Re: [PATCH 1/2] ALSA: core: Fix NULL module pointer assignment at card init

[Xu Yang <xu.yang_2@xxxxxxx>]

On Wed, May 22, 2024 at 09:04:39AM +0200, Takashi Iwai wrote:
> The commit 81033c6b584b ("ALSA: core: Warn on empty module")
> introduced a WARN_ON() for a NULL module pointer passed at snd_card
> object creation, and it also wraps the code around it with '#ifdef
> MODULE'.  This works in most cases, but the devils are always in
> details.  "MODULE" is defined when the target code (i.e. the sound
> core) is built as a module; but this doesn't mean that the caller is
> also built-in or not.  Namely, when only the sound core is built-in
> (CONFIG_SND=y) while the driver is a module (CONFIG_SND_USB_AUDIO=m),
> the passed module pointer is ignored even if it's non-NULL, and
> card->module remains as NULL.  This would result in the missing module
> reference up/down at the device open/close, leading to a race with the
> code execution after the module removal.
> 
> For addressing the bug, move the assignment of card->module again out
> of ifdef.  The WARN_ON() is still wrapped with ifdef because the
> module can be really NULL when all sound drivers are built-in.
> 
> Note that we keep 'ifdef MODULE' for WARN_ON(), otherwise it would
> lead to a false-positive NULL module check.  Admittedly it won't catch
> perfectly, i.e. no check is performed when CONFIG_SND=y.  But, it's no
> real problem as it's only for debugging, and the condition is pretty
> rare.
> 
> Fixes: 81033c6b584b ("ALSA: core: Warn on empty module")
> Reported-by: Xu Yang <xu.yang_2@xxxxxxx>
> Closes: https://lore.kernel.org/r/20240520170349.2417900-1-xu.yang_2@xxxxxxx
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>

For this patchset:
Tested-by: Xu Yang <xu.yang_2@xxxxxxx>

Thanks,
Xu Yang

> ---
>  sound/core/init.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sound/core/init.c b/sound/core/init.c
> index 6b127864a1a3..ac072614d1ea 100644
> --- a/sound/core/init.c
> +++ b/sound/core/init.c
> @@ -313,8 +313,8 @@ static int snd_card_init(struct snd_card *card, struct device *parent,
>  	card->number = idx;
>  #ifdef MODULE
>  	WARN_ON(!module);
> -	card->module = module;
>  #endif
> +	card->module = module;
>  	INIT_LIST_HEAD(&card->devices);
>  	init_rwsem(&card->controls_rwsem);
>  	rwlock_init(&card->ctl_files_rwlock);
> -- 
> 2.43.0
>