[PATCH] USB: gadget: dummy-hcd: Fix locking bug in RT-enabled kernels

[Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>]

Yunseong Kim and the syzbot fuzzer both reported a problem in
RT-enabled kernels caused by the way dummy-hcd mixes interrupt
management and spin-locking.  The pattern was:

	local_irq_save(flags);
	spin_lock(&dum->lock);
	...
	spin_unlock(&dum->lock);
	...		// calls usb_gadget_giveback_request()
	local_irq_restore(flags);

The code was written this way because usb_gadget_giveback_request()
needs to be called with interrupts disabled and the private lock not
held.

While this pattern works fine in non-RT kernels, it's not good when RT
is enabled.  RT kernels handle spinlocks much like mutexes; in particular,
spin_lock() may sleep.  But sleeping is not allowed while local
interrupts are disabled.

To fix the problem, rewrite the code to conform to the pattern used
elsewhere in dummy-hcd and other UDC drivers:

	spin_lock_irqsave(&dum->lock, flags);
	...
	spin_unlock(&dum->lock);
	usb_gadget_giveback_request(...);
	spin_lock(&dum->lock);
	...
	spin_unlock_irqrestore(&dum->lock, flags);

This approach satisfies the RT requirements.

Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Fixes: b4dbda1a22d2 ("USB: dummy-hcd: disable interrupts during req->complete")
Reported-by: Yunseong Kim <ysk@xxxxxxxxxxx>
Closes: <https://lore.kernel.org/linux-usb/5b337389-73b9-4ee4-a83e-7e82bf5af87a@xxxxxxxxxxx/>
Reported-by: syzbot+8baacc4139f12fa77909@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: <https://lore.kernel.org/linux-usb/68ac2411.050a0220.37038e.0087.GAE@xxxxxxxxxx/>
Tested-by: syzbot+8baacc4139f12fa77909@xxxxxxxxxxxxxxxxxxxxxxxxx
CC: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
CC: <stable@xxxxxxxxxxxxxxx>

---

 drivers/usb/gadget/udc/dummy_hcd.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Index: usb-devel/drivers/usb/gadget/udc/dummy_hcd.c
===================================================================
--- usb-devel.orig/drivers/usb/gadget/udc/dummy_hcd.c
+++ usb-devel/drivers/usb/gadget/udc/dummy_hcd.c
@@ -765,8 +765,7 @@ static int dummy_dequeue(struct usb_ep *
 	if (!dum->driver)
 		return -ESHUTDOWN;
 
-	local_irq_save(flags);
-	spin_lock(&dum->lock);
+	spin_lock_irqsave(&dum->lock, flags);
 	list_for_each_entry(iter, &ep->queue, queue) {
 		if (&iter->req != _req)
 			continue;
@@ -776,15 +775,16 @@ static int dummy_dequeue(struct usb_ep *
 		retval = 0;
 		break;
 	}
-	spin_unlock(&dum->lock);
 
 	if (retval == 0) {
 		dev_dbg(udc_dev(dum),
 				"dequeued req %p from %s, len %d buf %p\n",
 				req, _ep->name, _req->length, _req->buf);
+		spin_unlock(&dum->lock);
 		usb_gadget_giveback_request(_ep, _req);
+		spin_lock(&dum->lock);
 	}
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&dum->lock, flags);
 	return retval;
 }