USB: gadget: Fix obscure lockdep violation for udc_mutex

A recent commit expanding the scope of the udc_lock mutex in the
gadget core managed to cause an obscure and slightly bizarre lockdep
violation.  In abbreviated form:

======================================================
WARNING: possible circular locking dependency detected
5.19.0-rc7+ #12510 Not tainted
------------------------------------------------------
udevadm/312 is trying to acquire lock:
ffff80000aae1058 (udc_lock){+.+.}-{3:3}, at: usb_udc_uevent+0x54/0xe0

but task is already holding lock:
ffff000002277548 (kn->active#4){++++}-{0:0}, at: kernfs_seq_start+0x34/0xe0

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #3 (kn->active#4){++++}-{0:0}:
        lock_acquire+0x68/0x84
        __kernfs_remove+0x268/0x380
        kernfs_remove_by_name_ns+0x58/0xac
        sysfs_remove_file_ns+0x18/0x24
        device_del+0x15c/0x440

-> #2 (device_links_lock){+.+.}-{3:3}:
        lock_acquire+0x68/0x84
        __mutex_lock+0x9c/0x430
        mutex_lock_nested+0x38/0x64
        device_link_remove+0x3c/0xa0
        _regulator_put.part.0+0x168/0x190
        regulator_put+0x3c/0x54
        devm_regulator_release+0x14/0x20

-> #1 (regulator_list_mutex){+.+.}-{3:3}:
        lock_acquire+0x68/0x84
        __mutex_lock+0x9c/0x430
        mutex_lock_nested+0x38/0x64
        regulator_lock_dependent+0x54/0x284
        regulator_enable+0x34/0x80
        phy_power_on+0x24/0x130
        __dwc2_lowlevel_hw_enable+0x100/0x130
        dwc2_lowlevel_hw_enable+0x18/0x40
        dwc2_hsotg_udc_start+0x6c/0x2f0
        gadget_bind_driver+0x124/0x1f4

-> #0 (udc_lock){+.+.}-{3:3}:
        __lock_acquire+0x1298/0x20cc
        lock_acquire.part.0+0xe0/0x230
        lock_acquire+0x68/0x84
        __mutex_lock+0x9c/0x430
        mutex_lock_nested+0x38/0x64
        usb_udc_uevent+0x54/0xe0

Evidently this was caused by the scope of udc_mutex being too large.
The mutex is only meant to protect udc->driver along with a few other
things.  As far as I can tell, there's no reason for the mutex to be
held while the gadget core calls a gadget driver's ->bind or ->unbind
routine, or while a UDC is being started or stopped.  (This accounts
for link #1 in the chain above, where the mutex is held while the
dwc2_hsotg_udc is started as part of driver probing.)

Gadget drivers' ->disconnect callbacks are problematic.  Even though
usb_gadget_disconnect() will now acquire the udc_mutex, there's a
window in usb_gadget_bind_driver() between the times when the mutex is
released and the ->bind callback is invoked.  If a disconnect occurred
during that window, we could call the driver's ->disconnect routine
before its ->bind routine.  To prevent this from happening, it will be
necessary to prevent a UDC from connecting while it has no gadget
driver.  This should be done already but it doesn't seem to be;
currently usb_gadget_connect() has no check for this.  Such a check
will have to be added later.

Some degree of mutual exclusion is required in soft_connect_store(),
which can dereference udc->driver at arbitrary times since it is a
sysfs callback.  The solution here is to acquire the gadget's device
lock rather than the udc_mutex.  Since the driver core guarantees that
the device lock is always held during driver binding and unbinding,
this will make the accesses in soft_connect_store() mutually exclusive
with any changes to udc->driver.

Lastly, it turns out there is one place which should hold the
udc_mutex but currently does not: The function_show() routine needs
protection while it dereferences udc->driver.  The missing lock and
unlock calls are added.

