linux-kernel - [PATCH 3.16 14/83] net: sched: Fix a possible null-pointer dereference in dequeue

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Message-ID: <lsq.1574264230.473319350@decadent.org.uk>
Date:   Wed, 20 Nov 2019 15:37:24 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Jia-Ju Bai" <baijiaju1990@...il.com>,
        "Jiri Pirko" <jiri@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH 3.16 14/83] net: sched: Fix a possible null-pointer
 dereference in dequeue_func()

3.16.78-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jia-Ju Bai <baijiaju1990@...il.com>

commit 051c7b39be4a91f6b7d8c4548444e4b850f1f56c upstream.

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: 76e3cc126bb2 ("codel: Controlled Delay AQM")
Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
Reviewed-by: Jiri Pirko <jiri@...lanox.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- a/net/sched/sch_codel.c
+++ b/net/sched/sch_codel.c
@@ -68,7 +68,8 @@ static struct sk_buff *dequeue(struct co
 {
 	struct sk_buff *skb = __skb_dequeue(&sch->q);
 
-	prefetch(&skb->end); /* we'll need skb_shinfo() */
+	if (skb)
+		prefetch(&skb->end); /* we'll need skb_shinfo() */
 	return skb;
 }