Commit ed8b1d6d authored Feb 09, 2016 by Takashi Iwai

ALSA: timer: Fix race between stop and interrupt

A slave timer element also unlinks at snd_timer_stop() but it takes
only slave_active_lock.  When a slave is assigned to a master,
however, this may become a race against the master's interrupt
handling, eventually resulting in a list corruption.  The actual bug
could be seen with a syzkaller fuzzer test case in BugLink below.

As a fix, we need to take timeri->timer->lock when timer isn't NULL,
i.e. assigned to a master, while the assignment to a master itself is
protected by slave_active_lock.

BugLink: http://lkml.kernel.org/r/CACT4Y+Y_Bm+7epAb=8Wi=AaWd+DYS7qawX52qxdCfOfY49vozQ@mail.gmail.com


Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

parent 117159f0

sound/core/timer.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -518,9 +518,13 @@ static int _snd_timer_stop(struct snd_timer_instance *timeri, int event)
		spin_unlock_irqrestore(&slave_active_lock, flags);
		return -EBUSY;
		}
		if (timeri->timer)
		spin_lock(&timeri->timer->lock);
		timeri->flags &= ~SNDRV_TIMER_IFLG_RUNNING;
		list_del_init(&timeri->ack_list);
		list_del_init(&timeri->active_list);
		if (timeri->timer)
		spin_unlock(&timeri->timer->lock);
		spin_unlock_irqrestore(&slave_active_lock, flags);
		goto __end;
		}

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