patch "USB: Gadget: core: Help prevent panic during UVC unconfigure" added to usb-linus

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

USB: Gadget: core: Help prevent panic during UVC unconfigure

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 65dadb2beeb7360232b09ebc4585b54475dfee06 Mon Sep 17 00:00:00 2001
From: Alan Stern <[email protected]>
Date: Sat, 29 Jul 2023 10:59:38 -0400
Subject: USB: Gadget: core: Help prevent panic during UVC unconfigure

Avichal Rakesh reported a kernel panic that occurred when the UVC gadget driver was removed from a gadget's configuration. The panic involves a somewhat complicated interaction between the kernel driver and a userspace component (as described in the Link tag below), but the analysis did make one thing clear: The Gadget core should accomodate gadget drivers calling usb_gadget_deactivate() as part of their unbind procedure.

Currently this doesn't work. gadget_unbind_driver() calls driver->unbind() while holding the udc->connect_lock mutex, and usb_gadget_deactivate() attempts to acquire that mutex, which will result in a deadlock.

The simple fix is for gadget_unbind_driver() to release the mutex when invoking the ->unbind() callback. There is no particular reason for it to be holding the mutex at that time, and the mutex isn't held while the ->bind() callback is invoked. So we'll drop the mutex before performing the unbind callback and reacquire it afterward.

We'll also add a couple of comments to usb_gadget_activate() and usb_gadget_deactivate(). Because they run in process context they must not be called from a gadget driver's ->disconnect() callback, which (according to the kerneldoc for struct usb_gadget_driver in include/linux/usb/gadget.h) may run in interrupt context. This may help prevent similar bugs from arising in the future.

---
 drivers/usb/gadget/udc/core.c | 9 +++++++++
 1 file changed, 9 insertions(+)

index cd58f2a4e7f3..7d49d8a0b00c 100644
@@ -822,6 +822,9 @@ EXPORT_SYMBOL_GPL(usb_gadget_disconnect);
  * usb_gadget_activate() is called.  For example, user mode components may
  * need to be activated before the system can talk to hosts.
  *
+ * This routine may sleep; it must not be called in interrupt context
+ * (such as from within a gadget driver's disconnect() callback).
+ *
  * Returns zero on success, else negative errno.
  */
 int usb_gadget_deactivate(struct usb_gadget *gadget)

@@ -860,6 +863,8 @@ EXPORT_SYMBOL_GPL(usb_gadget_deactivate);
  * This routine activates gadget which was previously deactivated with
  * usb_gadget_deactivate() call. It calls usb_gadget_connect() if needed.
  *
+ * This routine may sleep; it must not be called in interrupt context.
+ *
  * Returns zero on success, else negative errno.
  */
 int usb_gadget_activate(struct usb_gadget *gadget)

@@ -1638,7 +1643,11 @@ static void gadget_unbind_driver(struct device *dev)
 	usb_gadget_disable_async_callbacks(udc);
 	if (gadget->irq)
 		synchronize_irq(gadget->irq);
+	mutex_unlock(&udc->connect_lock);
+
 	udc->driver->unbind(gadget);
+
+	mutex_lock(&udc->connect_lock);
 	usb_gadget_udc_stop_locked(udc);
 	mutex_unlock(&udc->connect_lock);
 

2.41.0