linux-kernel - [PATCH] ALSA: pcm: oss: Fix race at SNDCTL_DSP_SETTRIGGER There is a small race window at snd_pcm_oss_set_trigger() that is called from OSS PCM SNDCTL_DSP_SETTRIGGER ioctl; namely the function calls snd_pcm_oss_make_ready() at first, then takes the params_lock mutex for the rest. When the stream is set up again by another thread between them, it leads to inconsistency, and may result in unexpected results such as NULL dereference of OSS buffer as a fuzzer spotted recently.

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Date:   Wed, 20 Sep 2023 10:35:36 +0800
From:   Ma Ke <make_ruc2021@....com>
To:     perex@...ex.cz, tiwai@...e.com, mhocko@...e.com,
        akpm@...ux-foundation.org, make_ruc2021@....com,
        mgorman@...hsingularity.net, 42.hyeyoo@...il.com,
        surenb@...gle.com, alsa-devel@...a-project.org
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH] ALSA: pcm: oss: Fix race at SNDCTL_DSP_SETTRIGGER There is a small race window at snd_pcm_oss_set_trigger() that is called from OSS PCM SNDCTL_DSP_SETTRIGGER ioctl; namely the function calls snd_pcm_oss_make_ready() at first, then takes the params_lock mutex for the rest.  When the stream is set up again by another thread between them, it leads to inconsistency, and may result in unexpected results such as NULL dereference of OSS buffer as a fuzzer spotted recently.

The fix is simply to cover snd_pcm_oss_make_ready() call into the same
params_lock mutex with snd_pcm_oss_make_ready_locked() variant.

Signed-off-by: Ma Ke <make_ruc2021@....com>
---
 sound/core/oss/pcm_oss.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/sound/core/oss/pcm_oss.c b/sound/core/oss/pcm_oss.c
index 728c211142d1..9a830aeeba63 100644
--- a/sound/core/oss/pcm_oss.c
+++ b/sound/core/oss/pcm_oss.c
@@ -2083,21 +2083,14 @@ static int snd_pcm_oss_set_trigger(struct snd_pcm_oss_file *pcm_oss_file, int tr
 	psubstream = pcm_oss_file->streams[SNDRV_PCM_STREAM_PLAYBACK];
 	csubstream = pcm_oss_file->streams[SNDRV_PCM_STREAM_CAPTURE];
 
-	if (psubstream) {
-		err = snd_pcm_oss_make_ready(psubstream);
-		if (err < 0)
-			return err;
-	}
-	if (csubstream) {
-		err = snd_pcm_oss_make_ready(csubstream);
-		if (err < 0)
-			return err;
-	}
       	if (psubstream) {
       		runtime = psubstream->runtime;
 		cmd = 0;
 		if (mutex_lock_interruptible(&runtime->oss.params_lock))
 			return -ERESTARTSYS;
+		err = snd_pcm_oss_make_ready_locked(psubstream);
+		if (err < 0)
+			goto _skip1;
 		if (trigger & PCM_ENABLE_OUTPUT) {
 			if (runtime->oss.trigger)
 				goto _skip1;
@@ -2128,6 +2121,9 @@ static int snd_pcm_oss_set_trigger(struct snd_pcm_oss_file *pcm_oss_file, int tr
 		cmd = 0;
 		if (mutex_lock_interruptible(&runtime->oss.params_lock))
 			return -ERESTARTSYS;
+		err = snd_pcm_oss_make_ready_locked(csubstream);
+		if (err < 0)
+			goto _skip2;
 		if (trigger & PCM_ENABLE_INPUT) {
 			if (runtime->oss.trigger)
 				goto _skip2;
-- 
2.37.2