Nouveau - Mar 2022 - [PATCH] dispnv50: atom: fix an incorrect NULL check on list iterator

On Sun, 27 Mar 2022 at 08:39, Xiaomeng Tong <xiam0nd.tong at gmail.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?
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/

on Sun, 27 Mar 2022 16:59:28 +0100, Emil Velikov wrote:
> On Sun, 27 Mar 2022 at 08:39, Xiaomeng Tong <xiam0nd.tong at
gmail.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.

--
Xiaomeng Tong

On Mon, 28 Mar 2022 at 03:09, Xiaomeng Tong <xiam0nd.tong at gmail.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 at
gmail.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