linux-kernel - Re: [PATCH v3] net: sched: Fix a possible null-pointer dereference in dequeue

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Message-ID: <CAM_iQpU0L6hgzg1N9Ay=72Ee-2Ni-ovPJX8SyJaRDv7dbhZs_Q@mail.gmail.com>
Date:   Mon, 29 Jul 2019 10:23:53 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     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 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.