Link: https://lore.kernel.org/all/b2ba4245-9917-e399-94c8-03a383e7070e@samsung.com/
Fixes: 2191c00 ("USB: gadget: Fix use-after-free Read in usb_udc_uevent()")
Cc: Felipe Balbi <balbi@kernel.org>
Cc: stable@vger.kernel.org
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/YwkfhdxA/I2nOcK7@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/core.c

@@ -736,7 +736,10 @@ int usb_gadget_disconnect(struct usb_gadget *gadget)
 	ret = gadget->ops->pullup(gadget, 0);
 	if (!ret) {
 		gadget->connected = 0;
-		gadget->udc->driver->disconnect(gadget);
+		mutex_lock(&udc_lock);
+		if (gadget->udc->driver)
+			gadget->udc->driver->disconnect(gadget);
+		mutex_unlock(&udc_lock);
 	}
 
 out:
@@ -1489,7 +1492,6 @@ static int gadget_bind_driver(struct device *dev)
 
 	usb_gadget_udc_set_speed(udc, driver->max_speed);
 
-	mutex_lock(&udc_lock);
 	ret = driver->bind(udc->gadget, driver);
 	if (ret)
 		goto err_bind;
@@ -1499,7 +1501,6 @@ static int gadget_bind_driver(struct device *dev)
 		goto err_start;
 	usb_gadget_enable_async_callbacks(udc);
 	usb_udc_connect_control(udc);
-	mutex_unlock(&udc_lock);
 
 	kobject_uevent(&udc->dev.kobj, KOBJ_CHANGE);
 	return 0;
@@ -1512,6 +1513,7 @@ static int gadget_bind_driver(struct device *dev)
 		dev_err(&udc->dev, "failed to start %s: %d\n",
 			driver->function, ret);
 
+	mutex_lock(&udc_lock);
 	udc->driver = NULL;
 	driver->is_bound = false;
 	mutex_unlock(&udc_lock);
@@ -1529,14 +1531,14 @@ static void gadget_unbind_driver(struct device *dev)
 
 	kobject_uevent(&udc->dev.kobj, KOBJ_CHANGE);
 
-	mutex_lock(&udc_lock);
 	usb_gadget_disconnect(gadget);
 	usb_gadget_disable_async_callbacks(udc);
 	if (gadget->irq)
 		synchronize_irq(gadget->irq);
 	udc->driver->unbind(gadget);
 	usb_gadget_udc_stop(udc);
 
+	mutex_lock(&udc_lock);
 	driver->is_bound = false;
 	udc->driver = NULL;
 	mutex_unlock(&udc_lock);
@@ -1612,7 +1614,7 @@ static ssize_t soft_connect_store(struct device *dev,
 	struct usb_udc		*udc = container_of(dev, struct usb_udc, dev);
 	ssize_t			ret;
 
-	mutex_lock(&udc_lock);
+	device_lock(&udc->gadget->dev);
 	if (!udc->driver) {
 		dev_err(dev, "soft-connect without a gadget driver\n");
 		ret = -EOPNOTSUPP;
@@ -1633,7 +1635,7 @@ static ssize_t soft_connect_store(struct device *dev,
 
 	ret = n;
 out:
-	mutex_unlock(&udc_lock);
+	device_unlock(&udc->gadget->dev);
 	return ret;
 }
 static DEVICE_ATTR_WO(soft_connect);
@@ -1652,11 +1654,15 @@ static ssize_t function_show(struct device *dev, struct device_attribute *attr,
 			     char *buf)
 {
 	struct usb_udc		*udc = container_of(dev, struct usb_udc, dev);
-	struct usb_gadget_driver *drv = udc->driver;
+	struct usb_gadget_driver *drv;
+	int			rc = 0;
 
-	if (!drv || !drv->function)
-		return 0;
-	return scnprintf(buf, PAGE_SIZE, "%s\n", drv->function);
+	mutex_lock(&udc_lock);
+	drv = udc->driver;
+	if (drv && drv->function)
+		rc = scnprintf(buf, PAGE_SIZE, "%s\n", drv->function);
+	mutex_unlock(&udc_lock);
+	return rc;
 }
 static DEVICE_ATTR_RO(function);