patch "usb: gadget: dummy_hcd: fix gpf in gadget_setup" added to usb-testing

This is a note to let you know that I've just added the patch titled

usb: gadget: dummy_hcd: fix gpf in gadget_setup

to my usb git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-testing branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will be merged to the usb-next branch sometime soon, after it passes testing, and the merge window is open.

If you have any questions about this process, please let me know.

From 4a5d797a9f9c4f18585544237216d7812686a71f Mon Sep 17 00:00:00 2001
From: Anirudh Rayabharam <[email protected]>
Date: Mon, 19 Apr 2021 09:07:08 +0530
Subject: usb: gadget: dummy_hcd: fix gpf in gadget_setup

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.

---
 drivers/usb/gadget/udc/dummy_hcd.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

index ce24d4f28f2a..7db773c87379 100644
@@ -903,6 +903,21 @@ static int dummy_pullup(struct usb_gadget *_gadget, int value)
 	spin_lock_irqsave(&dum->lock, flags);
 	dum->pullup = (value != 0);
 	set_link_state(dum_hcd);
+	if (value == 0) {
+		/*
+		 * Emulate synchronize_irq(): wait for callbacks to finish.
+		 * This seems to be the best place to emulate the call to
+		 * synchronize_irq() that's in usb_gadget_remove_driver().
+		 * Doing it in dummy_udc_stop() would be too late since it
+		 * is called after the unbind callback and unbind shouldn't
+		 * be invoked until all the other callbacks are finished.
+		 */
+		while (dum->callback_usage > 0) {
+			spin_unlock_irqrestore(&dum->lock, flags);
+			usleep_range(1000, 2000);
+			spin_lock_irqsave(&dum->lock, flags);
+		}
+	}
 	spin_unlock_irqrestore(&dum->lock, flags);
 
 	usb_hcd_poll_rh_status(dummy_hcd_to_hcd(dum_hcd));

@@ -1004,14 +1019,6 @@ static int dummy_udc_stop(struct usb_gadget *g)
 	spin_lock_irq(&dum->lock);
 	dum->ints_enabled = 0;
 	stop_activity(dum);
-
-	/* emulate synchronize_irq(): wait for callbacks to finish */
-	while (dum->callback_usage > 0) {
-		spin_unlock_irq(&dum->lock);
-		usleep_range(1000, 2000);
-		spin_lock_irq(&dum->lock);
-	}
-
 	dum->driver = NULL;
 	spin_unlock_irq(&dum->lock);
 

2.31.1