Fix a general protection fault reported by syzbot due to a race between
gadget_setup() and gadget_unbind() in raw_gadget.

The gadget core is supposed to guarantee that there won't be any more
callbacks to the gadget driver once the driver's unbind routine is
called. That guarantee is enforced in usb_gadget_remove_driver as
follows:

        usb_gadget_disconnect(udc->gadget);
        if (udc->gadget->irq)
                synchronize_irq(udc->gadget->irq);
        udc->driver->unbind(udc->gadget);
        usb_gadget_udc_stop(udc);

usb_gadget_disconnect turns off the pullup resistor, telling the host
that the gadget is no longer connected and preventing the transmission
of any more USB packets. Any packets that have already been received
are sure to processed by the UDC driver's interrupt handler by the time
synchronize_irq returns.

But this doesn't work with dummy_hcd, because dummy_hcd doesn't use
interrupts; it uses a timer instead. It does have code to emulate the
effect of synchronize_irq, but that code doesn't get invoked at the
right time -- it currently runs in usb_gadget_udc_stop, after the unbind
callback instead of before. Indeed, there's no way for
usb_gadget_remove_driver to invoke this code before the unbind callback.

To fix this, move the synchronize_irq() emulation code to dummy_pullup
so that it runs before unbind. Also, add a comment explaining why it is
necessary to have it there.

Reported-by: syzbot+eb4674092e6cc8d9e0bd@syzkaller.appspotmail.com
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.com>
Link: https://lore.kernel.org/r/20210419033713.3021-1-mail@anirudhrb.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>