usb: gadget: Fix use-after-free bug by not setting udc->dev.driver

usb: gadget: Fix use-after-free bug by not setting udc->dev.driver

The syzbot fuzzer found a use-after-free bug:

BUG: KASAN: use-after-free in dev_uevent+0x712/0x780 drivers/base/core.c:2320
Read of size 8 at addr ffff88802b934098 by task udevd/3689

CPU: 2 PID: 3689 Comm: udevd Not tainted 5.17.0-rc4-syzkaller-00229-g4f12b742eb2b #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_address_description.constprop.0.cold+0x8d/0x303 mm/kasan/report.c:255
 __kasan_report mm/kasan/report.c:442 [inline]
 kasan_report.cold+0x83/0xdf mm/kasan/report.c:459
 dev_uevent+0x712/0x780 drivers/base/core.c:2320
 uevent_show+0x1b8/0x380 drivers/base/core.c:2391
 dev_attr_show+0x4b/0x90 drivers/base/core.c:2094

Although the bug manifested in the driver core, the real cause was a
race with the gadget core.  dev_uevent() does:

	if (dev->driver)
		add_uevent_var(env, "DRIVER=%s", dev->driver->name);

and between the test and the dereference of dev->driver, the gadget
core sets dev->driver to NULL.

The race wouldn't occur if the gadget core registered its devices on
a real bus, using the standard synchronization techniques of the
driver core.  However, it's not necessary to make such a large change
in order to fix this bug; all we need to do is make sure that
udc->dev.driver is always NULL.

In fact, there is no reason for udc->dev.driver ever to be set to
anything, let alone to the value it currently gets: the address of the
gadget's driver.  After all, a gadget driver only knows how to manage
a gadget, not how to manage a UDC.

This patch simply removes the statements in the gadget core that touch
udc->dev.driver.

Fixes: b90ed5eaa3b4 ("usb: gadget: introduce UDC Class")
CC: <stable@vger.kernel.org>
Reported-and-tested-by: syzbot+348b571beb5eeb70a582@syzkaller.appspotmail.com
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/YiQgukfFFbBnwJ/9@rowland.harvard.edu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>