linux-kernel - Re: [PATCH v2] net: sched: Fix a possible null-pointer dereference in dequeue

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Date:   Mon, 29 Jul 2019 10:18:54 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     jhs@...atatu.com, xiyou.wangcong@...il.com, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] net: sched: Fix a possible null-pointer dereference
 in dequeue_func()

Mon, Jul 29, 2019 at 10:00:18AM CEST, baijiaju1990@...il.com wrote:
>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")

Looks like this is something being there since the beginning:
commit 76e3cc126bb223013a6b9a0e2a51238d1ef2e409
Author: Eric Dumazet <edumazet@...gle.com>
Date:   Thu May 10 07:51:25 2012 +0000

    codel: Controlled Delay AQM


Please adjust "Fixes:".

The fix itself looks fine to me.
Reviewed-by: Jiri Pirko <jiri@...lanox.com>



>Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.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
>