1 files changed, 237 insertions, 0 deletions
diff --git a/daemon/tls_ephemeral_credentials.c b/daemon/tls_ephemeral_credentials.c
new file mode 100644
index 0000000..ff4682f
--- /dev/null
+++ b/daemon/tls_ephemeral_credentials.c
@@ -0,0 +1,237 @@
+/*
+ * Copyright (C) 2016 American Civil Liberties Union (ACLU)
+ * Copyright (C) CZ.NIC, z.s.p.o.
+ *
+ * Initial Author: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#include <sys/file.h>
+#include <unistd.h>
+#include <gnutls/gnutls.h>
+#include <gnutls/x509.h>
+#include <gnutls/crypto.h>
+
+#include "daemon/engine.h"
+#include "daemon/tls.h"
+
+#define EPHEMERAL_PRIVKEY_FILENAME "ephemeral_key.pem"
+#define INVALID_HOSTNAME "dns-over-tls.invalid"
+#define EPHEMERAL_CERT_EXPIRATION_SECONDS (60*60*24*90)
+
+/* This is an attempt to grab an exclusive, advisory, non-blocking
+ * lock based on a filename.  At the moment it's POSIX-only, but it
+ * should be abstract enough of an interface to make an implementation
+ * for non-posix systems if anyone cares. */
+typedef int lock_t;
+static bool _lock_is_invalid(lock_t lock)
+{
+	return lock == -1;
+}
+/* a blocking lock on a given filename */
+static lock_t _lock_filename(const char *fname)
+{
+	lock_t lockfd = open(fname, O_RDONLY|O_CREAT, 0400);
+	if (lockfd == -1)
+		return lockfd;
+	/* this should be a non-blocking lock */
+	if (flock(lockfd, LOCK_EX | LOCK_NB) != 0) {
+		close(lockfd);
+		return -1;
+	}
+	return lockfd; /* for cleanup later */
+}
+static void _lock_unlock(lock_t *lock, const char *fname)
+{
+	if (lock && !_lock_is_invalid(*lock)) {
+		flock(*lock, LOCK_UN);
+		close(*lock);
+		*lock = -1;
+		unlink(fname); /* ignore errors */
+	}
+}
+
+static gnutls_x509_privkey_t get_ephemeral_privkey (void)
+{
+	gnutls_x509_privkey_t privkey = NULL;
+	int err;
+	gnutls_datum_t data = { .data = NULL, .size = 0 };
+	lock_t lock;
+	int datafd = -1;
+
+	/* Take a lock to ensure that two daemons started concurrently
+	 * with a shared cache don't both create the same privkey: */
+	lock = _lock_filename(EPHEMERAL_PRIVKEY_FILENAME ".lock");
+	if (_lock_is_invalid(lock)) {
+		kr_log_error(TLS, "unable to lock lockfile " EPHEMERAL_PRIVKEY_FILENAME ".lock\n");
+		goto done;
+	}
+
+	if ((err = gnutls_x509_privkey_init (&privkey)) < 0) {
+		kr_log_error(TLS, "gnutls_x509_privkey_init() failed: %d (%s)\n",
+			     err, gnutls_strerror_name(err));
+		goto done;
+	}
+
+	/* read from cache file (we assume that we've chdir'ed
+	 * already, so we're just looking for the file in the
+	 * cachedir. */
+	datafd = open(EPHEMERAL_PRIVKEY_FILENAME, O_RDONLY);
+	if (datafd != -1) {
+		struct stat stat;
+		ssize_t bytes_read;
+		if (fstat(datafd, &stat)) {
+			kr_log_error(TLS, "unable to stat ephemeral private key " EPHEMERAL_PRIVKEY_FILENAME "\n");
+			goto bad_data;
+		}
+		data.data = gnutls_malloc(stat.st_size);
