[PATCH 3.18 08/56] ALSA: virmidi: Fix too long output trigger loop

From: Greg Kroah-Hartman
Date: Sun Aug 26 2018 - 02:46:28 EST

3.18-stable review patch. If anyone has any objections, please let me know.


From: Takashi Iwai <tiwai@xxxxxxx>

commit 50e9ffb1996a5d11ff5040a266585bad4ceeca0a upstream.

The virmidi output trigger tries to parse the all available bytes and
process sequencer events as much as possible. In a normal situation,
this is supposed to be relatively short, but a program may give a huge
buffer and it'll take a long time in a single spin lock, which may
eventually lead to a soft lockup.

This patch simply adds a workaround, a cond_resched() call in the loop
if applicable. A better solution would be to move the event processor
into a work, but let's put a duct-tape quickly at first.

Reported-and-tested-by: Dae R. Jeong <threeearcat@xxxxxxxxx>
Reported-by: syzbot+619d9f40141d826b097e@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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

--- a/sound/core/seq/seq_virmidi.c
+++ b/sound/core/seq/seq_virmidi.c
@@ -163,6 +163,7 @@ static void snd_virmidi_output_trigger(s
int count, res;
unsigned char buf[32], *pbuf;
unsigned long flags;
+ bool check_resched = !in_atomic();

if (up) {
vmidi->trigger = 1;
@@ -200,6 +201,15 @@ static void snd_virmidi_output_trigger(s
vmidi->event.type = SNDRV_SEQ_EVENT_NONE;
+ if (!check_resched)
+ continue;
+ /* do temporary unlock & cond_resched() for avoiding
+ * CPU soft lockup, which may happen via a write from
+ * a huge rawmidi buffer
+ */
+ spin_unlock_irqrestore(&substream->runtime->lock, flags);
+ cond_resched();
+ spin_lock_irqsave(&substream->runtime->lock, flags);
spin_unlock_irqrestore(&substream->runtime->lock, flags);