Re: [PATCH] usb: dwc3: fix race of usb_gadget_driver operation

On 2021-06-26 23:03, Alan Stern wrote:

On Sat, Jun 26, 2021 at 09:16:25AM +0800, linyyuan@codeaurora.org 
wrote:

quoted

On 2021-06-26 00:37, Alan Stern wrote:

quoted

On Fri, Jun 25, 2021 at 06:44:15PM +0800, Linyu Yuan wrote:

quoted

quoted

quoted

--- a/drivers/usb/dwc3/ep0.c
+++ b/drivers/usb/dwc3/ep0.c

@@ -597,10 +597,11 @@ static int dwc3_ep0_set_address(struct dwc3

*dwc, struct usb_ctrlrequest *ctrl)

 static int dwc3_ep0_delegate_req(struct dwc3 *dwc, struct
usb_ctrlrequest *ctrl)
 {
-	int ret;
+	int ret = 0;

 	spin_unlock(&dwc->lock);
-	ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
+	if (dwc->async_callbacks)
+		ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
 	spin_lock(&dwc->lock);

Here and in the other places, you should test dwc->async_callbacks
_before_ dropping the spinlock.  Otherwise there is a race (the flag
could be written at about the same time it is checked).

thanks for your comments,

if you think there is race here, how to make sure gadget_driver 
pointer is
safe,
this is closest place where we can confirm it is non-NULL by checking
async_callbacks ?

I explained this twice already: We know that gadget_driver is not
NULL because usb_gadget_remove_driver calls synchronize_irq before
doing usb_gadget_udc_stop.

Look at this timing diagram:

	CPU0				CPU1
	----				----
	IRQ happens for setup packet
	  Handler sees async_callbacks
	    is enabled
	  Handler unlocks dwc->lock
					usb_gadget_remove_driver runs
					  Disables async callbacks
					  Calls synchronize_irq
	  Handler calls dwc->		  . waits for IRQ handler to
	    gadget_driver->setup	  .   return
	  Handler locks dwc-lock	  .
	  ...				  .
	  Handler returns		  .
					  . synchronize_irq returns
					  Calls usb_gadget_udc_stop
					    dwc->gadget_driver is
					      set to NULL

As you can see, dwc->gadget_driver is non-NULL when CPU0 uses it,
even though async_callbacks gets cleared during the time when the
lock is released.

thanks for your patient explanation,
but from this part, seem it is synchronize_irq() help to avoid NULL 
pointer crash.

can you also explain how async_callbacks flag help here  ?

Alan Stern