ALSA: virmidi: Fix too long output trigger loop

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

diff --git a/sound/core/seq/seq_virmidi.c b/sound/core/seq/seq_virmidi.c
index 289ae6bb81d9d1f86f24b5bedc5a49feb8b00c0c..8ebbca554e995f47eff787d0ffd36fc244abf7d8 100644 (file)
--- a/sound/core/seq/seq_virmidi.c
+++ b/sound/core/seq/seq_virmidi.c
@@ -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);