feat: automatic TLS certificate reload

When running on Linux, monitor changes done to the TLS certificate and
key used by the https server.

When both change, react by updating the TLS configuration.

This allows to rotate the TLS certificate used by a Policy Server
without having to restart the process.

Signed-off-by: Flavio Castelli <fcastelli@suse.com>

Cargo.toml

@@ -55,6 +55,10 @@ jemalloc_pprof = "0.4.1"
 tikv-jemalloc-ctl = "0.5.4"
 rhai = { version = "1.19.0", features = ["sync"] }
 
+[target.'cfg(target_os = "linux")'.dependencies]
+inotify = "0.10"
+tokio-stream = "0.1.15"
+
 [dev-dependencies]
 mockall = "0.12"
 rstest = "0.21"

src/lib.rs

@@ -39,14 +39,18 @@ use tokio::{
 };
 use tower_http::trace::{self, TraceLayer};
 
+// This is required by certificate hot reload when using inotify, which is available only on linux
+#[cfg(target_os = "linux")]
+use tokio_stream::StreamExt;
+
 use crate::api::handlers::{
     audit_handler, pprof_get_cpu, pprof_get_heap, readiness_handler, validate_handler,
     validate_raw_handler,
 };
 use crate::api::state::ApiServerState;
 use crate::evaluation::precompiled_policy::{PrecompiledPolicies, PrecompiledPolicy};
 use crate::policy_downloader::{Downloader, FetchedPolicies};
-use config::Config;
+use config::{Config, TlsConfig};
 
 use tikv_jemallocator::Jemalloc;
 
@@ -193,9 +197,7 @@ impl PolicyServer {
         });
 
         let tls_config = if let Some(tls_config) = config.tls_config {
-            let rustls_config =
-                RustlsConfig::from_pem_file(tls_config.cert_file, tls_config.key_file).await?;
-            Some(rustls_config)
+            Some(create_tls_config_and_watch_certificate_changes(tls_config).await?)
         } else {
             None
         };
@@ -269,6 +271,80 @@ impl PolicyServer {
     }
 }
 
+/// There's no watching of the certificate files on non-linux platforms
+/// since we rely on inotify to watch for changes
+#[cfg(not(target_os = "linux"))]
+async fn create_tls_config_and_watch_certificate_changes(
+    tls_config: TlsConfig,
+) -> Result<RustlsConfig> {
+    let cfg = RustlsConfig::from_pem_file(tls_config.cert_file, tls_config.key_file).await?;
+    Ok(cfg)
+}
+
+/// Return the RustlsConfig and watch for changes in the certificate files
+/// using inotify.
+/// When a both the certificate and its key are changed, the RustlsConfig is reloaded,
+/// causing the https server to use the new certificate.
+///
+/// Relying on inotify is only available on linux
+#[cfg(target_os = "linux")]
+async fn create_tls_config_and_watch_certificate_changes(
+    tls_config: TlsConfig,
+) -> Result<RustlsConfig> {
+    let cert_file = tls_config.cert_file.clone();
+    let key_file = tls_config.key_file.clone();
+
+    let rust_config =
+        RustlsConfig::from_pem_file(tls_config.cert_file, tls_config.key_file).await?;
+    let reloadable_rust_config = rust_config.clone();
+
+    let inotify =
+        inotify::Inotify::init().map_err(|e| anyhow!("Cannot initialize inotify: {e}"))?;
+    let cert_watch = inotify
+        .watches()
+        .add(cert_file.clone(), inotify::WatchMask::MODIFY)
+        .map_err(|e| anyhow!("Cannot watch certificate file: {e}"))?;
+    let key_watch = inotify
+        .watches()
+        .add(key_file.clone(), inotify::WatchMask::MODIFY)
+        .map_err(|e| anyhow!("Cannot watch key file: {e}"))?;
+
+    let buffer = [0; 1024];
+    let mut stream = inotify
+        .into_event_stream(buffer)
+        .map_err(|e| anyhow!("Cannot create inotify event stream: {e}"))?;
+
+    tokio::spawn(async move {
+        let mut cert_changed = false;
+        let mut key_changed = false;
+
+        while let Some(event) = stream.next().await {
+            let event = event.unwrap();
+            if event.wd == cert_watch {
+                info!("TLS certificate file has been modified");
+                cert_changed = true;
+            }
+            if event.wd == key_watch {
+                info!("TLS key file has been modified");
+                key_changed = true;
+            }
+
+            if key_changed && cert_changed {
+                info!("reloading TLS certificate");
+
+                cert_changed = false;
+                key_changed = false;
+                reloadable_rust_config
+                    .reload_from_pem_file(cert_file.clone(), key_file.clone())
+                    .await
+                    .expect("Cannot reload TLS certificate"); //  we want to panic here
+            }
+        }
+    });
+
+    Ok(rust_config)
+}
+
 fn precompile_policies(
     engine: &wasmtime::Engine,
     fetched_policies: &FetchedPolicies,