netdev - [PATCH net] net/sched: fix NULL pointer dereference in qdisc

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Date:   Thu, 26 Oct 2017 20:26:48 +0200
From:   Davide Caratti <dcaratti@...hat.com>
To:     Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>
Cc:     Jiri Kosina <jkosina@...e.cz>, netdev@...r.kernel.org
Subject: [PATCH net] net/sched: fix NULL pointer dereference in qdisc_alloc()

the following script generates a NULL pointer dereference error:

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

upon creation of classful qdiscs, qdisc_alloc() dereferences dev_queue->dev
assuming it is not NULL. This is not true when eth0 has been added, but not
yet set administratively up; a bisect test proved that Linux started making
NULL exception with the above two commands after commit 59cc1f61f09c ("net:
sched:convert qdisc linked list to hashtable"). Let qdisc_alloc() return -1
(-ENOENT) when a NULL value of dev_queue->dev is seen, so that non-crashing
behaviour observable in Linux 4.8 is restored.

Fixes: 59cc1f61f09c ("net: sched:convert qdisc linked list to hashtable")
Signed-off-by: Davide Caratti <dcaratti@...hat.com>
---
 net/sched/sch_generic.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index bf8c81e07c70..5fb96f43d951 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -605,6 +605,14 @@ struct Qdisc *qdisc_alloc(struct netdev_queue *dev_queue,
 	int err = -ENOBUFS;
 	struct net_device *dev = dev_queue->dev;
 
+	/* dev_queue->dev can be NULL, if device has been registered but not
+	 * (yet) set administratively up: test it to avoid NULL dereference.
+	 */
+	if (!dev) {
+		err = -ENOENT;
+		goto errout;
+	}
+
 	p = kzalloc_node(size, GFP_KERNEL,
 			 netdev_queue_numa_node_read(dev_queue));
 
-- 
2.13.6