[PATCH v2] ALSA: control: Synchronize LED mode accesses

[Sreeraj S Kurup <sreekuttan2156239@gmail.com>]

mode_store() updates led->mode while holding
snd_ctl_led_mutex.

However, snd_ctl_led_set_state() and mode_show() read this
field without holding the same mutex.

This can cause a data race when userspace changes the mode
while another path reads it.

Read the mode under snd_ctl_led_mutex in
snd_ctl_led_set_state() and use the local value after
releasing the mutex.

Also protect the mode read in mode_show() with the mutex.

Fixes: cb17fe0045aa ("ALSA: control - add sysfs support to the LED trigger module")
Signed-off-by: Sreeraj S Kurup <sreekuttan2156239@gmail.com>
---
v2:
 - Add the Fixes trailer.

 sound/core/control_led.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/sound/core/control_led.c b/sound/core/control_led.c
index 501387287ae6..044430b2f1a2 100644
--- a/sound/core/control_led.c
+++ b/sound/core/control_led.c
@@ -140,6 +140,7 @@ static void snd_ctl_led_set_state(struct snd_card *card, unsigned int access,
 {
 	struct snd_ctl_led *led;
 	struct snd_ctl_led_ctl *lctl;
+	enum snd_ctl_led_mode mode;
 	int route;
 	bool found;
 
@@ -168,8 +169,9 @@ static void snd_ctl_led_set_state(struct snd_card *card, unsigned int access,
 				UPDATE_ROUTE(route, snd_ctl_led_get(lctl));
 			}
 		}
+		mode = led->mode;
 	}
-	switch (led->mode) {
+	switch (mode) {
 	case MODE_OFF:		route = 1; break;
 	case MODE_ON:		route = 0; break;
 	case MODE_FOLLOW_ROUTE:	if (route >= 0) route ^= 1; break;
@@ -382,6 +384,8 @@ static ssize_t mode_show(struct device *dev,
 	struct snd_ctl_led *led = container_of(dev, struct snd_ctl_led, dev);
 	const char *str = NULL;
 
+	guard(mutex)(&snd_ctl_led_mutex);
+
 	switch (led->mode) {
 	case MODE_FOLLOW_MUTE:	str = "follow-mute"; break;
 	case MODE_FOLLOW_ROUTE:	str = "follow-route"; break;
-- 
2.55.0