patch "USB: dummy-hcd: Fix erroneous synchronization change" added to usb-linus

[<gregkh@linuxfoundation.org>]

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

    USB: dummy-hcd: Fix erroneous synchronization change

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-linus 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 hopefully also be merged in Linus's tree for the
next -rc kernel release.

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


>From 7dbd8f4cabd96db5a50513de9d83a8105a5ffc81 Mon Sep 17 00:00:00 2001
From: Alan Stern <stern@rowland.harvard.edu>
Date: Tue, 26 Sep 2017 15:15:49 -0400
Subject: USB: dummy-hcd: Fix erroneous synchronization change

A recent change to the synchronization in dummy-hcd was incorrect.
The issue was that dummy_udc_stop() contained no locking and therefore
could race with various gadget driver callbacks, and the fix was to
add locking and issue the callbacks with the private spinlock held.

UDC drivers aren't supposed to do this.  Gadget driver callback
routines are allowed to invoke functions in the UDC driver, and these
functions will generally try to acquire the private spinlock.  This
would deadlock the driver.

The correct solution is to drop the spinlock before issuing callbacks,
and avoid races by emulating the synchronize_irq() call that all real
UDC drivers must perform in their ->udc_stop() routines after
disabling interrupts.  This involves adding a flag to dummy-hcd's
private structure to keep track of whether interrupts are supposed to
be enabled, and adding a counter to keep track of ongoing callbacks so
that dummy_udc_stop() can wait for them all to finish.

A real UDC driver won't receive disconnect, reset, suspend, resume, or
setup events once it has disabled interrupts.  dummy-hcd will receive
them but won't try to issue any gadget driver callbacks, which should
be just as good.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Fixes: f16443a034c7 ("USB: gadgetfs, dummy-hcd, net2280: fix locking for callbacks")
CC: <stable@vger.kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
---
 drivers/usb/gadget/udc/dummy_hcd.c | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/udc/dummy_hcd.c b/drivers/usb/gadget/udc/dummy_hcd.c
index b2ab9cc33fec..b17618a55f1b 100644
--- a/drivers/usb/gadget/udc/dummy_hcd.c
+++ b/drivers/usb/gadget/udc/dummy_hcd.c
@@ -255,11 +255,13 @@ struct dummy {
 	 */
 	struct dummy_ep			ep[DUMMY_ENDPOINTS];
 	int				address;
+	int				callback_usage;
 	struct usb_gadget		gadget;
 	struct usb_gadget_driver	*driver;
 	struct dummy_request		fifo_req;
 	u8				fifo_buf[FIFO_SIZE];
 	u16				devstatus;
+	unsigned			ints_enabled:1;
 	unsigned			udc_suspended:1;
 	unsigned			pullup:1;
 
@@ -441,18 +443,27 @@ static void set_link_state(struct dummy_hcd *dum_hcd)
 				(~dum_hcd->old_status) & dum_hcd->port_status;
 
 		/* Report reset and disconnect events to the driver */
-		if (dum->driver && (disconnect || reset)) {
+		if (dum->ints_enabled && (disconnect || reset)) {
 			stop_activity(dum);
+			++dum->callback_usage;
+			spin_unlock(&dum->lock);
 			if (reset)
 				usb_gadget_udc_reset(&dum->gadget, dum->driver);
 			else
 				dum->driver->disconnect(&dum->gadget);
+			spin_lock(&dum->lock);
+			--dum->callback_usage;
 		}
-	} else if (dum_hcd->active != dum_hcd->old_active) {
+	} else if (dum_hcd->active != dum_hcd->old_active &&
+			dum->ints_enabled) {
+		++dum->callback_usage;
+		spin_unlock(&dum->lock);
 		if (dum_hcd->old_active && dum->driver->suspend)
 			dum->driver->suspend(&dum->gadget);
 		else if (!dum_hcd->old_active &&  dum->driver->resume)
 			dum->driver->resume(&dum->gadget);
+		spin_lock(&dum->lock);
+		--dum->callback_usage;
 	}
 
 	dum_hcd->old_status = dum_hcd->port_status;
@@ -973,8 +984,11 @@ static int dummy_udc_start(struct usb_gadget *g,
 	 * can't enumerate without help from the driver we're binding.
 	 */
 
+	spin_lock_irq(&dum->lock);
 	dum->devstatus = 0;
 	dum->driver = driver;
+	dum->ints_enabled = 1;
+	spin_unlock_irq(&dum->lock);
 
 	return 0;
 }
@@ -985,6 +999,16 @@ static int dummy_udc_stop(struct usb_gadget *g)
 	struct dummy		*dum = dum_hcd->dum;
 
 	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);
 
@@ -1529,6 +1553,8 @@ static struct dummy_ep *find_endpoint(struct dummy *dum, u8 address)
 	if (!is_active((dum->gadget.speed == USB_SPEED_SUPER ?
 			dum->ss_hcd : dum->hs_hcd)))
 		return NULL;
+	if (!dum->ints_enabled)
+		return NULL;
 	if ((address & ~USB_DIR_IN) == 0)
 		return &dum->ep[0];
 	for (i = 1; i < DUMMY_ENDPOINTS; i++) {
@@ -1870,10 +1896,12 @@ static void dummy_timer(unsigned long _dum_hcd)
 			 * until setup() returns; no reentrancy issues etc.
 			 */
 			if (value > 0) {
+				++dum->callback_usage;
 				spin_unlock(&dum->lock);
 				value = dum->driver->setup(&dum->gadget,
 						&setup);
 				spin_lock(&dum->lock);
+				--dum->callback_usage;
 
 				if (value >= 0) {
 					/* no delays (max 64KB data stage) */
-- 
2.14.1