linux-kernel - [PATCH 4.9 21/42] ALSA: seq: oss: Fix data-race for max_midi

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Message-Id: <20220913140343.344878143@linuxfoundation.org>
Date:   Tue, 13 Sep 2022 16:07:52 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Abhishek Shah <abhishek.shah@...umbia.edu>,
        Takashi Iwai <tiwai@...e.de>
Subject: [PATCH 4.9 21/42] ALSA: seq: oss: Fix data-race for max_midi_devs access

From: Takashi Iwai <tiwai@...e.de>

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@...umbia.edu>
Cc: <stable@...r.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@...e.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 sound/core/seq/oss/seq_oss_midi.c |    2 ++
 1 file changed, 2 insertions(+)

--- 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);
 }

 /*