ALSA: seq: oss: Fix racy open/close of MIDI devices

Although snd_seq_oss_midi_open() and snd_seq_oss_midi_close() can be
called concurrently from different code paths, we have no proper data
protection against races.  Introduce open_mutex to each seq_oss_midi
object for avoiding the races.

Reported-by: "Gong, Sishuai" <sishuai@purdue.edu>
Closes: https://lore.kernel.org/r/7DC9AF71-F481-4ABA-955F-76C535661E33@purdue.edu
Link: https://lore.kernel.org/r/20230612125533.27461-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/seq/oss/seq_oss_midi.c b/sound/core/seq/oss/seq_oss_midi.c
index 07efb38..f2940b2 100644 (file)
--- a/sound/core/seq/oss/seq_oss_midi.c
+++ b/sound/core/seq/oss/seq_oss_midi.c
@@ -37,6 +37,7 @@ struct seq_oss_midi {
        struct snd_midi_event *coder;   /* MIDI event coder */
        struct seq_oss_devinfo *devinfo;        /* assigned OSSseq device */
        snd_use_lock_t use_lock;
+       struct mutex open_mutex;
 };
 
 
@@ -172,6 +173,7 @@ snd_seq_oss_midi_check_new_port(struct snd_seq_port_info *pinfo)
        mdev->flags = pinfo->capability;
        mdev->opened = 0;
        snd_use_lock_init(&mdev->use_lock);
+       mutex_init(&mdev->open_mutex);
 
        /* copy and truncate the name of synth device */
        strscpy(mdev->name, pinfo->name, sizeof(mdev->name));
@@ -322,15 +324,17 @@ snd_seq_oss_midi_open(struct seq_oss_devinfo *dp, int dev, int fmode)
        int perm;
        struct seq_oss_midi *mdev;
        struct snd_seq_port_subscribe subs;
+       int err;
 
        mdev = get_mididev(dp, dev);
        if (!mdev)
                return -ENODEV;
 
+       mutex_lock(&mdev->open_mutex);
        /* already used? */
        if (mdev->opened && mdev->devinfo != dp) {
-               snd_use_lock_free(&mdev->use_lock);
-               return -EBUSY;
+               err = -EBUSY;
+               goto unlock;
        }
 
        perm = 0;
@@ -340,14 +344,14 @@ snd_seq_oss_midi_open(struct seq_oss_devinfo *dp, int dev, int fmode)
                perm |= PERM_READ;
        perm &= mdev->flags;
        if (perm == 0) {
-               snd_use_lock_free(&mdev->use_lock);
-               return -ENXIO;
+               err = -ENXIO;
+               goto unlock;
        }
 
        /* already opened? */
        if ((mdev->opened & perm) == perm) {
-               snd_use_lock_free(&mdev->use_lock);
-               return 0;
+               err = 0;
+               goto unlock;
        }
 
        perm &= ~mdev->opened;
@@ -372,13 +376,17 @@ snd_seq_oss_midi_open(struct seq_oss_devinfo *dp, int dev, int fmode)
        }
 
        if (! mdev->opened) {
-               snd_use_lock_free(&mdev->use_lock);
-               return -ENXIO;
+               err = -ENXIO;
+               goto unlock;
        }
 
        mdev->devinfo = dp;
+       err = 0;
+
+ unlock:
+       mutex_unlock(&mdev->open_mutex);
        snd_use_lock_free(&mdev->use_lock);
-       return 0;
+       return err;
 }
 
 /*
@@ -393,10 +401,9 @@ snd_seq_oss_midi_close(struct seq_oss_devinfo *dp, int dev)
        mdev = get_mididev(dp, dev);
        if (!mdev)
                return -ENODEV;
-       if (! mdev->opened || mdev->devinfo != dp) {
-               snd_use_lock_free(&mdev->use_lock);
-               return 0;
-       }
+       mutex_lock(&mdev->open_mutex);
+       if (!mdev->opened || mdev->devinfo != dp)
+               goto unlock;
 
        memset(&subs, 0, sizeof(subs));
        if (mdev->opened & PERM_WRITE) {
@@ -415,6 +422,8 @@ snd_seq_oss_midi_close(struct seq_oss_devinfo *dp, int dev)
        mdev->opened = 0;
        mdev->devinfo = NULL;
 
+ unlock:
+       mutex_unlock(&mdev->open_mutex);
        snd_use_lock_free(&mdev->use_lock);
        return 0;
 }