Commit 50e9ffb1 authored Jul 26, 2018 by Takashi Iwai

ALSA: virmidi: Fix too long output trigger loop



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@gmail.com>
Reported-by:  <syzbot+619d9f40141d826b097e@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

parent 69756930

sound/core/seq/seq_virmidi.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -163,6 +163,7 @@ static void snd_virmidi_output_trigger(struct snd_rawmidi_substream *substream,
		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(struct snd_rawmidi_substream *substream,
		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);
		}
		out:
		spin_unlock_irqrestore(&substream->runtime->lock, flags);

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