usb: gadget: f_ecm: Always set current gadget in ecm_bind()

The gadget may change over bind/unbind cycles, so set it each time during
bind, not only the first time. Without it we get a use-after-free with
the following example:

cd /sys/kernel/config/usb_gadget/; mkdir -p mygadget; cd mygadget
mkdir -p configs/c.1/strings/0x409
echo "C1:Composite Device" > configs/c.1/strings/0x409/configuration
mkdir -p functions/ecm.usb0
ln -s functions/ecm.usb0 configs/c.1/
rmmod dummy_hcd
modprobe dummy_hcd

KASAN will complain shortly after the 'modprobe':

usb 2-1: New USB device found, idVendor=0000, idProduct=0000, bcdDevice= 6.01
usb 2-1: New USB device strings: Mfr=0, Product=0, SerialNumber=0
==================================================================
BUG: KASAN: use-after-free in gether_connect+0xb8/0x30c
Read of size 4 at addr cbef170c by task swapper/3/0

CPU: 3 PID: 0 Comm: swapper/3 Not tainted 6.1.0-rc3-00014-g41ff012f50cb-dirty torvalds#322
Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
 unwind_backtrace from show_stack+0x10/0x14
 show_stack from dump_stack_lvl+0x58/0x70
 dump_stack_lvl from print_report+0x134/0x4d4
 print_report from kasan_report+0x78/0x10c
 kasan_report from gether_connect+0xb8/0x30c
 gether_connect from ecm_set_alt+0x124/0x254
 ecm_set_alt from composite_setup+0xb98/0x2b18
 composite_setup from configfs_composite_setup+0x80/0x98
 configfs_composite_setup from dummy_timer+0x8f0/0x14a0 [dummy_hcd]
 ...

Signed-off-by: Sascha Hauer <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/usb/gadget/function/f_ecm.c

@@ -685,7 +685,7 @@ ecm_bind(struct usb_configuration *c, struct usb_function *f)
 	struct usb_composite_dev *cdev = c->cdev;
 	struct f_ecm		*ecm = func_to_ecm(f);
 	struct usb_string	*us;
-	int			status;
+	int			status = 0;
 	struct usb_ep		*ep;
 
 	struct f_ecm_opts	*ecm_opts;
@@ -695,23 +695,19 @@ ecm_bind(struct usb_configuration *c, struct usb_function *f)
 
 	ecm_opts = container_of(f->fi, struct f_ecm_opts, func_inst);
 
-	/*
-	 * in drivers/usb/gadget/configfs.c:configfs_composite_bind()
-	 * configurations are bound in sequence with list_for_each_entry,
-	 * in each configuration its functions are bound in sequence
-	 * with list_for_each_entry, so we assume no race condition
-	 * with regard to ecm_opts->bound access
-	 */
+	mutex_lock(&ecm_opts->lock);
+
+	gether_set_gadget(ecm_opts->net, cdev->gadget);
+
 	if (!ecm_opts->bound) {
-		mutex_lock(&ecm_opts->lock);
-		gether_set_gadget(ecm_opts->net, cdev->gadget);
 		status = gether_register_netdev(ecm_opts->net);
-		mutex_unlock(&ecm_opts->lock);
-		if (status)
-			return status;
 		ecm_opts->bound = true;
 	}
 
+	mutex_unlock(&ecm_opts->lock);
+	if (status)
+		return status;
+
 	ecm_string_defs[1].s = ecm->ethaddr;
 
 	us = usb_gstrings_attach(cdev, ecm_strings,