linux-kernel - Re: [PATCH] dispnv50: atom: fix an incorrect NULL check on list iterator

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Date:   Tue, 29 Mar 2022 11:59:06 +0100
From:   Emil Velikov <emil.l.velikov@...il.com>
To:     Xiaomeng Tong <xiam0nd.tong@...il.com>
Cc:     Dave Airlie <airlied@...ux.ie>, Ben Skeggs <bskeggs@...hat.com>,
        Daniel Vetter <daniel@...ll.ch>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        Karol Herbst <kherbst@...hat.com>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        Lyude <lyude@...hat.com>,
        ML nouveau <nouveau@...ts.freedesktop.org>,
        "# 3.13+" <stable@...r.kernel.org>, yangyingliang@...wei.com
Subject: Re: [PATCH] dispnv50: atom: fix an incorrect NULL check on list iterator

On Mon, 28 Mar 2022 at 03:09, Xiaomeng Tong <xiam0nd.tong@...il.com> wrote:
>
> on Sun, 27 Mar 2022 16:59:28 +0100, Emil Velikov wrote:
> > On Sun, 27 Mar 2022 at 08:39, Xiaomeng Tong <xiam0nd.tong@...il.com> wrote:
> > >
> > > The bug is here:
> > >         return encoder;
> > >
> > > The list iterator value 'encoder' will *always* be set and non-NULL
> > > by drm_for_each_encoder_mask(), so it is incorrect to assume that the
> > > iterator value will be NULL if the list is empty or no element found.
> > > Otherwise it will bypass some NULL checks and lead to invalid memory
> > > access passing the check.
> > >
> > > To fix this bug, just return 'encoder' when found, otherwise return
> > > NULL.
> > >
> >
> > Isn't this covered by the upcoming list* iterator rework [1] or is
> > this another iterator glitch?
>
> Actually, it is a part of the upcoming work.
>
> > IMHO we should be looking at fixing the implementation and not the
> > hundreds of users through the kernel.
> >
> > HTH
> > -Emil
> > [1] https://lwn.net/Articles/887097/
>
> Yes, you are right. This has also been taken into account by the upcoming
> list iterator rework to avoid a lot uesr' changes as much as possible.
>
> However, this patch is fixing a potential bug caused by incorrect use of
> list iterator outside the loop, which can not be fixed by the implementation
> itself.
>

I see, thanks for the information o/

-Emil