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Subject[PATCH] sound/oss: fix deadlock in sequencer_ioctl(SNDCTL_SEQ_OUTOFBAND)
Date
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>
---
sound/oss/sequencer.c | 12 ++----------
1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/sound/oss/sequencer.c b/sound/oss/sequencer.c
index c0eea1dfe90f..f19da4b47c1d 100644
--- a/sound/oss/sequencer.c
+++ b/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:
--
1.9.1


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