sound/oss: fix deadlock in sequencer_ioctl(SNDCTL_SEQ_OUTOFBAND)

commit bc26d4d0 upstream.

A deadlock can be initiated by userspace via ioctl(SNDCTL_SEQ_OUTOFBAND)
on /dev/sequencer with TMR_ECHO midi event.

In this case the control flow is:
sound_ioctl()
-> case SND_DEV_SEQ:
   case SND_DEV_SEQ2:
     sequencer_ioctl()
     -> case SNDCTL_SEQ_OUTOFBAND:
          spin_lock_irqsave(&lock,flags);
          play_event();
          -> case EV_TIMING:
               seq_timing_event()
               -> case TMR_ECHO:
                    seq_copy_to_input()
                    -> spin_lock_irqsave(&lock,flags);

It seems that spin_lock_irqsave() around play_event() is not necessary,
because the only other call location in seq_startplay() makes the call
without acquiring spinlock.

So, the patch just removes spinlocks around play_event().
By the way, it removes unreachable code in seq_timing_event(),
since (seq_mode == SEQ_2) case is handled in the beginning.

Compile tested only.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

sound/oss/sequencer.c

@@ -681,13 +681,8 @@ static int seq_timing_event(unsigned char *event_rec)
 			break;
 
 		case TMR_ECHO:
-			if (seq_mode == SEQ_2)
-				seq_copy_to_input(event_rec, 8);
-			else
-			{
-				parm = (parm << 8 | SEQ_ECHO);
-				seq_copy_to_input((unsigned char *) &parm, 4);
-			}
+			parm = (parm << 8 | SEQ_ECHO);
+			seq_copy_to_input((unsigned char *) &parm, 4);
 			break;
 
 		default:;
@@ -1324,7 +1319,6 @@ int sequencer_ioctl(int dev, struct file *file, unsigned int cmd, void __user *a
 	int mode = translate_mode(file);
 	struct synth_info inf;
 	struct seq_event_rec event_rec;
-	unsigned long flags;
 	int __user *p = arg;
 
 	orig_dev = dev = dev >> 4;
@@ -1479,9 +1473,7 @@ int sequencer_ioctl(int dev, struct file *file, unsigned int cmd, void __user *a
 		case SNDCTL_SEQ_OUTOFBAND:
 			if (copy_from_user(&event_rec, arg, sizeof(event_rec)))
 				return -EFAULT;
-			spin_lock_irqsave(&lock,flags);
 			play_event(event_rec.arr);
-			spin_unlock_irqrestore(&lock,flags);
 			return 0;
 
 		case SNDCTL_MIDI_INFO: