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ALSA: Use del_timer_sync() before freeing timer

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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: [email protected]
Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Signed-off-by: Steven Rostedt (Google) <[email protected]>
Reviewed-by: Guenter Roeck <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
This commit is contained in:
Steven Rostedt (Google) 2022-10-26 23:12:36 -04:00 • committed by Greg Kroah-Hartman
commit d853b43808
1 changed files with 1 additions and 6 deletions
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7
sound/synth/emux/emux.c
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@ -125,15 +125,10 @@ 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);