Reject empty schedules (num_entries == 0) so next_entry is always valid and
RCU readers/timer logic never walk an empty list. taprio enforces the same
constraint on schedules (sch_taprio.c, commit 09dbdf28f9f9fa).

Fixes: a51c328df310 ("net: qos: introduce a gate control flow action")
Signed-off-by: Paul Moses <redacted>
Cc: stable@vger.kernel.org
---
  net/sched/act_gate.c | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/net/sched/act_gate.c b/net/sched/act_gate.c
index 48ff378bb051a..e4134b9a4a314 100644
--- a/net/sched/act_gate.c
+++ b/net/sched/act_gate.c

@@ -509,6 +509,12 @@ static int tcf_gate_init(struct net *net, struct nlattr *nla,
  		cycletime_ext = nla_get_u64(tb[TCA_GATE_CYCLE_TIME_EXT]);
  	p->tcfg_cycletime_ext = cycletime_ext;
  
+	if (p->num_entries == 0) {
+		NL_SET_ERR_MSG(extack, "The entry list is empty");
+		err = -EINVAL;
+		goto release_mem;
+	}