect targets a CID that the H2G transport (e.g.
> +vhost-vsock) does not serve, the connection is routed via the G2H transport
> +(e.g. virtio-vsock) instead. This allows a host running both nested VMs
> +(via vhost-vsock) and sibling VMs reachable through the hypervisor (e.g.
> +Nitro Enclaves) to address both using a single CID space, without requiring
> +applications to set ``VMADDR_FLAG_TO_HOST``.
> +
> +When the fallback is taken, ``VMADDR_FLAG_TO_HOST`` is automatically set on
> +the remote address so that userspace can determine the path via
> +``getpeername()``.
> +
> +Values:
> +
> +	- 0 - Connections to CIDs < 3 get handled by G2H, others by H2G.
> +	- 1 - Connections to CIDs not owned by H2G fall back to G2H. (default)
> diff --git a/drivers/misc/vmw_vmci/vmci_context.c b/drivers/misc/vmw_vmci/vmci_context.c
> index 19ca00feed6e..577296784df5 100644
> --- a/drivers/misc/vmw_vmci/vmci_context.c
> +++ b/drivers/misc/vmw_vmci/vmci_context.c
> @@ -364,6 +364,7 @@ bool vmci_ctx_exists(u32 cid)
>  	rcu_read_unlock();
>  	return exists;
>  }
> +EXPORT_SYMBOL_GPL(vmci_ctx_exists);
>  
>  /*
>   * Retrieves VMCI context corresponding to the given cid.
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index 054f7a718f50..319e3a690108 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -91,6 +91,18 @@ static struct vhost_vsock *vhost_vsock_get(u32 guest_cid, struct net *net)
>  	return NULL;
>  }
>  
> +static bool vhost_transport_has_remote_cid(struct vsock_sock *vsk, u32 cid)
> +{
> +	struct sock *sk = sk_vsock(vsk);
> +	struct net *net = sock_net(sk);
> +	bool found;
> +
> +	rcu_read_lock();
> +	found = vhost_vsock_get(cid, net) != NULL;
> +	rcu_read_unlock();
> +	return found;
> +}
> +
>  static void
>  vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
>  			    struct vhost_virtqueue *vq)
> @@ -424,6 +436,7 @@ static struct virtio_transport vhost_transport = {
>  		.module                   = THIS_MODULE,
>  
>  		.get_local_cid            = vhost_transport_get_local_cid,
> +		.has_remote_cid           = vhost_transport_has_remote_cid,
>  
>  		.init                     = virtio_transport_do_socket_init,
>  		.destruct                 = virtio_transport_destruct,
> diff --git a/include/linux/vmw_vmci_api.h b/include/linux/vmw_vmci_api.h
> index 41764a684423..c412d17c572f 100644
> --- a/include/linux/vmw_vmci_api.h
> +++ b/include/linux/vmw_vmci_api.h
> @@ -37,6 +37,7 @@ int vmci_doorbell_create(struct vmci_handle *handle, u32 flags,
>  int vmci_doorbell_destroy(struct vmci_handle handle);
>  u32 vmci_get_context_id(void);
>  bool vmci_is_context_owner(u32 context_id, kuid_t uid);
> +bool vmci_ctx_exists(u32 cid);
>  int vmci_register_vsock_callback(vmci_vsock_cb callback);
>  
>  int vmci_event_subscribe(u32 event,
> diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
> index 533d8e75f7bb..0aeb25642827 100644
> --- a/include/net/af_vsock.h
> +++ b/include/net/af_vsock.h
> @@ -179,6 +179,9 @@ struct vsock_transport {
>  	/* Addressing. */
>  	u32 (*get_local_cid)(void);
>  
> +	/* Check if this transport serves a specific remote CID. */
> +	bool (*has_remote_cid)(struct vsock_sock *vsk, u32 remote_cid);
> +
>  	/* Read a single skb */
>  	int (*read_skb)(struct vsock_sock *, skb_read_actor_t);
>  
> diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
> index 2f7d94d682cb..b41bc734d6c0 100644
> --- a/net/vmw_vsock/af_vsock.c
> +++ b/net/vmw_vsock/af_vsock.c
> @@ -210,6 +210,8 @@ static const struct vsock_transport *transport_dgram;
>  static const struct vsock_transport *transport_local;
>  static DEFINE_MUTEX(vsock_register_mutex);
>  
> +static int vsock_g2h_fallback = 1;
> +
>  /**** UTILS ****/
>  
>  /* Each bound VSocket is stored in the bind hash table and each connected
> @@ -547,7 +549,8 @@ static void vsock_deassign_transport(struct vsock_sock *vsk)
>   *    g2h is not loaded, will use local transport;
>   *  - remote CID <= VMADDR_CID_HOST or h2g is not loaded or remote flags field
>   *    includes VMADDR_FLAG_TO_HOST flag value, will use guest->host transport;
> - *  - remote CID > VMADDR_CID_HOST will use host->guest transport;
> + *  - remote CID > VMADDR_CID_HOST will use host->guest transport if h2g has
> + *    registered that CID, otherwise will use guest->host transport (overlay);
>   */
>  int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
>  {
> @@ -584,6 +587,12 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
>  		else if (remote_cid <= VMADDR_CID_HOST || !transport_h2g ||
>  			 (remote_flags & VMADDR_FLAG_TO_HOST))
>  			new_transport = transport_g2h;
> +		else if (vsock_g2h_fallback &&
> +			 transport_h2g->has_remote_cid &&
> +			 !transport_h2g->has_remote_cid(vsk, remote_cid)) {
> +			vsk->remote_addr.svm_flags |= VMADDR_FLAG_TO_HOST;
> +			new_transport = transport_g2h;
> +		}
>  		else
>  			new_transport = transport_h2g;
>  		break;
> @@ -2879,6 +2888,15 @@ static struct ctl_table vsock_table[] = {
>  		.mode		= 0644,
>  		.proc_handler	= vsock_net_child_mode_string
>  	},
> +	{
> +		.procname	= "g2h_fallback",
> +		.data		= &vsock_g2h_fallback,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= SYSCTL_ZERO,
> +		.extra2		= SYSCTL_ONE,
> +	},
>  };
>  
>  static int __net_init vsock_sysctl_register(struct net *net)
> diff --git a/net/vmw_vsock/vmci_transport.c b/net/vmw_vsock/vmci_transport.c
> index 4296ca1183f1..de3dff52c566 100644
> --- a/net/vmw_vsock/vmci_transport.c
> +++ b/net/vmw_vsock/vmci_transport.c
> @@ -2045,6 +2045,11 @@ static u32 vmci_transport_get_local_cid(void)
>  	return vmci_get_context_id();
>  }
>  
> +static bool vmci_transport_has_remote_cid(struct vsock_sock *vsk, u32 cid)
> +{
> +	return vmci_ctx_exists(cid);
> +}
> +
>  static struct vsock_transport vmci_transport = {
>  	.module = THIS_MODULE,
>  	.init = vmci_transport_socket_init,
> @@ -2074,6 +2079,7 @@ static struct vsock_transport vmci_transport = {
>  	.notify_send_post_enqueue = vmci_transport_notify_send_post_enqueue,
>  	.shutdown = vmci_transport_shutdown,
>  	.get_local_cid = vmci_transport_get_local_cid,
> +	.has_remote_cid = vmci_transport_has_remote_cid,
>  };
>  
>  static bool vmci_check_transport(struct vsock_sock *vsk)
> -- 
> 2.47.1
> 
> 
> 
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