[PATCH 4.19 20/86] ALSA: info: Drop WARN_ON() from buffer NULL sanity check

[Greg Kroah-Hartman]

From: Takashi Iwai <tiwai@xxxxxxx>

commit 60379ba08532eca861e933b389526a4dc89e0c42 upstream.

snd_info_get_line() has a sanity check of NULL buffer -- both buffer
itself being NULL and buffer->buffer being NULL.  Basically both
checks are valid and necessary, but the problem is that it's with
snd_BUG_ON() macro that triggers WARN_ON().  The latter condition
(NULL buffer->buffer) can be met arbitrarily by user since the buffer
is allocated at the first write, so it means that user can trigger
WARN_ON() at will.

This patch addresses it by simply moving buffer->buffer NULL check out
of snd_BUG_ON() so that spurious WARNING is no longer triggered.

Reported-by: syzbot+e42d0746c3c3699b6061@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20200717084023.5928-1-tiwai@xxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 sound/core/info.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/sound/core/info.c
+++ b/sound/core/info.c
@@ -634,7 +634,9 @@ int snd_info_get_line(struct snd_info_bu
 {
 	int c = -1;
 
-	if (snd_BUG_ON(!buffer || !buffer->buffer))
+	if (snd_BUG_ON(!buffer))
+		return 1;
+	if (!buffer->buffer)
 		return 1;
 	if (len <= 0 || buffer->stop || buffer->error)
 		return 1;