linux-kernel - Re: [PATCH] drivers/media/rc: Ensure usb_submit_urb() is not called if write is in progress

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Date:   Sat, 3 Sep 2022 15:08:23 +0530
From:   Gautam Menghani <gautammenghani201@...il.com>
To:     Sean Young <sean@...s.org>
Cc:     mchehab@...nel.org, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+0c3cb6dc05fbbdc3ad66@...kaller.appspotmail.com,
        hdanton@...a.com
Subject: Re: [PATCH] drivers/media/rc: Ensure usb_submit_urb() is not called
 if write is in progress

On Sat, Sep 03, 2022 at 08:49:56AM +0100, Sean Young wrote:
> On Fri, Sep 02, 2022 at 11:42:41PM +0530, Gautam Menghani wrote:
> > On Sun, Aug 14, 2022 at 07:55:42PM +0530, Gautam Menghani wrote:
> > > The warning "URB submitted while active" is reported if the function
> > > send_packet() in imon.c is called if a write is already is in progress.
> > > Add a check to return -EBUSY in case a write is already is in progress.
> > > Also, mark tx.busy as false after transmission is completed.
> > > 
> > > Fixes: 21677cfc562a ("V4L/DVB: ir-core: add imon driver")
> > > Cc: hdanton@...a.com
> > > Suggested-by: hdanton@...a.com
> > > 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 | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/drivers/media/rc/imon.c b/drivers/media/rc/imon.c
> > > index 735b925da998..a5b997c2c7e2 100644
> > > --- a/drivers/media/rc/imon.c
> > > +++ b/drivers/media/rc/imon.c
> > > @@ -598,6 +598,8 @@ static int send_packet(struct imon_context *ictx)
> > >  	int retval = 0;
> > >  	struct usb_ctrlrequest *control_req = NULL;
> > >  
> > > +	if (ictx->tx.busy)
> > > +		return -EBUSY;
> > >  	/* Check if we need to use control or interrupt urb */
> > >  	if (!ictx->tx_control) {
> > >  		pipe = usb_sndintpipe(ictx->usbdev_intf0,
> > > @@ -654,6 +656,7 @@ static int send_packet(struct imon_context *ictx)
> > >  			pr_err_ratelimited("task interrupted\n");
> > >  		}
> > >  		mutex_lock(&ictx->lock);
> > > +		ictx->tx.busy = false;
> > >  
> > >  		retval = ictx->tx.status;
> > >  		if (retval)
> > > -- 
> > > 2.34.1
> > > 
> > Hi,
> > 
> > Please review the above fix and let me know if any changes are needed.
> 
> Greg has pointed out issues with this patch: there is no locking.
> 
> Thanks,
> 
> Sean

Hi,

I am a bit confused about the locking part. All the calls to send_packet() are
made with ictx->lock held. So will the following be sufficient?

lockdep_assert_held(&ictx->lock);
if (ictx->tx.busy)
	return -EBUSY;

Or do we need to add a mutex/spin lock inside ictx->tx structure? 

Or should we instead wait for the the tx to be completed as follows:
wait_for_completion(&ictx->tx.finished);

Please advise.

Thanks,
Gautam