USB: gadget: Fix use-after-free Read in usb_udc_uevent()

The syzbot fuzzer found a race between uevent callbacks and gadget
driver unregistration that can cause a use-after-free bug:

---------------------------------------------------------------
BUG: KASAN: use-after-free in usb_udc_uevent+0x11f/0x130
drivers/usb/gadget/udc/core.c:1732
Read of size 8 at addr ffff888078ce2050 by task udevd/2968

CPU: 1 PID: 2968 Comm: udevd Not tainted 5.19.0-rc4-next-20220628-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google
06/29/2022
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_address_description mm/kasan/report.c:317 [inline]
 print_report.cold+0x2ba/0x719 mm/kasan/report.c:433
 kasan_report+0xbe/0x1f0 mm/kasan/report.c:495
 usb_udc_uevent+0x11f/0x130 drivers/usb/gadget/udc/core.c:1732
 dev_uevent+0x290/0x770 drivers/base/core.c:2424
---------------------------------------------------------------

The bug occurs because usb_udc_uevent() dereferences udc->driver but
does so without acquiring the udc_lock mutex, which protects this
field.  If the gadget driver is unbound from the udc concurrently with
uevent processing, the driver structure may be accessed after it has
been deallocated.

To prevent the race, we make sure that the routine holds the mutex
around the racing accesses.

Link: <https://lore.kernel.org/all/0000000000004de90405a719c951@google.com>
CC: stable@vger.kernel.org # fc274c1e
Reported-and-tested-by: syzbot+b0de012ceb1e2a97891b@syzkaller.appspotmail.com
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/YtlrnhHyrHsSky9m@rowland.harvard.eduSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/core.c

@@ -1728,14 +1728,15 @@ static int usb_udc_uevent(struct device *dev, struct kobj_uevent_env *env)
 		return ret;
 	}
 
-	if (udc->driver) {
+	mutex_lock(&udc_lock);
+	if (udc->driver)
 		ret = add_uevent_var(env, "USB_UDC_DRIVER=%s",
 				udc->driver->function);
+	mutex_unlock(&udc_lock);
 	if (ret) {
 		dev_err(dev, "failed to add uevent USB_UDC_DRIVER\n");
 		return ret;
 	}
-	}
 
 	return 0;
 }