bestool 2.2.2

//! The certificate endpoint Caddy asks during a handshake.
//!
//! Caddy's `get_certificate http` manager issues an ordinary GET carrying the
//! name from the client's server name indication, the signature schemes and
//! cipher suites the client offered, and the local address it connected to. A
//! `200` carrying the chain and its key as PEM is served; a `204` declines and
//! hands the handshake back to Caddy, which issues for the name itself.
//!
//! Anything else ends the handshake: Caddy reads an error as the manager having
//! been unable to serve a certificate it was responsible for, and does not fall
//! back to its own issuance. So the endpoint declines wherever it can, and fails
//! only when it cannot answer at all — which is the refused caller, deliberately.
//!
//! Every answer comes from state the daemon already holds in memory. The
//! handshake is blocked until it answers and Caddy applies no timeout to the
//! request, so the handler reaches no network, opens no key store and reads no
//! file.
//!
//! spec: TLSD

use std::{collections::BTreeSet, net::SocketAddr, sync::Arc, time::Duration};

use axum::{
	body::Body,
	extract::{ConnectInfo, Query, State},
	http::{HeaderValue, StatusCode, header::CONTENT_TYPE},
	response::{IntoResponse, Response},
};
use miette::{IntoDiagnostic as _, Result, WrapErr as _};
use serde::Deserialize;
use serde_json::Value;
use tracing::{debug, warn};

use super::peer;
use crate::alertd::http_server::ServerState;

/// What Caddy's certificate manager passes.
///
/// Only the server name is acted on. The signature schemes and cipher suites
/// beside it are what the client offered, which a manager choosing between
/// several chains for one name would use; there is one chain per name here, so
/// they go unread rather than being made into a reason to refuse a well-formed
/// request.
#[derive(Debug, Deserialize)]
pub struct CertificateQuery {
	#[serde(default)]
	pub server_name: String,
}

/// Both ends of an accepted connection, which together name the caller's socket
/// in the kernel's table.
///
/// Axum's own connect info carries only the far end; the near end comes off the
/// accepted stream.
#[derive(Debug, Clone, Copy)]
pub struct Endpoints {
	pub remote: SocketAddr,
	pub local: SocketAddr,
}

impl
	axum::extract::connect_info::Connected<axum::serve::IncomingStream<'_, tokio::net::TcpListener>>
	for Endpoints
{
	fn connect_info(stream: axum::serve::IncomingStream<'_, tokio::net::TcpListener>) -> Self {
		let remote = *stream.remote_addr();
		// A connected socket always knows its own address; where it somehow
		// does not, this names a pair no entry in the kernel's table can match,
		// so the caller is refused rather than waved through.
		let local = stream.io().local_addr().unwrap_or(remote);
		Self { remote, local }
	}
}

/// Route handler for `/certificate`.
pub async fn handle_certificate(
	State(state): State<Arc<ServerState>>,
	ConnectInfo(ends): ConnectInfo<Endpoints>,
	Query(query): Query<CertificateQuery>,
) -> Response {
	let Some(certificates) = state.certificates.as_ref() else {
		// Nothing is collecting on this host, so there is no name this endpoint
		// is responsible for: decline, and let Caddy issue as it always has.
		return StatusCode::NO_CONTENT.into_response();
	};

	// The endpoint hands out a private key, so it identifies its caller rather
	// than accepting any connection that reaches it. A caller that is not
	// permitted is refused, and that refusal is a failure rather than a decline:
	// it is a misconfiguration to correct, not a name for Caddy to begin issuing
	// for itself.
	if let Err(err) =
		peer::caller_permitted(certificates.permitted(), ends.remote, ends.local).await
	{
		warn!(peer = %ends.remote, %err, "refused a certificate request");
		return (StatusCode::FORBIDDEN, format!("{err}")).into_response();
	}

