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-rw-r--r--src/timesync/timesyncd-manager.c35
1 files changed, 15 insertions, 20 deletions
diff --git a/src/timesync/timesyncd-manager.c b/src/timesync/timesyncd-manager.c
index 4352858..23480d4 100644
--- a/src/timesync/timesyncd-manager.c
+++ b/src/timesync/timesyncd-manager.c
@@ -25,6 +25,7 @@
#include "network-util.h"
#include "ratelimit.h"
#include "resolve-private.h"
+#include "random-util.h"
#include "socket-util.h"
#include "string-util.h"
#include "strv.h"
@@ -77,13 +78,6 @@ static double ts_to_d(const struct timespec *ts) {
return ts->tv_sec + (1.0e-9 * ts->tv_nsec);
}
-static uint32_t graceful_add_offset_1900_1970(time_t t) {
- /* Adds OFFSET_1900_1970 to t and returns it as 32bit value. This is handles overflows
- * gracefully in a deterministic and well-defined way by cutting off the top bits. */
- uint64_t a = (uint64_t) t + OFFSET_1900_1970;
- return (uint32_t) (a & UINT64_C(0xFFFFFFFF));
-}
-
static int manager_timeout(sd_event_source *source, usec_t usec, void *userdata) {
_cleanup_free_ char *pretty = NULL;
Manager *m = ASSERT_PTR(userdata);
@@ -125,20 +119,22 @@ static int manager_send_request(Manager *m) {
}
/*
- * Set transmit timestamp, remember it; the server will send that back
- * as the origin timestamp and we have an indication that this is the
- * matching answer to our request.
- *
- * The actual value does not matter, We do not care about the correct
- * NTP UINT_MAX fraction; we just pass the plain nanosecond value.
+ * Generate a random number as transmit timestamp, to ensure we get
+ * a full 64 bits of entropy to make it hard for off-path attackers
+ * to inject random time to us.
*/
- assert_se(clock_gettime(CLOCK_BOOTTIME, &m->trans_time_mon) >= 0);
- assert_se(clock_gettime(CLOCK_REALTIME, &m->trans_time) >= 0);
- ntpmsg.trans_time.sec = htobe32(graceful_add_offset_1900_1970(m->trans_time.tv_sec));
- ntpmsg.trans_time.frac = htobe32(m->trans_time.tv_nsec);
+ random_bytes(&m->request_nonce, sizeof(m->request_nonce));
+ ntpmsg.trans_time = m->request_nonce;
server_address_pretty(m->current_server_address, &pretty);
+ /*
+ * Record the transmit timestamp. This should be as close as possible to
+ * the send-to to ensure the timestamp is reasonably accurate
+ */
+ assert_se(clock_gettime(CLOCK_BOOTTIME, &m->trans_time_mon) >= 0);
+ assert_se(clock_gettime(CLOCK_REALTIME, &m->trans_time) >= 0);
+
len = sendto(m->server_socket, &ntpmsg, sizeof(ntpmsg), MSG_DONTWAIT, &m->current_server_address->sockaddr.sa, m->current_server_address->socklen);
if (len == sizeof(ntpmsg)) {
m->pending = true;
@@ -456,9 +452,8 @@ static int manager_receive_response(sd_event_source *source, int fd, uint32_t re
m->missed_replies = 0;
- /* check our "time cookie" (we just stored nanoseconds in the fraction field) */
- if (be32toh(ntpmsg.origin_time.sec) != graceful_add_offset_1900_1970(m->trans_time.tv_sec) ||
- be32toh(ntpmsg.origin_time.frac) != (unsigned long) m->trans_time.tv_nsec) {
+ /* check the transmit request nonce was properly returned in the origin_time field */
+ if (ntpmsg.origin_time.sec != m->request_nonce.sec || ntpmsg.origin_time.frac != m->request_nonce.frac) {
log_debug("Invalid reply; not our transmit time. Ignoring.");
return 0;
}