ALSA: seq: Fix copy_from_user() call inside lock

The event handler in the virmidi sequencer code takes a read-lock for
the linked list traverse, while it's calling snd_seq_dump_var_event()
in the loop.  The latter function may expand the user-space data
depending on the event type.  It eventually invokes copy_from_user(),
which might be a potential dead-lock.

The sequencer core guarantees that the user-space data is passed only
with atomic=0 argument, but snd_virmidi_dev_receive_event() ignores it
and always takes read-lock().  For avoiding the problem above, this
patch introduces rwsem for non-atomic case, while keeping rwlock for
atomic case.

Also while we're at it: the superfluous irq flags is dropped in
snd_virmidi_input_open().

Reported-by: Jia-Ju Bai <baijiaju1990@163.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

2 files changed, 20 insertions, 8 deletions

diff --git a/include/sound/seq_virmidi.h b/include/sound/seq_virmidi.h
index a03acd0d398a..695257ae64ac 100644
--- a/include/sound/seq_virmidi.h
+++ b/include/sound/seq_virmidi.h
@@ -60,6 +60,7 @@ struct snd_virmidi_dev {
         int port;                       /* created/attached port */
         unsigned int flags;             /* SNDRV_VIRMIDI_* */
         rwlock_t filelist_lock;
+        struct rw_semaphore filelist_sem;
         struct list_head filelist;
 };
 
diff --git a/sound/core/seq/seq_virmidi.c b/sound/core/seq/seq_virmidi.c
index 8d93a4021c78..f48a4cd24ffc 100644
--- a/sound/core/seq/seq_virmidi.c
+++ b/sound/core/seq/seq_virmidi.c
@@ -77,13 +77,17 @@ static void snd_virmidi_init_event(struct snd_virmidi *vmidi,
  * decode input event and put to read buffer of each opened file
  */
 static int snd_virmidi_dev_receive_event(struct snd_virmidi_dev *rdev,
-                                         struct snd_seq_event *ev)
+                                         struct snd_seq_event *ev,
+                                         bool atomic)
 {
         struct snd_virmidi *vmidi;
         unsigned char msg[4];
         int len;
 
-        read_lock(&rdev->filelist_lock);
+        if (atomic)
+                read_lock(&rdev->filelist_lock);
+        else
+                down_read(&rdev->filelist_sem);
         list_for_each_entry(vmidi, &rdev->filelist, list) {
                 if (!vmidi->trigger)
                         continue;
@@ -97,7 +101,10 @@ static int snd_virmidi_dev_receive_event(struct snd_virmidi_dev *rdev,
                                 snd_rawmidi_receive(vmidi->substream, msg, len);
                 }
         }
-        read_unlock(&rdev->filelist_lock);
+        if (atomic)
+                read_unlock(&rdev->filelist_lock);
+        else
+                up_read(&rdev->filelist_sem);
 
         return 0;
 }
@@ -115,7 +122,7 @@ int snd_virmidi_receive(struct snd_rawmidi *rmidi, struct snd_seq_event *ev)
         struct snd_virmidi_dev *rdev;
 
         rdev = rmidi->private_data;
-        return snd_virmidi_dev_receive_event(rdev, ev);
+        return snd_virmidi_dev_receive_event(rdev, ev, true);
 }
 #endif  /*  0  */
 
@@ -130,7 +137,7 @@ static int snd_virmidi_event_input(struct snd_seq_event *ev, int direct,
         rdev = private_data;
         if (!(rdev->flags & SNDRV_VIRMIDI_USE))
                 return 0; /* ignored */
-        return snd_virmidi_dev_receive_event(rdev, ev);
+        return snd_virmidi_dev_receive_event(rdev, ev, atomic);
 }
 
 /*
@@ -209,7 +216,6 @@ static int snd_virmidi_input_open(struct snd_rawmidi_substream *substream)
         struct snd_virmidi_dev *rdev = substream->rmidi->private_data;
         struct snd_rawmidi_runtime *runtime = substream->runtime;
         struct snd_virmidi *vmidi;
-        unsigned long flags;
 
         vmidi = kzalloc(sizeof(*vmidi), GFP_KERNEL);
         if (vmidi == NULL)
@@ -223,9 +229,11 @@ static int snd_virmidi_input_open(struct snd_rawmidi_substream *substream)
         vmidi->client = rdev->client;
         vmidi->port = rdev->port;       
         runtime->private_data = vmidi;
-        write_lock_irqsave(&rdev->filelist_lock, flags);
+        down_write(&rdev->filelist_sem);
+        write_lock_irq(&rdev->filelist_lock);
         list_add_tail(&vmidi->list, &rdev->filelist);
-        write_unlock_irqrestore(&rdev->filelist_lock, flags);
+        write_unlock_irq(&rdev->filelist_lock);
+        up_write(&rdev->filelist_sem);
         vmidi->rdev = rdev;
         return 0;
 }
@@ -264,9 +272,11 @@ 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;
 
+        down_write(&rdev->filelist_sem);
         write_lock_irq(&rdev->filelist_lock);
         list_del(&vmidi->list);
         write_unlock_irq(&rdev->filelist_lock);
+        up_write(&rdev->filelist_sem);
         snd_midi_event_free(vmidi->parser);
         substream->runtime->private_data = NULL;
         kfree(vmidi);
@@ -520,6 +530,7 @@ int snd_virmidi_new(struct snd_card *card, int device, struct snd_rawmidi **rrmi
         rdev->rmidi = rmidi;
         rdev->device = device;
         rdev->client = -1;
+        init_rwsem(&rdev->filelist_sem);
         rwlock_init(&rdev->filelist_lock);
         INIT_LIST_HEAD(&rdev->filelist);
         rdev->seq_mode = SNDRV_VIRMIDI_SEQ_DISPATCH;