Drivers: hv: vmbus: Enforce 'VMBus version >= 5.2' on isolated guests

Restrict the protocol version(s) that will be negotiated with the host
to be 5.2 or greater if the guest is running isolated.  This reduces the
footprint of the code that will be exercised by Confidential VMs and
hence the exposure to bugs and vulnerabilities.
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20210201144814.2701-4-parri.andrea@gmail.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

drivers/hv/connection.c

@@ -244,6 +244,13 @@ int vmbus_connect(void)
 			break;
 	}
 
+	if (hv_is_isolation_supported() && version < VERSION_WIN10_V5_2) {
+		pr_err("Invalid VMBus version %d.%d (expected >= %d.%d) from the host supporting isolation\n",
+		       version >> 16, version & 0xFFFF, VERSION_WIN10_V5_2 >> 16, VERSION_WIN10_V5_2 & 0xFFFF);
+		ret = -EINVAL;
+		goto cleanup;
+	}
+
 	vmbus_proto_version = version;
 	pr_info("Vmbus version:%d.%d\n",
 		version >> 16, version & 0xFFFF);