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.
Reported-and-tested-by: Avichal Rakesh <arakesh@xxxxxxxxxx>
Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Fixes: 286d9975a838 ("usb: gadget: udc: core: Prevent soft_connect_store() race")
Link: https://lore.kernel.org/linux-usb/4d7aa3f4-22d9-9f5a-3d70-1bd7148ff4ba@xxxxxxxxxx/
Cc: Badhri Jagan Sridharan <badhri@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
drivers/usb/gadget/udc/core.c | 9 +++++++++
1 file changed, 9 insertions(+)
Index: usb-devel/drivers/usb/gadget/udc/core.c
===================================================================
--- usb-devel.orig/drivers/usb/gadget/udc/core.c
+++ usb-devel/drivers/usb/gadget/udc/core.c
@@ -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)
@@ -1639,7 +1644,11 @@ static void gadget_unbind_driver(struct
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);
