[PATCH 6.1 473/473] ALSA: timer: Set lower bound of start tick time

Greg Kroah-Hartman
2024-06-06T14:21:04Z

From: 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.

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>
[ backport note: the error handling is changed, as the original commit
is based on the recent cleanup with guard() in commit beb45974dd49
-- tiwai ]
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
sound/core/timer.c | 10 ++++++++++
1 file changed, 10 insertions(+)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -553,6 +553,16 @@ static int snd_timer_start1(struct snd_t
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)