linux-kernel - [PATCH] ALSA: aloop: Avoid pointer dereference before null-check

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Date:   Wed, 27 Nov 2019 05:06:22 -0600
From:   Andrew Gabbasov <andrew_gabbasov@...tor.com>
To:     <alsa-devel@...a-project.org>, <linux-kernel@...r.kernel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        Eugeniu Rosca <erosca@...adit-jv.com>,
        Timo Wischer <twischer@...adit-jv.com>,
        Andrew Gabbasov <andrew_gabbasov@...tor.com>
Subject: [PATCH] ALSA: aloop: Avoid pointer dereference before null-check

Static analysis tools (cppcheck and PVS Studio) report an error
in loopback_snd_timer_period_elapsed() regarding dpcm_play pointer
dereference earlier than its null-check. And although this is a result
of a formal check, and the pointer correctness is also protected
by having a corresponding bit set in the "running" mask, re-ordering
of the lines can imake the code even formally correct and eliminate
those static analysis error reports.

Fixes: 26c53379f98d ("ALSA: aloop: Support selection of snd_timer instead of jiffies")
Reported-by: Eugeniu Rosca <erosca@...adit-jv.com>
Signed-off-by: Andrew Gabbasov <andrew_gabbasov@...tor.com>
---
 sound/drivers/aloop.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c
index 0ebfbe70db00..6bb46423f5ae 100644
--- a/sound/drivers/aloop.c
+++ b/sound/drivers/aloop.c
@@ -727,10 +727,6 @@ static void loopback_snd_timer_period_elapsed(struct loopback_cable *cable,
 
 	dpcm_play = cable->streams[SNDRV_PCM_STREAM_PLAYBACK];
 	dpcm_capt = cable->streams[SNDRV_PCM_STREAM_CAPTURE];
-	substream_play = (running & (1 << SNDRV_PCM_STREAM_PLAYBACK)) ?
-			dpcm_play->substream : NULL;
-	substream_capt = (running & (1 << SNDRV_PCM_STREAM_CAPTURE)) ?
-			dpcm_capt->substream : NULL;
 
 	if (event == SNDRV_TIMER_EVENT_MSTOP) {
 		if (!dpcm_play ||
@@ -741,6 +737,10 @@ static void loopback_snd_timer_period_elapsed(struct loopback_cable *cable,
 		}
 	}
 
+	substream_play = (running & (1 << SNDRV_PCM_STREAM_PLAYBACK)) ?
+			dpcm_play->substream : NULL;
+	substream_capt = (running & (1 << SNDRV_PCM_STREAM_CAPTURE)) ?
+			dpcm_capt->substream : NULL;
 	valid_runtime = (running & (1 << SNDRV_PCM_STREAM_PLAYBACK)) ?
 				dpcm_play->substream->runtime :
 				dpcm_capt->substream->runtime;
-- 
2.21.0