linux-kernel - Re: [PATCH] drivers/media/rc: Fix a race condition in send

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Date:   Mon, 31 Oct 2022 10:20:09 +0530
From:   Gautam Menghani <gautammenghani201@...il.com>
To:     sean@...s.org, mchehab@...nel.org
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+0c3cb6dc05fbbdc3ad66@...kaller.appspotmail.com
Subject: Re: [PATCH] drivers/media/rc: Fix a race condition in send_packet()

On Wed, Oct 19, 2022 at 10:32:14AM +0530, Gautam Menghani wrote:
> The function send_packet() has a race condition as follows:
> func send_packet()
> {
>     // do work
>     call usb_submit_urb()
>     mutex_unlock()
>     wait_for_event_interruptible()  <-- lock gone
>     mutex_lock()
> }
> 
> func vfd_write()
> {
>     mutex_lock()
>     call send_packet()  <- prev call is not completed
>     mutex_unlock()
> }
> 
> When the mutex is unlocked and the function send_packet() waits for the
> call to complete, vfd_write() can start another call, which leads to the
> "URB submitted while active" warning in usb_submit_urb(). 
> Fix this by removing the mutex_unlock() call in send_packet() and using
> mutex_lock_interruptible().
> 
> Fixes: 21677cfc562a ("V4L/DVB: ir-core: add imon driver")
> Link: https://syzkaller.appspot.com/bug?id=e378e6a51fbe6c5cc43e34f131cc9a315ef0337e
> Reported-by: syzbot+0c3cb6dc05fbbdc3ad66@...kaller.appspotmail.com
> 
> Signed-off-by: Gautam Menghani <gautammenghani201@...il.com>
> ---
>  drivers/media/rc/imon.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/media/rc/imon.c b/drivers/media/rc/imon.c
> index 735b925da998..91d8056666ec 100644
> --- a/drivers/media/rc/imon.c
> +++ b/drivers/media/rc/imon.c
> @@ -646,15 +646,14 @@ static int send_packet(struct imon_context *ictx)
>  		pr_err_ratelimited("error submitting urb(%d)\n", retval);
>  	} else {
>  		/* Wait for transmission to complete (or abort) */
> -		mutex_unlock(&ictx->lock);
>  		retval = wait_for_completion_interruptible(
>  				&ictx->tx.finished);
>  		if (retval) {
>  			usb_kill_urb(ictx->tx_urb);
>  			pr_err_ratelimited("task interrupted\n");
>  		}
> -		mutex_lock(&ictx->lock);
>  
> +		ictx->tx.busy = false;
>  		retval = ictx->tx.status;
>  		if (retval)
>  			pr_err_ratelimited("packet tx failed (%d)\n", retval);
> @@ -955,7 +954,8 @@ static ssize_t vfd_write(struct file *file, const char __user *buf,
>  	if (ictx->disconnected)
>  		return -ENODEV;
>  
> -	mutex_lock(&ictx->lock);
> +	if (mutex_lock_interruptible(&ictx->lock))
> +		return -ERESTARTSYS;
>  
>  	if (!ictx->dev_present_intf0) {
>  		pr_err_ratelimited("no iMON device present\n");
> -- 
> 2.34.1
> 

Hi,

Please review the above fix and let me know if any changes are required.

Thanks,
Gautam