	// A request naming no name names nothing this is responsible for, so it
	// declines rather than failing: a client that offered no server name
	// indication is Caddy's to answer from its own default, and an error here
	// would end that handshake instead of handing it back.
	if query.server_name.is_empty() {
		debug!("a certificate request carried no server_name");
		return StatusCode::NO_CONTENT.into_response();
	}

	match certificates.serve(&query.server_name).await {
		Some((chain, key)) => {
			debug!(name = %query.server_name, "serving a canopy-issued chain");
			let mut body = chain;
			if !body.ends_with('\n') {
				body.push('\n');
			}
			body.push_str(&key);
			let mut response = Response::new(Body::from(body));
			response.headers_mut().insert(
				CONTENT_TYPE,
				HeaderValue::from_static("application/x-pem-file"),
			);
			response
		}
		None => {
			// Record the name so an order follows, then decline. Caddy asks only
			// for names it is configured to serve, so this is immediacy between
			// passes rather than a route of its own.
			certificates.note_wanted(&query.server_name).await;
			StatusCode::NO_CONTENT.into_response()
		}
	}
}

/// The admin API a local Caddy answers on.
const CONFIG_URL: &str = "http://localhost:2019/config/";
const CONFIG_TIMEOUT: Duration = Duration::from_secs(3);

/// The site addresses Caddy is configured to serve, read from its live admin
/// configuration.
///
/// This is where the names to certify come from: the daemon orders for them
/// ahead of any client arriving, because a certificate is obtained before it is
/// needed rather than while a client waits.
///
/// spec: TLS#which-names-are-certified
pub async fn caddy_subjects(client: &reqwest::Client) -> Result<BTreeSet<String>> {
	let response = client
		.get(CONFIG_URL)
		.timeout(CONFIG_TIMEOUT)
		.send()
		.await
		.into_diagnostic()
		.wrap_err("reading the caddy admin API at localhost:2019")?;
	if !response.status().is_success() {
		return Err(miette::miette!(
			"the caddy admin API answered {}",
			response.status()
		));
	}
	let config: Value = response
		.json()
		.await
		.into_diagnostic()
		.wrap_err("parsing caddy's configuration")?;
	Ok(active_subjects(&config))
}

/// Every hostname Caddy considers active, gathered from its configuration: route
/// `host` matchers (anywhere, including subroutes), plus TLS `automate` and
/// automation-policy `subjects`.
///
/// The same reading `caddy_certs` takes, for the same reason: the live
/// configuration is what says which names the host answers on.
fn active_subjects(config: &Value) -> BTreeSet<String> {
	fn walk(value: &Value, out: &mut BTreeSet<String>) {
		match value {
			Value::Object(map) => {
				for (key, val) in map {
					if matches!(key.as_str(), "host" | "automate" | "subjects")
						&& let Some(arr) = val.as_array()
					{
						out.extend(
							arr.iter()
								.filter_map(|v| v.as_str())
								.map(|s| s.trim_end_matches('.').to_ascii_lowercase()),
						);
					}
					walk(val, out);
				}
			}
			Value::Array(arr) => arr.iter().for_each(|v| walk(v, out)),
			_ => {}
		}
	}
	let mut out = BTreeSet::new();
	walk(config, &mut out);
	out
}

#[cfg(test)]
mod tests {
	use super::*;

	#[test]
	fn the_names_to_certify_come_from_caddys_live_configuration() {
		let config = serde_json::json!({
			"apps": {
				"http": { "servers": { "srv0": { "routes": [
					{ "match": [{ "host": ["a.example.com", "B.example.com."] }],
					  "handle": [{ "handler": "subroute", "routes": [
						{ "match": [{ "host": ["nested.example.com"] }] }
					  ]}] }
				]}}},
				"tls": {
					"certificates": { "automate": ["auto.example.com"] },
					"automation": { "policies": [{ "subjects": ["policy.example.com"] }] }
				}
			}
		});
		let subjects = active_subjects(&config);
		for host in [
			"a.example.com",
			"b.example.com",
			"nested.example.com",
			"auto.example.com",
			"policy.example.com",
		] {
			assert!(subjects.contains(host), "missing {host}");
		}
	}

	#[test]
	fn a_configuration_naming_no_site_names_nothing() {
		assert!(active_subjects(&serde_json::json!({"apps": {}})).is_empty());
	}

	/// The endpoint over a real loopback connection, which is the only way to
	/// exercise the caller check: it reads the connection out of the kernel's
	/// table, so there has to be a connection.
	#[cfg(target_os = "linux")]
	mod over_a_connection {
		use std::sync::Arc;

		use axum::{Router, routing::get};

		use super::super::{Endpoints, handle_certificate};
		use crate::alertd::certificates::{CertificateState, peer};

