[added to the 4.1 stable tree] ALSA: seq: Fix race at closing in virmidi driver

From: Takashi Iwai <redacted>

This patch has been added to the 4.1 stable tree. If you have any
objections, please let us know.

===============

[ Upstream commit 2d1b5c08366acd46c35a2e9aba5d650cb5bf5c19 ]

The virmidi driver has an open race at closing its assigned rawmidi
device, and this may lead to use-after-free in
snd_seq_deliver_single_event().

Plug the hole by properly protecting the linked list deletion and
calling in the right order in snd_virmidi_input_close().

BugLink: http://lkml.kernel.org/r/CACT4Y+Zd66+w12fNN85-425cVQT=K23kWbhnCEcMB8s3us-Frw@mail.gmail.com
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <redacted>
Signed-off-by: Takashi Iwai <redacted>
Signed-off-by: Sasha Levin <redacted>
---
 sound/core/seq/seq_virmidi.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/sound/core/seq/seq_virmidi.c b/sound/core/seq/seq_virmidi.c
index 56e0f4cd..f297592 100644
--- a/sound/core/seq/seq_virmidi.c
+++ b/sound/core/seq/seq_virmidi.c

@@ -254,9 +254,13 @@ static int snd_virmidi_output_open(struct snd_rawmidi_substream *substream)
  */
 static int snd_virmidi_input_close(struct snd_rawmidi_substream *substream)
 {
+	struct snd_virmidi_dev *rdev = substream->rmidi->private_data;
 	struct snd_virmidi *vmidi = substream->runtime->private_data;
-	snd_midi_event_free(vmidi->parser);
+
+	write_lock_irq(&rdev->filelist_lock);
 	list_del(&vmidi->list);
+	write_unlock_irq(&rdev->filelist_lock);
+	snd_midi_event_free(vmidi->parser);
 	substream->runtime->private_data = NULL;
 	kfree(vmidi);
 	return 0;

-- 
2.5.0