diff --git a/virtualizationservice/src/composite/config.rs b/virtualizationservice/src/composite/config.rs
new file mode 100644
index 0000000..1a915ba
--- /dev/null
+++ b/virtualizationservice/src/composite/config.rs
@@ -0,0 +1,44 @@
+// Copyright 2021, The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! JSON configuration for running `mk_cdisk`.
+
+use anyhow::{Context, Error};
+use serde::{Deserialize, Serialize};
+use std::io::Write;
+use std::path::PathBuf;
+
+#[derive(Clone, Debug, Default, Deserialize, Eq, PartialEq, Serialize)]
+pub struct Config {
+    /// The set of partitions to be assembled into a composite image.
+    pub partitions: Vec<Partition>,
+}
+
+/// A partition to be assembled into a composite image.
+#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
+pub struct Partition {
+    /// A label for the partition.
+    pub label: String,
+    /// The filename of the partition image.
+    pub path: PathBuf,
+    /// Whether the partition should be writable.
+    #[serde(default)]
+    pub writable: bool,
+}
+
+impl Config {
+    pub fn write_json(&self, writer: impl Write) -> Result<(), Error> {
+        serde_json::to_writer(writer, self).context("Failed to write config JSON for mk_cdisk")
+    }
+}