ANDROID: usb: gadget: configfs: fix null ptr in android_disconnect

There's a race between usb_gadget_udc_stop() which is likely
to set the gadget driver to NULL in the udc driver and this drivers
gadget disconnect fn which likely checks for the gadget driver to
a null ptr. It happens that unbind (doing set_gadget_data(NULL))
is called before the gadget driver is set to NULL and the udc driver
calls disconnect fn which results in cdev being a null ptr.

As a workaround we check cdev in android_disconnect() to prevent
the following panic:

Unable to handle kernel NULL pointer dereference at virtual address 000000a8
pgd = ffffff800940a000
[000000a8] *pgd=00000000be1fe003, *pud=00000000be1fe003, *pmd=0000000000000000
Internal error: Oops: 96000046 [#1] PREEMPT SMP
CPU: 7 PID: 1134 Comm: kworker/u16:3 Tainted: G S 4.9.41-g75cd2a0231ea-dirty #4
Hardware name: HiKey960 (DT)
Workqueue: events_power_efficient event_work
task: ffffffc0b5f4f000 task.stack: ffffffc0b5b94000
PC is at android_disconnect+0x54/0xa4
LR is at android_disconnect+0x54/0xa4
pc : [<ffffff8008855938>] lr : [<ffffff8008855938>] pstate: 80000185
sp : ffffffc0b5b97bf0
x29: ffffffc0b5b97bf0 x28: 0000000000000003
x27: ffffffc0b5181c54 x26: ffffffc0b5181c68
x25: ffffff8008dc1000 x24: ffffffc0b5181d70
x23: ffffff8008dc18a0 x22: ffffffc0b5f5a018
x21: ffffffc0b5894ad8 x20: 0000000000000000
x19: ffffff8008ddaec8 x18: 0000000000000000
x17: 0000000000000000 x16: 0000000000000000
x15: 0000000000000000 x14: 00000000007c9ccd
x13: 0000000000000000 x12: 0000000000000000
x11: 0000000000000001 x10: 0000000000000001
x9 : ffffff800930f1a8 x8 : ffffff800932a133
x7 : 0000000000000000 x6 : 0000000000000000
x5 : ffffffc0b5b97a50 x4 : ffffffc0be19f090
x3 : 0000000000000000 x2 : ffffff80091ca000
x1 : 000000000000002f x0 : 000000000000002f

This happened on a hikey960 with the following backtrace:

[<ffffff8008855938>] android_disconnect+0x54/0xa4
[<ffffff80089def38>] dwc3_disconnect_gadget.part.19+0x114.888119]
[<ffffff80087f7d48>] dwc3_gadget_suspend+0x6c/0x70
[<ffffff80087ee674>] dwc3_suspend_device+0x58/0xa0
[<ffffff80087fb418>] dwc3_otg_work+0x214/0x474
[<ffffff80087fdc74>] event_work+0x3bc/0x5ac
[<ffffff80080e5d88>] process_one_work+0x14c/0x43c
[<ffffff80080e60d4>] worker_thread+0x5c/0x438
[<ffffff80080ece68>] kthread+0xec/0x100
[<ffffff8008083680>] ret_from_fork+0x10/0x50

dwc3_otg_work tries to handle a switch from host to device mode
and therefore calls disconnect on the gadget driver.

To reproduce the issue it is enaugh to enable tethering (rndis gadget),
unplug and plug in again the usb connector which causes the change
from device to host and back to device mode.

Signed-off-by: Danilo Krummrich <danilokrummrich@gmail.com>

drivers/usb/gadget/configfs.c

@@ -1524,6 +1524,18 @@ static void android_disconnect(struct usb_gadget *gadget)
 	struct usb_composite_dev        *cdev = get_gadget_data(gadget);
 	struct gadget_info *gi = container_of(cdev, struct gadget_info, cdev);
 
+	/* FIXME: There's a race between usb_gadget_udc_stop() which is likely
+	 * to set the gadget driver to NULL in the udc driver and this drivers
+	 * gadget disconnect fn which likely checks for the gadget driver to
+	 * be a null ptr. It happens that unbind (doing set_gadget_data(NULL))
+	 * is called before the gadget driver is set to NULL and the udc driver
+	 * calls disconnect fn which results in cdev being a null ptr.
+	 */
+	if (cdev == NULL) {
+		WARN(1, "%s: gadget driver already disconnected\n", __func__);
+		return;
+	}
+
 	/* accessory HID support can be active while the
 		accessory function is not actually enabled,
 		so we need to inform it when we are disconnected.