ALSA: seq: oss: Fix data-race for max_midi_devs access

commit 22dec134dbfa825b963f8a1807ad19b943e46a56 upstream.

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@columbia.edu>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/CAEHB2493pZRXs863w58QWnUTtv3HHfg85aYhLn5HJHCwxqtHQg@mail.gmail.com
Link: https://lore.kernel.org/r/20220823072717.1706-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/sound/core/seq/oss/seq_oss_midi.c b/sound/core/seq/oss/seq_oss_midi.c
index b7bef25b34cca7f39abb640c6df57bb4d9a7a602..838c3c8b403cb37e15ffc51b9e2bb294aa21cdb8 100644 (file)
--- a/sound/core/seq/oss/seq_oss_midi.c
+++ b/sound/core/seq/oss/seq_oss_midi.c
@@ -280,7 +280,9 @@ 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);
 }
 
 /*