+		if (data.data == NULL) {
+			kr_log_error(TLS, "unable to allocate memory for reading ephemeral private key\n");
+			goto bad_data;
+		}
+		data.size = stat.st_size;
+		bytes_read = read(datafd, data.data, stat.st_size);
+		if (bytes_read != stat.st_size) {
+			kr_log_error(TLS, "unable to read ephemeral private key\n");
+			goto bad_data;
+		}
+		if ((err = gnutls_x509_privkey_import (privkey, &data, GNUTLS_X509_FMT_PEM)) < 0) {
+			kr_log_error(TLS, "gnutls_x509_privkey_import() failed: %d (%s)\n",
+				     err, gnutls_strerror_name(err));
+			/* goto bad_data; */
+		bad_data:
+			close(datafd);
+			datafd = -1;
+		}
+		if (data.data != NULL) {
+			gnutls_free(data.data);
+			data.data = NULL;
+		}
+	}
+	if (datafd == -1) {
+		/* if loading failed, then generate ... */
+#if GNUTLS_VERSION_NUMBER >= 0x030500
+		if ((err = gnutls_x509_privkey_generate(privkey, GNUTLS_PK_ECDSA, GNUTLS_CURVE_TO_BITS(GNUTLS_ECC_CURVE_SECP256R1), 0)) < 0) {
+#else
+		if ((err = gnutls_x509_privkey_generate(privkey, GNUTLS_PK_RSA, gnutls_sec_param_to_pk_bits(GNUTLS_PK_RSA, GNUTLS_SEC_PARAM_MEDIUM), 0)) < 0) {
+#endif
+			kr_log_error(TLS, "gnutls_x509_privkey_init() failed: %d (%s)\n",
+				     err, gnutls_strerror_name(err));
+			gnutls_x509_privkey_deinit(privkey);
+			goto done;
+		}
+		/* ... and save */
+		kr_log_info(TLS, "Stashing ephemeral private key in " EPHEMERAL_PRIVKEY_FILENAME "\n");
+		if ((err = gnutls_x509_privkey_export2(privkey, GNUTLS_X509_FMT_PEM, &data)) < 0) {
+			kr_log_error(TLS, "gnutls_x509_privkey_export2() failed: %d (%s), not storing\n",
+				     err, gnutls_strerror_name(err));
+		} else {
+			datafd = open(EPHEMERAL_PRIVKEY_FILENAME, O_WRONLY|O_CREAT, 0600);
+			if (datafd == -1) {
+				kr_log_error(TLS, "failed to open " EPHEMERAL_PRIVKEY_FILENAME " to store the ephemeral key\n");
+			} else {
+				ssize_t bytes_written;
+				bytes_written = write(datafd, data.data, data.size);
+				if (bytes_written != data.size)
+					kr_log_error(TLS, "failed to write %d octets to "
+						     EPHEMERAL_PRIVKEY_FILENAME
+						     " (%zd written)\n",
+						     data.size, bytes_written);
+			}
+		}
+	}
+ done:
+	_lock_unlock(&lock, EPHEMERAL_PRIVKEY_FILENAME ".lock");
+	if (datafd != -1) {
+		close(datafd);
+	}
+	if (data.data != NULL) {
+		gnutls_free(data.data);
+	}
+	return privkey;
+}
+
+static gnutls_x509_crt_t get_ephemeral_cert(gnutls_x509_privkey_t privkey, const char *servicename, time_t invalid_before, time_t valid_until)
+{
+	gnutls_x509_crt_t cert = NULL;
+	int err;
+	/* need a random buffer of bytes */
+	uint8_t serial[16];
+	gnutls_rnd(GNUTLS_RND_NONCE, serial, sizeof(serial));
+	/* clear the left-most bit to avoid signedness confusion: */
+	serial[0] &= 0x7f;
+	size_t namelen = strlen(servicename);
+
+#define gtx(fn, ...)							\
