1 files changed, 245 insertions, 0 deletions
diff --git a/daemon/tls_session_ticket-srv.c b/daemon/tls_session_ticket-srv.c
new file mode 100644
index 0000000..b198903
--- /dev/null
+++ b/daemon/tls_session_ticket-srv.c
@@ -0,0 +1,245 @@
+/*  Copyright (C) CZ.NIC, z.s.p.o. <knot-resolver@labs.nic.cz>
+ *  SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#include <inttypes.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/time.h>
+
+#include <gnutls/gnutls.h>
+#include <gnutls/crypto.h>
+#include <uv.h>
+
+#include "lib/utils.h"
+
+/* Style: "local/static" identifiers are usually named tst_* */
+
+/** The number of seconds between synchronized rotation of TLS session ticket key. */
+#define TST_KEY_LIFETIME 4096
+
+/** Value from gnutls:lib/ext/session_ticket.c
+ * Beware: changing this needs to change the hashing implementation. */
+#define SESSION_KEY_SIZE 64
+
+/** Compile-time support for setting the secret. */
+/* This is not secure with TLS <= 1.2 but TLS 1.3 and secure configuration
+ * is not available in GnuTLS yet. See https://gitlab.com/gnutls/gnutls/issues/477 */
+#define TLS_SESSION_RESUMPTION_SYNC (GNUTLS_VERSION_NUMBER >= 0x030603)
+#if TLS_SESSION_RESUMPTION_SYNC
+	#define TST_HASH GNUTLS_DIG_SHA3_512
+#else
+	#define TST_HASH abort()
+#endif
+
+#if GNUTLS_VERSION_NUMBER < 0x030400
+	/* It's of little use anyway.  We may get the secret through lua,
+	 * which creates a copy outside of our control. */
+	#define gnutls_memset memset
+#endif
+
+/** Fields are internal to tst_key_* functions. */
+typedef struct tls_session_ticket_ctx {
+	uv_timer_t timer;	/**< timer for rotation of the key */
+	unsigned char key[SESSION_KEY_SIZE]; /**< the key itself */
+	bool has_secret;	/**< false -> key is random for each epoch */
+	uint16_t hash_len;	/**< length of `hash_data` */
+	char hash_data[];	/**< data to hash to obtain `key`;
+				 *   it's `time_t epoch` and then the secret string */
+} tst_ctx_t;
+
+/** Check invariants, based on gnutls version. */
+static bool tst_key_invariants(void)
+{
+	static int result = 0; /*< cache for multiple invocations */
+	if (result) return result > 0;
+	bool ok = true;
+	#if TLS_SESSION_RESUMPTION_SYNC
+		/* SHA3-512 output size may never change, but let's check it anyway :-) */
+		ok = ok && gnutls_hash_get_len(TST_HASH) == SESSION_KEY_SIZE;
+	#endif
+	/* The ticket key size might change in a different gnutls version. */
+	gnutls_datum_t key = { 0, 0 };
+	ok = ok && gnutls_session_ticket_key_generate(&key) == 0
+		&& key.size == SESSION_KEY_SIZE;
+	free(key.data);
+	result = ok ? 1 : -1;
+	return ok;
+}
+
+/** Create the internal structures and copy the secret. Beware: secret must be kept secure. */
+static tst_ctx_t * tst_key_create(const char *secret, size_t secret_len, uv_loop_t *loop)
+{
+	const size_t hash_len = sizeof(time_t) + secret_len;
+	if (kr_fails_assert(!secret_len || (secret && hash_len >= secret_len && hash_len <= UINT16_MAX))) {
+		return NULL;
+		/* reasonable secret_len is best enforced in config API */
+	}
+	if (kr_fails_assert(tst_key_invariants()))
+		return NULL;
+	#if !TLS_SESSION_RESUMPTION_SYNC
+		if (secret_len) {
+			kr_log_error(TLS, "session ticket: secrets were not enabled at compile-time (your GnuTLS version is not supported)\n");
+			return NULL; /* ENOTSUP */
+		}
+	#endif
+
+	tst_ctx_t *ctx = malloc(sizeof(*ctx) + hash_len); /* can be slightly longer */
+	if (!ctx) return NULL;
+	ctx->has_secret = secret_len > 0;
+	ctx->hash_len = hash_len;
+	if (secret_len) {
+		memcpy(ctx->hash_data + sizeof(time_t), secret, secret_len);
+	}
+
+	if (uv_timer_init(loop, &ctx->timer) != 0) {
+		free(ctx);
+		return NULL;
+	}
+	ctx->timer.data = ctx;
+	return ctx;
+}
+
+/** Random variant of secret rotation: generate into key_tmp and copy. */
+static int tst_key_get_random(tst_ctx_t *ctx)
+{
+	gnutls_datum_t key_tmp = { NULL, 0 };
+	int err = gnutls_session_ticket_key_generate(&key_tmp);
+	if (err) return kr_error(err);
+	if (kr_fails_assert(key_tmp.size == SESSION_KEY_SIZE))
+		return kr_error(EFAULT);
+	memcpy(ctx->key, key_tmp.data, SESSION_KEY_SIZE);
+	gnutls_memset(key_tmp.data, 0, SESSION_KEY_SIZE);
+	free(key_tmp.data);
+	return kr_ok();
+}
+
