linux-kernel - Re: [PATCH] sound/oss: fix deadlock in sequencer_ioctl(SNDCTL_SEQ

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Date:	Sat, 18 Apr 2015 09:06:33 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Alexey Khoroshilov <khoroshilov@...ras.ru>
Cc:	Jaroslav Kysela <perex@...ex.cz>, alsa-devel@...a-project.org,
	linux-kernel@...r.kernel.org, ldv-project@...uxtesting.org,
	security@...nel.org
Subject: Re: [PATCH] sound/oss: fix deadlock in sequencer_ioctl(SNDCTL_SEQ_OUTOFBAND)

At Sat, 18 Apr 2015 02:53:25 +0300,
Alexey Khoroshilov wrote:
> 
> 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@...ras.ru>

Applied, thanks.


Takashi

> ---
>  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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