Subject: [PATCH 33/37] net_sched: avoid matching qdisc with zero handle

From: Cong Wang <[email-masked]>
Date: Fri, 27 Oct 2017 22:08:56 -0700

[ Upstream commit 50317fce2cc70a2bbbc4b42c31bbad510382a53c ]

Davide found the following script triggers a NULL pointer
dereference:

ip l a name eth0 type dummy
tc q a dev eth0 parent :1 handle 1: htb

This is because for a freshly created netdevice noop_qdisc
is attached and when passing 'parent :1', kernel actually
tries to match the major handle which is 0 and noop_qdisc
has handle 0 so is matched by mistake. Commit 69012ae425d7
tries to fix a similar bug but still misses this case.

Handle 0 is not a valid one, should be just skipped. In
fact, kernel uses it as TC_H_UNSPEC.

Fixes: 69012ae425d7 ("net: sched: fix handling of singleton qdiscs with qdisc_hash")
Fixes: 59cc1f61f09c ("net: sched:convert qdisc linked list to hashtable")
Reported-by: Davide Caratti <[email-masked]>
Cc: Jiri Kosina <[email-masked]>
Cc: Eric Dumazet <[email-masked]>
Cc: Jamal Hadi Salim <[email-masked]>
Signed-off-by: Cong Wang <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/sched/sch_api.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index 4fb5a3222d0d..7935db0d787c 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -307,6 +307,8 @@ struct Qdisc *qdisc_lookup(struct net_device *dev, u32 handle)
 {
 	struct Qdisc *q;
 
+	if (!handle)
+		return NULL;
 	q = qdisc_match_from_root(dev->qdisc, handle);
 	if (q)
 		goto out;
-- 
2.13.6