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/*
* 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 ()
{
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] &= 0x8f;
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;
}
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