net_sched: fix estimator lock selection for mq child qdiscs

When new child qdiscs are attached to the mq qdisc, they are actually
attached as root qdiscs to the device queues. The lock selection for
new estimators incorrectly picks the root lock of the existing and
to be replaced qdisc, which results in a use-after-free once the old
qdisc has been destroyed.

Mark mq qdisc instances with a new flag and treat qdiscs attached to
mq as children similar to regular root qdiscs.

Additionally prevent estimators from being attached to the mq qdisc
itself since it only updates its byte and packet counters during dumps.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sch_generic.h

@@ -46,6 +46,7 @@ struct Qdisc
 #define TCQ_F_THROTTLED		2
 #define TCQ_F_INGRESS		4
 #define TCQ_F_CAN_BYPASS	8
+#define TCQ_F_MQROOT		16
 #define TCQ_F_WARN_NONWC	(1 << 16)
 	int			padded;
 	struct Qdisc_ops	*ops;

net/sched/sch_api.c

@@ -733,7 +733,8 @@ static struct lock_class_key qdisc_rx_lock;
 
 static struct Qdisc *
 qdisc_create(struct net_device *dev, struct netdev_queue *dev_queue,
-	     u32 parent, u32 handle, struct nlattr **tca, int *errp)
+	     struct Qdisc *p, u32 parent, u32 handle,
+	     struct nlattr **tca, int *errp)
 {
 	int err;
 	struct nlattr *kind = tca[TCA_KIND];
@@ -810,24 +811,21 @@ qdisc_create(struct net_device *dev, struct netdev_queue *dev_queue,
 		if (tca[TCA_RATE]) {
 			spinlock_t *root_lock;
 
+			err = -EOPNOTSUPP;
+			if (sch->flags & TCQ_F_MQROOT)
+				goto err_out4;
+
 			if ((sch->parent != TC_H_ROOT) &&
-			    !(sch->flags & TCQ_F_INGRESS))
+			    !(sch->flags & TCQ_F_INGRESS) &&
+			    (!p || !(p->flags & TCQ_F_MQROOT)))
 				root_lock = qdisc_root_sleeping_lock(sch);
 			else
 				root_lock = qdisc_lock(sch);
 
 			err = gen_new_estimator(&sch->bstats, &sch->rate_est,
 						root_lock, tca[TCA_RATE]);
-			if (err) {
-				/*
-				 * Any broken qdiscs that would require
-				 * a ops->reset() here? The qdisc was never
-				 * in action so it shouldn't be necessary.
-				 */
-				if (ops->destroy)
-					ops->destroy(sch);
-				goto err_out3;
-			}
+			if (err)
+				goto err_out4;
 		}
 
 		qdisc_list_add(sch);
@@ -843,6 +841,15 @@ qdisc_create(struct net_device *dev, struct netdev_queue *dev_queue,
 err_out:
 	*errp = err;
 	return NULL;
+
+err_out4:
+	/*
+	 * Any broken qdiscs that would require a ops->reset() here?
+	 * The qdisc was never in action so it shouldn't be necessary.
+	 */
+	if (ops->destroy)
+		ops->destroy(sch);
+	goto err_out3;
 }
 
 static int qdisc_change(struct Qdisc *sch, struct nlattr **tca)
@@ -867,13 +874,16 @@ static int qdisc_change(struct Qdisc *sch, struct nlattr **tca)
 	qdisc_put_stab(sch->stab);
 	sch->stab = stab;
 
-	if (tca[TCA_RATE])
+	if (tca[TCA_RATE]) {
 		/* NB: ignores errors from replace_estimator
 		   because change can't be undone. */
+		if (sch->flags & TCQ_F_MQROOT)
+			goto out;
 		gen_replace_estimator(&sch->bstats, &sch->rate_est,
 					    qdisc_root_sleeping_lock(sch),
 					    tca[TCA_RATE]);
-
+	}
+out:
 	return 0;
 }
 
@@ -1097,7 +1107,7 @@ static int tc_modify_qdisc(struct sk_buff *skb, struct nlmsghdr *n, void *arg)
 	if (!(n->nlmsg_flags&NLM_F_CREATE))
 		return -ENOENT;
 	if (clid == TC_H_INGRESS)
-		q = qdisc_create(dev, &dev->rx_queue,
+		q = qdisc_create(dev, &dev->rx_queue, p,
 				 tcm->tcm_parent, tcm->tcm_parent,
 				 tca, &err);
 	else {
@@ -1106,7 +1116,7 @@ static int tc_modify_qdisc(struct sk_buff *skb, struct nlmsghdr *n, void *arg)
 		if (p && p->ops->cl_ops && p->ops->cl_ops->select_queue)
 			ntx = p->ops->cl_ops->select_queue(p, tcm);
 
-		q = qdisc_create(dev, netdev_get_tx_queue(dev, ntx),
+		q = qdisc_create(dev, netdev_get_tx_queue(dev, ntx), p,
 				 tcm->tcm_parent, tcm->tcm_handle,
 				 tca, &err);
 	}

net/sched/sch_mq.c

@@ -64,6 +64,7 @@ static int mq_init(struct Qdisc *sch, struct nlattr *opt)
 		priv->qdiscs[ntx] = qdisc;
 	}
 
+	sch->flags |= TCQ_F_MQROOT;
 	return 0;
 
 err: