Refuse prepared-lower child mount crossings

crates/lazaret-prepared-lower/tests/prepared_lower.rs

@@ -22,6 +22,8 @@ use sha2::{Digest, Sha256};
 use tar::{Builder, EntryType, Header};
 use tempfile::TempDir;
 
+const MOUNT_CHILD_ENV: &str = "LAZARET_PREPARED_LOWER_MOUNT_CHILD";
+
 const OCI_TAR: &str = "application/vnd.oci.image.layer.v1.tar";
 const OCI_GZIP: &str = "application/vnd.oci.image.layer.v1.tar+gzip";
 const OCI_ZSTD: &str = "application/vnd.oci.image.layer.v1.tar+zstd";
@@ -83,6 +85,64 @@ fn preexisting_fixed_children_require_exact_private_mode() {
     }
 }
 
+#[test]
+#[allow(clippy::disallowed_methods, clippy::disallowed_types)]
+fn retained_fixed_child_bind_mount_is_refused_when_mount_setup_is_available() {
+    if std::env::var_os(MOUNT_CHILD_ENV).is_some() {
+        let directory = TempDir::new().unwrap();
+        let root = directory.path().join("storage");
+        let source = directory.path().join("source");
+        std::fs::create_dir(&root).unwrap();
+        std::fs::create_dir(&source).unwrap();
+        let catalog = Catalog::new(root.join("catalog.redb"), root.join("storage-catalog.lock"));
+        catalog.initialize().unwrap();
+        PreparedLowerStore::initialize(&root, catalog.clone()).unwrap();
+        let target = root.join("artifacts");
+        if let Err(error) = rustix::mount::mount_bind(&source, &target) {
+            eprintln!(
+                "skipping physical mount-crossing assertion: bind mount unavailable: {error}"
+            );
+            return;
+        }
+        let result = PreparedLowerStore::initialize(&root, catalog);
+        rustix::mount::unmount(&target, rustix::mount::UnmountFlags::DETACH).unwrap();
+        assert!(result.is_err());
+        return;
+    }
+
+    let output = std::process::Command::new("unshare")
+        .args([
+            "--user",
+            "--map-root-user",
+            "--mount",
+            "--propagation",
+            "private",
+            "--",
+        ])
+        .arg(std::env::current_exe().unwrap())
+        .args([
+            "--exact",
+            "retained_fixed_child_bind_mount_is_refused_when_mount_setup_is_available",
+        ])
+        .env(MOUNT_CHILD_ENV, "1")
+        .output();
+    let output = match output {
+        Ok(output) => output,
+        Err(error) => {
+            eprintln!("skipping physical mount-crossing assertion: unshare unavailable: {error}");
+            return;
+        }
+    };
+    if !output.status.success() {
+        let stderr = String::from_utf8_lossy(&output.stderr);
+        if stderr.contains("Operation not permitted") {
+            eprintln!("skipping physical mount-crossing assertion: {stderr}");
+            return;
+        }
+        panic!("mount-crossing child failed: {stderr}");
+    }
+}
+
 #[derive(Clone)]
 struct Entry<'a> {
     path: &'a str,