linux-kernel - Re: [PATCH] ALSA: ac97: Fix possible NULL dereference in snd_ac97

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Date:   Wed, 23 Aug 2023 19:18:53 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     Su Hui <suhui@...china.com>, Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, Arnd Bergmann <arnd@...db.de>,
        maciej.szmigiero@...cle.com, yangyingliang@...wei.com,
        alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] ALSA: ac97: Fix possible NULL dereference in
 snd_ac97_mixer

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@...china.com>
>>> ---
>>>    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))
>>
>