+/** Recompute the session ticket key, if epoch has changed or forced. */
+static int tst_key_update(tst_ctx_t *ctx, time_t epoch, bool force_update)
+{
+	if (kr_fails_assert(ctx && ctx->hash_len >= sizeof(epoch)))
+		return kr_error(EINVAL);
+	/* documented limitation: time_t and endianness must match
+	 * on instances sharing a secret */
+	if (!force_update && memcmp(ctx->hash_data, &epoch, sizeof(epoch)) == 0) {
+		return kr_ok(); /* we are up to date */
+	}
+	memcpy(ctx->hash_data, &epoch, sizeof(epoch));
+
+	if (!ctx->has_secret) {
+		return tst_key_get_random(ctx);
+	}
+	/* Otherwise, deterministic variant of secret rotation, if supported. */
+	#if !TLS_SESSION_RESUMPTION_SYNC
+		kr_assert(!ENOTSUP);
+		return kr_error(ENOTSUP);
+	#else
+		int err = gnutls_hash_fast(TST_HASH, ctx->hash_data,
+					   ctx->hash_len, ctx->key);
+		return err == 0 ? kr_ok() : kr_error(err);
+	#endif
+}
+
+/** Free all resources of the key (securely). */
+static void tst_key_destroy(uv_handle_t *timer)
+{
+	if (kr_fails_assert(timer))
+		return;
+	tst_ctx_t *ctx = timer->data;
+	if (kr_fails_assert(ctx))
+		return;
+	gnutls_memset(ctx, 0, offsetof(tst_ctx_t, hash_data) + ctx->hash_len);
+	free(ctx);
+}
+
+static void tst_key_check(uv_timer_t *timer, bool force_update);
+static void tst_timer_callback(uv_timer_t *timer)
+{
+	tst_key_check(timer, false);
+}
+
+/** Update the ST key if needed and reschedule itself via the timer. */
+static void tst_key_check(uv_timer_t *timer, bool force_update)
+{
+	tst_ctx_t *stst = (tst_ctx_t *)timer->data;
+	/* Compute the current epoch. */
+	struct timeval now;
+	if (gettimeofday(&now, NULL)) {
+		kr_log_error(TLS, "session ticket: gettimeofday failed, %s\n",
+				strerror(errno));
+		return;
+	}
+	uv_update_time(timer->loop); /* to have sync. between real and mono time */
+	const time_t epoch = now.tv_sec / TST_KEY_LIFETIME;
+	/* Update the key; new sessions will fetch it from the location.
+	 * Old ones hopefully can't get broken by that; documentation
+	 * for gnutls_session_ticket_enable_server() doesn't say. */
+	int err = tst_key_update(stst, epoch, force_update);
+	if (err) {
+		kr_log_error(TLS, "session ticket: failed rotation, %s\n",
+				kr_strerror(err));
+		if (kr_fails_assert(err != kr_error(EINVAL)))
+			return;
+	}
+	/* Reschedule. */
+	const time_t tv_sec_next = (epoch + 1) * TST_KEY_LIFETIME;
+	const uint64_t ms_until_second = 1000 - (now.tv_usec + 501) / 1000;
+	const uint64_t remain_ms = (tv_sec_next - now.tv_sec - 1) * (uint64_t)1000
+				 + ms_until_second + 1;
+	/* ^ +1 because we don't want to wake up half a millisecond before the epoch! */
+	if (kr_fails_assert(remain_ms < (TST_KEY_LIFETIME + 1 /*rounding tolerance*/) * 1000))
+		return;
+	kr_log_debug(TLS, "session ticket: epoch %"PRIu64
+			", scheduling rotation check in %"PRIu64" ms\n",
+			(uint64_t)epoch, remain_ms);
+	err = uv_timer_start(timer, &tst_timer_callback, remain_ms, 0);
+	if (kr_fails_assert(err == 0)) {
+		kr_log_error(TLS, "session ticket: failed to schedule, %s\n",
+				uv_strerror(err));
+		return;
+	}
+}
+
+/* Implementation for prototypes from ./tls.h */
+
+void tls_session_ticket_enable(struct tls_session_ticket_ctx *ctx, gnutls_session_t session)
+{
+	if (kr_fails_assert(ctx && session))
+		return;
+	const gnutls_datum_t gd = {
+		.size = SESSION_KEY_SIZE,
+		.data = ctx->key,
+	};
+	int err = gnutls_session_ticket_enable_server(session, &gd);
+	if (err) {
+		kr_log_error(TLS, "failed to enable session tickets: %s (%d)\n",
+				gnutls_strerror_name(err), err);
+		/* but continue without tickets */
+	}
+}
+
+tst_ctx_t * tls_session_ticket_ctx_create(uv_loop_t *loop, const char *secret,
+					  size_t secret_len)
+{
+	if (kr_fails_assert(loop && (!secret_len || secret)))
+		return NULL;
+	#if GNUTLS_VERSION_NUMBER < 0x030500
+		/* We would need different SESSION_KEY_SIZE; avoid an error. */
+		return NULL;
+	#endif
+	tst_ctx_t *ctx = tst_key_create(secret, secret_len, loop);
+	if (ctx) {
+		tst_key_check(&ctx->timer, true);
+	}
+	return ctx;
+}
+
+void tls_session_ticket_ctx_destroy(tst_ctx_t *ctx)
+{
+	if (ctx == NULL) {
+		return;
+	}
+	uv_close((uv_handle_t *)&ctx->timer, &tst_key_destroy);
+}
+