Re: [PATCH] ALSA: ac97: Fix possible NULL dereference in snd_ac97_mixer

[Christophe JAILLET]

Le 23/08/2023 à 16:37, Takashi Iwai a écrit :
> On Tue, 22 Aug 2023 22:07:40 +0200,
> Christophe JAILLET wrote:
> > Le 15/06/2023 à 04:17, Su Hui a écrit :
> > > smatch error:
> > > sound/pci/ac97/ac97_codec.c:2354 snd_ac97_mixer() error:
> > > we previously assumed 'rac97' could be null (see line 2072)
> > >
> > > remove redundant assignment, return error if rac97 is NULL.
> >
> > Hi,
> >
> > why is the assigment redundant?
>
> It's misleading, yeah.  Basically all callers are with non-NULL, hence
> we took rather make it mandatory.  Maybe it should have been with
> WARN_ON() to catch the NULL argument for an out-of-tree stuff.
>
> > Should an error occur, the 'struct snd_ac97 **' parameter was garanted
> > to be set to NULL, now it is left as-is.
> >
> > I've checked all callers and apparently this is fine because the
> > probes fail if snd_ac97_mixer() returns an error.
> >
> > However, some drivers with several mixers seem to rely on the value
> > being NULL in case of error.
> >
> > See [1] as an example of such code that forces a NULL value on its
> > own, to be sure.
> >
> > So, wouldn't it be safer to leave a "*rac97 = NULL;" just after the
> > added sanity check?
>
> Yes, we need the NULL initialization.
> Care to submit an additional fix patch?

Hi,

Su Hui already did.

CJ

> thanks,
>
> Takashi
>
> > CJ
> >
> >
> > [1]:
> > https://elixir.bootlin.com/linux/v6.5-rc7/source/sound/pci/atiixp.c#L1438
> >
> > > Fixes: da3cec35dd3c ("ALSA: Kill snd_assert() in sound/pci/*")
> > > Signed-off-by: Su Hui <suhui@xxxxxxxxxxxx>
> > > ---
> > >    sound/pci/ac97/ac97_codec.c | 4 ++--
> > >    1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/sound/pci/ac97/ac97_codec.c b/sound/pci/ac97/ac97_codec.c
> > > index 9afc5906d662..80a65b8ad7b9 100644
> > > --- a/sound/pci/ac97/ac97_codec.c
> > > +++ b/sound/pci/ac97/ac97_codec.c
> > > @@ -2069,8 +2069,8 @@ int snd_ac97_mixer(struct snd_ac97_bus *bus, struct snd_ac97_template *template,
> > >    		.dev_disconnect =	snd_ac97_dev_disconnect,
> > >    	};
> > >    -	if (rac97)
> > > -		*rac97 = NULL;
> > > +	if (!rac97)
> > > +		return -EINVAL;
> > >    	if (snd_BUG_ON(!bus || !template))
> > >    		return -EINVAL;
> > >    	if (snd_BUG_ON(template->num >= 4))