+	if ((err = fn ( __VA_ARGS__ )) != GNUTLS_E_SUCCESS) {		\
+		kr_log_error(TLS, #fn "() failed: %d (%s)\n",	\
+			     err, gnutls_strerror_name(err));		\
+		goto bad; }
+
+	gtx(gnutls_x509_crt_init, &cert);
+	gtx(gnutls_x509_crt_set_activation_time, cert, invalid_before);
+	gtx(gnutls_x509_crt_set_ca_status, cert, 0);
+	gtx(gnutls_x509_crt_set_expiration_time, cert, valid_until);
+	gtx(gnutls_x509_crt_set_key, cert, privkey);
+	gtx(gnutls_x509_crt_set_key_purpose_oid, cert, GNUTLS_KP_TLS_WWW_CLIENT, 0);
+	gtx(gnutls_x509_crt_set_key_purpose_oid, cert, GNUTLS_KP_TLS_WWW_SERVER, 0);
+	gtx(gnutls_x509_crt_set_key_usage, cert, GNUTLS_KEY_DIGITAL_SIGNATURE);
+	gtx(gnutls_x509_crt_set_serial, cert, serial, sizeof(serial));
+	gtx(gnutls_x509_crt_set_subject_alt_name, cert, GNUTLS_SAN_DNSNAME, servicename, namelen, GNUTLS_FSAN_SET);
+	gtx(gnutls_x509_crt_set_dn_by_oid,cert, GNUTLS_OID_X520_COMMON_NAME, 0, servicename, namelen);
+	gtx(gnutls_x509_crt_set_version, cert, 3);
+	gtx(gnutls_x509_crt_sign2,cert, cert, privkey, GNUTLS_DIG_SHA256, 0); /* self-sign, since it doesn't look like we can just stub-sign */
+#undef gtx
+
+	return cert;
+bad:
+	gnutls_x509_crt_deinit(cert);
+	return NULL;
+}
+
+struct tls_credentials * tls_get_ephemeral_credentials(struct engine *engine)
+{
+	struct tls_credentials *creds = NULL;
+	gnutls_x509_privkey_t privkey = NULL;
+	gnutls_x509_crt_t cert = NULL;
+	int err;
+	time_t now = time(NULL);
+
+	creds = calloc(1, sizeof(*creds));
+	if (!creds) {
+		kr_log_error(TLS, "failed to allocate memory for ephemeral credentials\n");
+		return NULL;
+	}
+	if ((err = gnutls_certificate_allocate_credentials(&(creds->credentials))) < 0) {
+		kr_log_error(TLS, "failed to allocate memory for ephemeral credentials\n");
+		goto failure;
+	}
+
+	creds->valid_until = now + EPHEMERAL_CERT_EXPIRATION_SECONDS;
+	creds->ephemeral_servicename = strdup(engine_get_hostname(engine));
+	if (creds->ephemeral_servicename == NULL) {
+		kr_log_error(TLS, "could not get server's hostname, using '" INVALID_HOSTNAME "' instead\n");
+		if ((creds->ephemeral_servicename = strdup(INVALID_HOSTNAME)) == NULL) {
+			kr_log_error(TLS, "failed to allocate memory for ephemeral credentials\n");
+			goto failure;
+		}
+	}
+	if ((privkey = get_ephemeral_privkey()) == NULL) {
+		goto failure;
+	}
+	if ((cert = get_ephemeral_cert(privkey, creds->ephemeral_servicename, now - 60*15, creds->valid_until)) == NULL) {
+		goto failure;
+	}
+	if ((err = gnutls_certificate_set_x509_key(creds->credentials, &cert, 1, privkey)) < 0) {
+		kr_log_error(TLS, "failed to set up ephemeral credentials\n");
+		goto failure;
+	}
+	gnutls_x509_privkey_deinit(privkey);
+	gnutls_x509_crt_deinit(cert);
+	return creds;
+ failure:
+	gnutls_x509_privkey_deinit(privkey);
+	gnutls_x509_crt_deinit(cert);
+	tls_credentials_free(creds);
+	return NULL;
+}