ALSA: timer: Set lower bound of start tick time

Currently ALSA timer doesn't have the lower limit of the start tick
time, and it allows a very small size, e.g. 1 tick with 1ns resolution
for hrtimer.  Such a situation may lead to an unexpected RCU stall,
where  the callback repeatedly queuing the expire update, as reported
by fuzzer.

This patch introduces a sanity check of the timer start tick time, so
that the system returns an error when a too small start size is set.
As of this patch, the lower limit is hard-coded to 100us, which is
small enough but can still work somehow.

Reported-by: syzbot+43120c2af6ca2938cc38@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/000000000000fa00a1061740ab6d@google.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20240514182745.4015-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/timer.c b/sound/core/timer.c
index 4d2ee99c12a3fb2a5af3dcebc4bc9a77efadcccb..d104adc75a8b0ebad1a43687b9006a39d7af4658 100644 (file)
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -544,6 +544,14 @@ static int snd_timer_start1(struct snd_timer_instance *timeri,
                             SNDRV_TIMER_IFLG_START))
                return -EBUSY;
 
+       /* check the actual time for the start tick;
+        * bail out as error if it's way too low (< 100us)
+        */
+       if (start) {
+               if ((u64)snd_timer_hw_resolution(timer) * ticks < 100000)
+                       return -EINVAL;
+       }
+
        if (start)
                timeri->ticks = timeri->cticks = ticks;
        else if (!timeri->cticks)