netdev - Re: [PATCH 3/5] net: sched: taprio: Fix null pointer deref bug

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Message-ID: <CAM_iQpUYAXdcrEixKyL=S9mUdBRNit-N5RKLWQ5E8QfJSowBqw@mail.gmail.com>
Date:   Mon, 22 Apr 2019 12:36:29 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     "Guedes, Andre" <andre.guedes@...el.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "Gomes, Vinicius" <vinicius.gomes@...el.com>
Subject: Re: [PATCH 3/5] net: sched: taprio: Fix null pointer deref bug

On Mon, Apr 22, 2019 at 12:24 PM Guedes, Andre <andre.guedes@...el.com> wrote:
>
>
> > On Apr 22, 2019, at 11:07 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > On Mon, Apr 22, 2019 at 11:04 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >>
> >> On Fri, Apr 19, 2019 at 6:06 PM Andre Guedes <andre.guedes@...el.com> wrote:
> >>>
> >>> If 'entry' is NULL we WARN_ON() but dereference the pointer anyway,
> >>> generating a null pointer dereference bug. This patch fixes should_
> >>> restart_cycle() so we return if the pointer is NULL.
> >>
> >> Hmm, while you are on it, how is it possible to have entry==NULL
> >> for should_restart_cycle(). It is only called in advance_sched()
> >> where entry is already checked against NULL right before it,
> >> so for me, entry is always NULL at the point of calling
> >> should_restart_cycle().
> >
> > I meant, 'entry' is always non-NULL here... I typed too fast.
>
> Your assessment is correct. I believe the WARN_ON() was added as a defensive practice to prevent null pointer dereference in case someone misuse that helper in the future.

Yeah, so we can just remove it. :)