grpc: make the mTLS private keys user-readable only

Fixes ElementsProject#6064
Reported-by: denis2342 <@denis2342>

Changelog-Changed: grpc: The mTLS private keys are no longer group-readable

plugins/grpc-plugin/src/tls.rs

@@ -59,6 +59,8 @@ fn generate_or_load_identity(
     filename: &str,
     parent: Option<&Identity>,
 ) -> Result<Identity> {
+    use std::io::Write;
+    use std::os::unix::fs::PermissionsExt;
     // Just our naming convention here.
     let cert_path = directory.join(format!("{}.pem", filename));
     let key_path = directory.join(format!("{}-key.pem", filename));
@@ -70,7 +72,18 @@ fn generate_or_load_identity(
             &key_path
         );
         let keypair = KeyPair::generate(&rcgen::PKCS_ECDSA_P256_SHA256)?;
-        std::fs::write(&key_path, keypair.serialize_pem())?;
+
+        // Create the file, but make it user-readable only:
+        let mut file = std::fs::File::create(&key_path)?;
+        let mut perms = std::fs::metadata(&key_path)?.permissions();
+        perms.set_mode(0o600);
+        std::fs::set_permissions(&key_path, perms)?;
+
+	// Only after changing the permissions we can write the
+	// private key
+        file.write_all(keypair.serialize_pem().as_bytes())?;
+        drop(file);
+
         debug!(
             "Generating a new certificate for key {:?} at {:?}",
             &key_path, &cert_path

tests/test_cln_rs.py

@@ -181,6 +181,11 @@ def test_grpc_generate_certificate(node_factory):
     assert contents[-2] != files[-2].open().read()
     assert contents[-1] != files[-1].open().read()
 
+    keys = [f for f in files if f.name.endswith('-key.pem')]
+    modes = [f.stat().st_mode for f in keys]
+    private = [m % 8 == 0 and (m // 8) % 8 == 0 for m in modes]
+    assert all(private)
+
 
 def test_grpc_no_auto_start(node_factory):
     """Ensure that we do not start cln-grpc unless a port is configured.