Re: [PATCH 0/6] usbip fixes to crashes found by syzbot

[Shuah Khan]

On 3/17/21 12:21 AM, Tetsuo Handa wrote:
> Shuah, this driver is getting more and more cryptic and buggy.
> Please explain the strategy for serialization before you write patches.
>
> > - Fix attach_store() to check usbip_event_happened() before
> >    waking up threads.
>
> No, this helps nothing.
>
> > diff --git a/drivers/usb/usbip/vhci_sysfs.c b/drivers/usb/usbip/vhci_sysfs.c
> > index c4b4256e5dad3..f0a770adebd97 100644
> > --- a/drivers/usb/usbip/vhci_sysfs.c
> > +++ b/drivers/usb/usbip/vhci_sysfs.c
> > @@ -418,6 +418,15 @@ static ssize_t attach_store(struct device *dev, struct device_attribute *attr,
> >   	spin_unlock_irqrestore(&vhci->lock, flags);
> >   	/* end the lock */
> >   
> > +	if (usbip_event_happened(&vdev->ud)) {
> > +		/*
> > +		 * something went wrong - event handler shutting
> > +		 * the connection and doing reset - bail out
> > +		 */
> > +		dev_err(dev, "Event happended - handler is active\n");
> > +		return -EAGAIN;
> > +	}
> > +
>
> detach_store() can queue shutdown event as soon as reaching "/* end the lock */" line
> but attach_store() might be preempted immediately after verifying that
> usbip_event_happened() was false (i.e. at this location) in order to wait for
> shutdown event posted by detach_store() to be processed.

Yes. I haven't sent the patch for that reason. I am trying to test a
solution. I haven't come up with a solution yet.

Holding event_lock isn't the right solution. I am not going to accept
that. This is a window that gets triggered by syzbot injecting errors
in a sequence. Fixing this should be done taking other moving parts of
the driver into account.

thanks,
-- Shuah