netdev - [Patch net] net_sched: avoid matching qdisc with zero handle

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Message-Id: <20171028050856.18393-1-xiyou.wangcong@gmail.com>
Date:   Fri, 27 Oct 2017 22:08:56 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     netdev@...r.kernel.org
Cc:     dcaratti@...hat.com, Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Kosina <jkosina@...e.cz>,
        Eric Dumazet <edumazet@...gle.com>,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: [Patch net] net_sched: avoid matching qdisc with zero handle

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 <dcaratti@...hat.com>
Cc: Jiri Kosina <jkosina@...e.cz>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Jamal Hadi Salim <jhs@...atatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
---
 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 c6deb74e3d2f..22bc6fc48311 100644
--- a/net/sched/sch_api.c
+++ b/net/sched/sch_api.c
@@ -301,6 +301,8 @@ 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;
-- 
2.13.0