		/// Serve the certificate route on an ephemeral loopback port, and answer
		/// from `state`.
		async fn serve(state: Option<Arc<CertificateState>>) -> String {
			let mut server = crate::alertd::http_server::test_utils::create_test_state().await;
			Arc::get_mut(&mut server).unwrap().certificates = state;

			let app = Router::new()
				.route("/certificate", get(handle_certificate))
				.with_state(server);
			let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
			let base = format!("http://{}", listener.local_addr().unwrap());
			tokio::spawn(async move {
				let _ = axum::serve(
					listener,
					app.into_make_service_with_connect_info::<Endpoints>(),
				)
				.await;
			});
			base
		}

		/// This process's own uid, read the same way the endpoint reads a
		/// caller's: without libc, which the workspace forbids reaching for.
		fn own_uid() -> u32 {
			std::fs::read_to_string("/proc/self/status")
				.unwrap()
				.lines()
				.find_map(|line| line.strip_prefix("Uid:"))
				.and_then(|rest| rest.split_whitespace().next()?.parse().ok())
				.unwrap()
		}

		async fn state_holding(name: Option<&str>) -> Arc<CertificateState> {
			let state = Arc::new(CertificateState::new(
				std::env::temp_dir().join("a3-delivery-test"),
				peer::Permitted::new(Some(own_uid())),
			));
			if let Some(name) = name {
				state.held.write().await.insert(
					name.to_owned(),
					crate::alertd::certificates::Held {
						chain: "-----BEGIN CERTIFICATE-----\nchain\n-----END CERTIFICATE-----"
							.into(),
						key_pem: "-----BEGIN PRIVATE KEY-----\nkey\n-----END PRIVATE KEY-----\n"
							.into(),
						not_after: None,
						usable: true,
						revoked: false,
					},
				);
			}
			state
		}

		/// The handler reaches no network, so canopy being unreachable — here,
		/// absent entirely — cannot stall or fail a handshake.
		#[tokio::test]
		async fn a_held_chain_is_served_from_memory_with_its_key() {
			let base = serve(Some(state_holding(Some("app.example.com")).await)).await;
			let response = reqwest::get(format!("{base}/certificate?server_name=app.example.com"))
				.await
				.unwrap();

			assert_eq!(response.status(), 200);
			let body = response.text().await.unwrap();
			assert!(body.contains("BEGIN CERTIFICATE"), "{body}");
			assert!(body.contains("BEGIN PRIVATE KEY"), "{body}");
		}

		/// A decline, not an error: an error would take down a name Caddy would
		/// otherwise have issued for itself.
		#[tokio::test]
		async fn nothing_held_declines_with_no_content() {
			let base = serve(Some(state_holding(None).await)).await;
			let response = reqwest::get(format!("{base}/certificate?server_name=app.example.com"))
				.await
				.unwrap();
			assert_eq!(response.status(), 204);
		}

		/// A request naming no name is a decline too. An error would end the
		/// handshake with no fallback, and a client that offered no server name
		/// indication is Caddy's to answer from its own default.
		#[tokio::test]
		async fn a_request_with_no_server_name_declines() {
			let base = serve(Some(state_holding(Some("app.example.com")).await)).await;
			let response = reqwest::get(format!("{base}/certificate?server_name="))
				.await
				.unwrap();
			assert_eq!(response.status(), 204);

			let response = reqwest::get(format!("{base}/certificate")).await.unwrap();
			assert_eq!(response.status(), 204);
		}

		/// A refused caller is a failure rather than a decline: it is a
		/// misconfiguration to correct, not a name for Caddy to begin issuing
		/// for itself.
		#[tokio::test]
		async fn a_caller_that_is_not_permitted_is_refused() {
			if own_uid() == 0 {
				// The superuser is always permitted, so there is no refusal to
				// observe from a test running as root.
				return;
			}
			let state = Arc::new(CertificateState::new(
				std::env::temp_dir().join("a3-delivery-test"),
				// Someone else entirely, and not root.
				peer::Permitted::new(Some(own_uid().wrapping_add(1))),
			));
			let base = serve(Some(state)).await;
			let response = reqwest::get(format!("{base}/certificate?server_name=app.example.com"))
				.await
				.unwrap();
			assert_eq!(response.status(), 403);
		}
	}
}