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 protected]> > > --- > > 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 > > > > > Amazon Web Services Development Center Germany GmbH > Tamara-Danz-Str. 13 > 10243 Berlin > Geschaeftsfuehrung: Christof Hellmis, Andreas Stieger > Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B > Sitz: Berlin > Ust-ID: DE 365 538 597
