[PATCH v2] net: sched: Fix a possible null-pointer dereference in dequeue_func()

[Jia-Ju Bai <baijiaju1990@gmail.com>]

In dequeue_func(), there is an if statement on line 74 to check whether
skb is NULL:
    if (skb)

When skb is NULL, it is used on line 77:
    prefetch(&skb->end);

Thus, a possible null-pointer dereference may occur.

To fix this bug, skb->end is used when skb is not NULL.

This bug is found by a static analysis tool STCheck written by us.

Fixes: 79bdc4c862af ("codel: generalize the implementation")
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
---
v2:
* Add a fix tag.
  Thank Jiri Pirko for helpful advice.

---
 net/sched/sch_codel.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/sched/sch_codel.c b/net/sched/sch_codel.c
index 25ef172c23df..30169b3adbbb 100644
--- a/net/sched/sch_codel.c
+++ b/net/sched/sch_codel.c
@@ -71,10 +71,10 @@ static struct sk_buff *dequeue_func(struct codel_vars *vars, void *ctx)
 	struct Qdisc *sch = ctx;
 	struct sk_buff *skb = __qdisc_dequeue_head(&sch->q);
 
-	if (skb)
+	if (skb) {
 		sch->qstats.backlog -= qdisc_pkt_len(skb);
-
-	prefetch(&skb->end); /* we'll need skb_shinfo() */
+		prefetch(&skb->end); /* we'll need skb_shinfo() */
+	}
 	return skb;
 }
 
-- 
2.17.0