[PATCH 1/2] ALSA: seq: oss: Fix data-race for max_midi_devs access

[Takashi Iwai]

ALSA OSS sequencer refers to a global variable max_midi_devs at
creating a new port, storing it to its own field.  Meanwhile this
variable may be changed by other sequencer events at
snd_seq_oss_midi_check_exit_port() in parallel, which may cause a data
race.

OTOH, this data race itself is almost harmless, as the access to the
MIDI device is done via get_mdev() and it's protected with a refcount,
hence its presence is guaranteed.

Though, it's sill better to address the data-race from the code sanity
POV, and this patch adds the proper spinlock for the protection.

Reported-by: Abhishek Shah <abhishek.shah at columbia.edu>
Cc: <stable at vger.kernel.org>
Link: https://lore.kernel.org/r/CAEHB2493pZRXs863w58QWnUTtv3HHfg85aYhLn5HJHCwxqtHQg@mail.gmail.com
Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 sound/core/seq/oss/seq_oss_midi.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sound/core/seq/oss/seq_oss_midi.c b/sound/core/seq/oss/seq_oss_midi.c
index 1e3bf086f867..07efb38f58ac 100644
--- a/sound/core/seq/oss/seq_oss_midi.c
+++ b/sound/core/seq/oss/seq_oss_midi.c
@@ -270,7 +270,9 @@ snd_seq_oss_midi_clear_all(void)
 void
 snd_seq_oss_midi_setup(struct seq_oss_devinfo *dp)
 {
+	spin_lock_irq(&register_lock);
 	dp->max_mididev = max_midi_devs;
+	spin_unlock_irq(&register_lock);
 }
 
 /*
-- 
2.35.3