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timesyncd: make the transmit timestamp in requests fully random

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This improves security against off-path attackers, and avoids leaking
the current system time.

(cherry picked from commit 678bd12cfc1a7f3f0d074ac9c52f0b06ec601618)
(cherry picked from commit 310405ff82bfc1fe767a34d6cb99def940e0ef23)
(cherry picked from commit 30b2741e95b8446eb309cf786638b6daeace25b1)
This commit is contained in:
David Venhoek 2024-01-26 10:40:03 +01:00 committed by Luca Boccassi
parent 612d6c576d
commit 103e29d164
2 changed files with 16 additions and 20 deletions
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35
src/timesync/timesyncd-manager.c
View File

@ -26,6 +26,7 @@
#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);
@ -125,19 +119,21 @@ 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.
*/
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);
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);
server_address_pretty(m->current_server_address, &pretty);
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)) {
@ -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;
}

1
src/timesync/timesyncd-manager.h
View File

@ -71,6 +71,7 @@ struct Manager {
/* last sent packet */
struct timespec trans_time_mon;
struct timespec trans_time;
struct ntp_ts request_nonce;
usec_t retry_interval;
usec_t connection_retry_usec;
bool pending;