[PATCH 6.6.y] ALSA: pcm: oss: Fix data race at accessing runtime.oss.trigger

[Artem Dinaburg <artem@trailofbits.com>]

From: Takashi Iwai <tiwai@suse.de>

[ Upstream commit 901ac0ff15edf9503162e2cf6579bd11a30f1ed4 ]

Currently the runtime.oss.trigger field may be accessed concurrently
without protection, which may lead to the data race.  And, in this
case, it may lead to more severe problem because it's a bit field; as
writing the data, it may overwrite other bit fields as well, which
confuses the operation completely, as spotted by fuzzing.

Fix it by covering runtime.oss.trigger bit fled also with the existing
params_lock mutex in both snd_pcm_oss_get_trigger() and
snd_pcm_oss_poll().

Reported-and-tested-by: Jaeyoung Chung <jjy600901@snu.ac.kr>
Closes: https://lore.kernel.org/20260423145330.210035-1-jjy600901@snu.ac.kr
Cc: <stable@vger.kernel.org>
Link: https://patch.msgid.link/20260424112205.123703-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Assisted-by: LLM
[ Backport to 6.6.y: Replace upstream mutex guards with explicit
  params_lock operations and matching unlocks; the target has no scoped
  locking helpers. ]
Signed-off-by: Artem Dinaburg <artem@trailofbits.com>
---
Hi,

Please consider this backport of the fix for CVE-2026-46157 to 6.6.y.

The code changes are identical to upstream, except explicit params_lock
operations replace upstream mutex guards; see the bracketed backport note
above.

The complete fix is already present in 6.12.y, 6.18.y and 7.2.y.

Build-tested only as part of a cumulative 6.6.y series: x86_64 objects
(allmodconfig, CONFIG_WERROR=y); no runtime testing.

Thanks,
Artem Dinaburg

 sound/core/oss/pcm_oss.c |   32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/sound/core/oss/pcm_oss.c b/sound/core/oss/pcm_oss.c
index 23ecbd70ccf653aa70a2b08aecf781a194afcc66..ce63fe4ecf707670a1a4daab307f2258b00697d8 100644
--- a/sound/core/oss/pcm_oss.c
+++ b/sound/core/oss/pcm_oss.c
@@ -2161,10 +2161,18 @@ static int snd_pcm_oss_get_trigger(struct snd_pcm_oss_file *pcm_oss_file)
 
 	psubstream = pcm_oss_file->streams[SNDRV_PCM_STREAM_PLAYBACK];
 	csubstream = pcm_oss_file->streams[SNDRV_PCM_STREAM_CAPTURE];
-	if (psubstream && psubstream->runtime && psubstream->runtime->oss.trigger)
-		result |= PCM_ENABLE_OUTPUT;
-	if (csubstream && csubstream->runtime && csubstream->runtime->oss.trigger)
-		result |= PCM_ENABLE_INPUT;
+	if (psubstream && psubstream->runtime) {
+		mutex_lock(&psubstream->runtime->oss.params_lock);
+		if (psubstream->runtime->oss.trigger)
+			result |= PCM_ENABLE_OUTPUT;
+		mutex_unlock(&psubstream->runtime->oss.params_lock);
+	}
+	if (csubstream && csubstream->runtime) {
+		mutex_lock(&csubstream->runtime->oss.params_lock);
+		if (csubstream->runtime->oss.trigger)
+			result |= PCM_ENABLE_INPUT;
+		mutex_unlock(&csubstream->runtime->oss.params_lock);
+	}
 	return result;
 }
 
@@ -2839,6 +2847,18 @@ static int snd_pcm_oss_capture_ready(struct snd_pcm_substream *substream)
 						runtime->oss.period_frames;
 }
 
+static bool need_input_retrigger(struct snd_pcm_runtime *runtime)
+{
+	bool ret;
+
+	mutex_lock(&runtime->oss.params_lock);
+	ret = runtime->oss.trigger;
+	if (ret)
+		runtime->oss.trigger = 0;
+	mutex_unlock(&runtime->oss.params_lock);
+	return ret;
+}
+
 static __poll_t snd_pcm_oss_poll(struct file *file, poll_table * wait)
 {
 	struct snd_pcm_oss_file *pcm_oss_file;
@@ -2871,11 +2891,11 @@ static __poll_t snd_pcm_oss_poll(struct file *file, poll_table * wait)
 		    snd_pcm_oss_capture_ready(csubstream))
 			mask |= EPOLLIN | EPOLLRDNORM;
 		snd_pcm_stream_unlock_irq(csubstream);
-		if (ostate != SNDRV_PCM_STATE_RUNNING && runtime->oss.trigger) {
+		if (ostate != SNDRV_PCM_STATE_RUNNING &&
+		    need_input_retrigger(runtime)) {
 			struct snd_pcm_oss_file ofile;
 			memset(&ofile, 0, sizeof(ofile));
 			ofile.streams[SNDRV_PCM_STREAM_CAPTURE] = pcm_oss_file->streams[SNDRV_PCM_STREAM_CAPTURE];
-			runtime->oss.trigger = 0;
 			snd_pcm_oss_set_trigger(&ofile, PCM_ENABLE_INPUT);
 		}
 	}