[PATCH 6.8.y-and-older] ALSA: timer: Set lower bound of start tick time

[Takashi Iwai <tiwai@suse.de>]

commit 4a63bd179fa8d3fcc44a0d9d71d941ddd62f0c4e upstream.

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.

[ backport note: the error handling is changed, as the original commit
  is based on the recent cleanup with guard() in commit beb45974dd49
  -- tiwai ]

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>
---

Greg, this is an alternative fix to the original cherry-pick; apply
to 6.8.y and older stable kernels.  Thanks!


 sound/core/timer.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/sound/core/timer.c b/sound/core/timer.c
index e6e551d4a29e..a0b515981ee9 100644
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -553,6 +553,16 @@ static int snd_timer_start1(struct snd_timer_instance *timeri,
 		goto unlock;
 	}
 
+	/* 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) {
+			result = -EINVAL;
+			goto unlock;
+		}
+	}
+
 	if (start)
 		timeri->ticks = timeri->cticks = ticks;
 	else if (!timeri->cticks)
-- 
2.43.0