[PATCH 5.15 005/132] ALSA: Use del_timer_sync() before freeing timer

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From: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

commit f0a868788fcbf63cdab51f5adcf73b271ede8164 upstream.

The current code for freeing the emux timer is extremely dangerous:

  CPU0				CPU1
  ----				----
snd_emux_timer_callback()
			    snd_emux_free()
			      spin_lock(&emu->voice_lock)
			      del_timer(&emu->tlist); <-- returns immediately
			      spin_unlock(&emu->voice_lock);
			      [..]
			      kfree(emu);

  spin_lock(&emu->voice_lock);

 [BOOM!]

Instead just use del_timer_sync() which will wait for the timer to finish
before continuing. No need to check if the timer is active or not when
doing so.

This doesn't fix the race of a possible re-arming of the timer, but at
least it won't use the data that has just been freed.

[ Fixed unused variable warning by tiwai ]

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Reviewed-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20221026231236.6834b551@xxxxxxxxxxxxxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 sound/synth/emux/emux.c |    7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

--- a/sound/synth/emux/emux.c
+++ b/sound/synth/emux/emux.c
@@ -126,15 +126,10 @@ EXPORT_SYMBOL(snd_emux_register);
  */
 int snd_emux_free(struct snd_emux *emu)
 {
-	unsigned long flags;
-
 	if (! emu)
 		return -EINVAL;
 
-	spin_lock_irqsave(&emu->voice_lock, flags);
-	if (emu->timer_active)
-		del_timer(&emu->tlist);
-	spin_unlock_irqrestore(&emu->voice_lock, flags);
+	del_timer_sync(&emu->tlist);
 
 	snd_emux_proc_free(emu);
 	snd_emux_delete_virmidi(emu);





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