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Message-ID: <7c260c52-5f73-2efb-1477-cfcca0971a59@gmail.com>
Date: Mon, 5 Aug 2019 10:05:46 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: Cong Wang <xiyou.wangcong@...il.com>,
Jia-Ju Bai <baijiaju1990@...il.com>
Cc: Jamal Hadi Salim <jhs@...atatu.com>, Jiri Pirko <jiri@...nulli.us>,
David Miller <davem@...emloft.net>,
Linux Kernel Network Developers <netdev@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] net: sched: Fix a possible null-pointer dereference in
dequeue_func()
On 7/29/19 7:23 PM, Cong Wang wrote:
> On Mon, Jul 29, 2019 at 1:24 AM Jia-Ju Bai <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.
>
> It doesn't dereference the pointer, it simply calculates the address
> and passes it to gcc builtin prefetch. Both are fine when it is NULL,
> as prefetching a NULL address should be okay for kernel.
>
> So your changelog is misleading and it doesn't fix any bug,
> although it does very slightly make the code better.
>
Original code was intentional.
A prefetch() need to be done as early as possible.
At the time of commit 76e3cc126bb223013a6b9a0e2a51238d1ef2e409
this was pretty clear :
+static struct sk_buff *dequeue(struct codel_vars *vars, struct Qdisc *sch)
+{
+ struct sk_buff *skb = __skb_dequeue(&sch->q);
+
+ prefetch(&skb->end); /* we'll need skb_shinfo() */
+ return skb;
+}
Really static analysis should learn about prefetch(X) being legal for _any_ value of X
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