net_sched: Fix qdisc_notify()

Ben Pfaff reported a kernel oops and provided a test program to
reproduce it.

https://kerneltrap.org/mailarchive/linux-netdev/2010/5/21/6277805

tc_fill_qdisc() should not be called for builtin qdisc, or it
dereference a NULL pointer to get device ifindex.

Fix is to always use tc_qdisc_dump_ignore() before calling
tc_fill_qdisc().
Reported-by: NBen Pfaff <blp@nicira.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_api.c

@@ -1195,6 +1195,11 @@ static int tc_fill_qdisc(struct sk_buff *skb, struct Qdisc *q, u32 clid,
 	return -1;
 }
 
+static bool tc_qdisc_dump_ignore(struct Qdisc *q)
+{
+	return (q->flags & TCQ_F_BUILTIN) ? true : false;
+}
+
 static int qdisc_notify(struct net *net, struct sk_buff *oskb,
 			struct nlmsghdr *n, u32 clid,
 			struct Qdisc *old, struct Qdisc *new)
@@ -1206,11 +1211,11 @@ static int qdisc_notify(struct net *net, struct sk_buff *oskb,
 	if (!skb)
 		return -ENOBUFS;
 
-	if (old && old->handle) {
+	if (old && !tc_qdisc_dump_ignore(old)) {
 		if (tc_fill_qdisc(skb, old, clid, pid, n->nlmsg_seq, 0, RTM_DELQDISC) < 0)
 			goto err_out;
 	}
-	if (new) {
+	if (new && !tc_qdisc_dump_ignore(new)) {
 		if (tc_fill_qdisc(skb, new, clid, pid, n->nlmsg_seq, old ? NLM_F_REPLACE : 0, RTM_NEWQDISC) < 0)
 			goto err_out;
 	}
@@ -1223,11 +1228,6 @@ static int qdisc_notify(struct net *net, struct sk_buff *oskb,
 	return -EINVAL;
 }
 
-static bool tc_qdisc_dump_ignore(struct Qdisc *q)
-{
-	return (q->flags & TCQ_F_BUILTIN) ? true : false;
-}
-
 static int tc_dump_qdisc_root(struct Qdisc *root, struct sk_buff *skb,
 			      struct netlink_callback *cb,
 			      int *q_idx_p, int s_q_idx)