[PATCH v2 2/2] vsock/vmci: enforce per-netns mode on the datagram receive path

["Bartłomiej Dmitruk" <bartlomiej.dmitruk@isec.pl>]

The send hook vmci_transport_dgram_allow() refuses datagrams when the socket's
netns is not in global mode (vsock_net_mode_global()), but the receive path
(vmci_transport_recv_dgram_cb() -> vmci_transport_allow_dgram()) did not, so a
socket bound in a non-global (local) netns received datagrams from peers it
could never send to, defeating namespace isolation.

Add the check at the start of the receive decision, before the
VMADDR_CID_HYPERVISOR short-circuit, so the hypervisor CID is covered too
(matching the send side).

This was found by code inspection; I do not have VMCI hardware to test on
(compile-tested only).

Fixes: eafb64f40ca4 ("vsock: add netns to vsock core")
Signed-off-by: Bartłomiej Dmitruk <bartlomiej.dmitruk@isec.pl>
Assisted-by: Claude (Anthropic)
---
v2: perform the netns check first (before the hypervisor early return) so
    hypervisor datagrams are also gated in local mode -- the Sashiko AI review
    noted v1 left that path partially bypassed.
v1: https://lore.kernel.org/netdev/20260917220225.56200-1-bartlomiej.dmitruk@isec.pl/

diff --git a/net/vmw_vsock/vmci_transport.c b/net/vmw_vsock/vmci_transport.c
--- a/net/vmw_vsock/vmci_transport.c
+++ b/net/vmw_vsock/vmci_transport.c
@@ -532,6 +532,14 @@
 static bool vmci_transport_allow_dgram(struct vsock_sock *vsock, u32 peer_cid)
 {
 	u64 access;
+
+	/* Enforce the per-netns mode first, symmetrically with the send hook
+	 * vmci_transport_dgram_allow(): a socket in a non-global (local) netns
+	 * must not receive datagrams it could never send (this also covers the
+	 * hypervisor CID).
+	 */
+	if (!vsock_net_mode_global(vsock))
+		return false;
 
 	if (VMADDR_CID_HYPERVISOR == peer_cid)
 		return true;