Re: [PATCH] ASoC: topology: Avoid card NULL deref in snd_soc_tplg_component_remove()

["Amadeusz Sławiński" <amadeuszx.slawinski@linux.intel.com>]

On 6/3/2022 10:14 PM, Dean Gehnert wrote:
> Don't deference card in comp->card->snd_card before checking for NULL card.
> 
> During the unloading of ASoC kernel modules, there is a kernel oops in
> snd_soc_tplg_component_remove() that happens because comp->card is set to
> NULL in soc_cleanup_component().
> 
> Cc: Liam Girdwood <lgirdwood@gmail.com>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Jaroslav Kysela <perex@perex.cz>
> Cc: Takashi Iwai <tiwai@suse.com>
> Cc: alsa-devel@alsa-project.org
> Cc: linux-kernel@vger.kernel.org
> Cc: stable@vger.kernel.org
> Fixes: 7e567b5ae063 ("ASoC: topology: Add missing rwsem around snd_ctl_remove() calls")
> Signed-off-by: Dean Gehnert <deang@tpi.com>
> ---
>   sound/soc/soc-topology.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/sound/soc/soc-topology.c b/sound/soc/soc-topology.c
> index 3f9d314fba16..cf0efe1147c2 100644
> --- a/sound/soc/soc-topology.c
> +++ b/sound/soc/soc-topology.c
> @@ -2613,15 +2613,18 @@ EXPORT_SYMBOL_GPL(snd_soc_tplg_component_load);
>   /* remove dynamic controls from the component driver */
>   int snd_soc_tplg_component_remove(struct snd_soc_component *comp)
>   {
> -	struct snd_card *card = comp->card->snd_card;
> +	struct snd_card *card;
>   	struct snd_soc_dobj *dobj, *next_dobj;
>   	int pass;
>   
>   	/* process the header types from end to start */
>   	for (pass = SOC_TPLG_PASS_END; pass >= SOC_TPLG_PASS_START; pass--) {
>   
> +		card = (comp->card) ? comp->card->snd_card : NULL;
> +
>   		/* remove mixer controls */
> -		down_write(&card->controls_rwsem);
> +		if (card)
> +			down_write(&card->controls_rwsem);
>   		list_for_each_entry_safe(dobj, next_dobj, &comp->dobj_list,
>   			list) {

I'm pretty sure that quite a lot of operations in this 
list_for_each_entry_safe() loop require existing card...

And trying to investigate more closely, there is no 
soc_cleanup_component() mentioned in commit message, for quite a few 
kernel versions - seems to have been removed during v5.5-rc1.

I would say to not merge this, unless problem can be reproduced with 
latest kernel and even then would consider if it is a correct fix.

>   
> @@ -2660,7 +2663,8 @@ int snd_soc_tplg_component_remove(struct snd_soc_component *comp)
>   				break;
>   			}
>   		}
> -		up_write(&card->controls_rwsem);
> +		if (card)
> +			up_write(&card->controls_rwsem);
>   	}
>   
>   	/* let caller know if FW can be freed when no objects are left */