taprio takes base-time as an unbounded signed 64-bit value. A base time
in the future is used as the schedule start unchanged, and
setup_first_end_time() then adds the cycle time, the first interval and
the gate durations of the first entry to it. With a start close to
KTIME_MAX these sums overflow, and the software schedule starts with end
and gate close times that lie far in the past.

If this is the first schedule, the qdisc timer is armed for its future
start. With an operational schedule running, taprio_start_sched() keeps
the earlier operational expiry instead. A large cycle-time-extension
can then trigger an early handover to the pending admin schedule.
advance_sched() uses the invalid entry end as the next expiry and can
keep restarting inside the same timer interrupt.

Before a software schedule is initialized and published, check that the
computed start is not negative and leaves room for every timestamp
initialized from it, and reject the schedule with -ERANGE otherwise. Full
offload and txtime-assist do not use the software timer and are not
affected. Schedules with a reasonable base time behave as before.

Fixes: 5a781ccbd19e ("tc: Add support for configuring the taprio scheduler")
Assisted-by: Codex:gpt-6.1-sol
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Krystian Kaniewski <krystianmkaniewski@gmail.com>
---
 net/sched/sch_taprio.c | 46 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 299234a5f0fe6..1f753911cdfec 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -1238,6 +1238,48 @@ static int taprio_get_start_time(struct Qdisc *sch,
 	return 0;
 }
 
+static int taprio_validate_start_time(struct taprio_sched *q,
+				      const struct sched_gate_list *sched,
+				      ktime_t start,
+				      struct netlink_ext_ack *extack)
+{
+	int num_tc = netdev_get_num_tc(qdisc_dev(q->root));
+	const struct sched_entry *first, *entry;
+	u64 offset = 0, span;
+	int tc;
+
+	if (TXTIME_ASSIST_IS_ENABLED(q->flags) ||
+	    FULL_OFFLOAD_IS_ENABLED(q->flags))
+		return 0;
+
+	first = list_first_entry(&sched->entries, struct sched_entry, list);
+
+	/* setup_first_end_time() adds the cycle time, the first interval and
+	 * the finite gate durations of the first entry to the start, and
+	 * setup_txtime() adds the offset of every entry. None of these sums
+	 * may overflow.
+	 */
+	span = max_t(u64, sched->cycle_time, first->interval);
+	list_for_each_entry(entry, &sched->entries, list) {
+		span = max(span, offset);
+		offset += entry->interval;
+	}
+
+	for (tc = 0; tc < num_tc; tc++) {
+		if (first->gate_duration[tc] == sched->cycle_time)
+			continue;
+		span = max(span, first->gate_duration[tc]);
+	}
+
+	if (start < 0 || span > KTIME_MAX ||
+	    (u64)start > KTIME_MAX - span) {
+		NL_SET_ERR_MSG(extack, "Schedule timing is out of range");
+		return -ERANGE;
+	}
+
+	return 0;
+}
+
 static void setup_first_end_time(struct taprio_sched *q,
 				 struct sched_gate_list *sched, ktime_t base)
 {
@@ -1958,6 +2000,10 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 		goto unlock;
 	}
 
+	err = taprio_validate_start_time(q, new_admin, start, extack);
+	if (err)
+		goto unlock;
+
 	setup_txtime(q, new_admin, start);
 
 	if (TXTIME_ASSIST_IS_ENABLED(q->flags)) {
-- 
2.53.0