Re: [PATCH v3] net: sched: Fix a possible null-pointer dereference in dequeue_func()

[Eric Dumazet <eric.dumazet@gmail.com>]

On 7/29/19 7:23 PM, Cong Wang wrote:
> On Mon, Jul 29, 2019 at 1:24 AM Jia-Ju Bai <baijiaju1990@gmail.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