[RFC net-next v1 1/6] taprio: Fix potencial use of invalid memory during dequeue()

[Vinicius Costa Gomes <vinicius.gomes@intel.com>]

Right now, this isn't a problem, but the next commit allows schedules
to be added during runtime. When a new schedule transitions from the
inactive to the active state ("admin" -> "oper") the previous one is
free'd, if it's free'd just after the RCU read lock is released, we
may access an invalid 'entry'.

So, we should take care to protect the dequeue() flow, so all the
places that access the entries are protected by the RCU read lock.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
---
 net/sched/sch_taprio.c | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index c7041999eb5d..1fb5eae46bc6 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -124,8 +124,8 @@ static struct sk_buff *taprio_dequeue(struct Qdisc *sch)
 {
 	struct taprio_sched *q = qdisc_priv(sch);
 	struct net_device *dev = qdisc_dev(sch);
+	struct sk_buff *skb = NULL;
 	struct sched_entry *entry;
-	struct sk_buff *skb;
 	u32 gate_mask;
 	int i;
 
@@ -137,10 +137,9 @@ static struct sk_buff *taprio_dequeue(struct Qdisc *sch)
 	 * "AdminGateSates"
 	 */
 	gate_mask = entry ? entry->gate_mask : TAPRIO_ALL_GATES_OPEN;
-	rcu_read_unlock();
 
 	if (!gate_mask)
-		return NULL;
+		goto done;
 
 	for (i = 0; i < dev->num_tx_queues; i++) {
 		struct Qdisc *child = q->qdiscs[i];
@@ -170,26 +169,35 @@ static struct sk_buff *taprio_dequeue(struct Qdisc *sch)
 		 * guard band ...
 		 */
 		if (gate_mask != TAPRIO_ALL_GATES_OPEN &&
-		    ktime_after(guard, entry->close_time))
-			return NULL;
+		    ktime_after(guard, entry->close_time)) {
+			skb = NULL;
+			goto done;
+		}
 
 		/* ... and no budget. */
 		if (gate_mask != TAPRIO_ALL_GATES_OPEN &&
-		    atomic_sub_return(len, &entry->budget) < 0)
-			return NULL;
+		    atomic_sub_return(len, &entry->budget) < 0) {
+			skb = NULL;
+			goto done;
+		}
 
 		skb = child->ops->dequeue(child);
 		if (unlikely(!skb))
-			return NULL;
+			goto done;
+
 
 		qdisc_bstats_update(sch, skb);
 		qdisc_qstats_backlog_dec(sch, skb);
 		sch->q.qlen--;
 
-		return skb;
+		goto done;
 	}
 
-	return NULL;
+done:
+	rcu_read_unlock();
+
+	return skb;
+
 }
 
 static bool should_restart_cycle(const struct taprio_sched *q,
-- 
2.21.0