ALSA: core: Fix NULL module pointer assignment at card init

The commit 81033c6b ("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: 81033c6b ("ALSA: core: Warn on empty module")
Reported-by: Xu Yang <xu.yang_2@nxp.com>
Closes: https://lore.kernel.org/r/20240520170349.2417900-1-xu.yang_2@nxp.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Xu Yang <xu.yang_2@nxp.com>
Link: https://lore.kernel.org/r/20240522070442.17786-1-tiwai@suse.de

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);