usb: gadget: u_serial: Disable ep before setting port to null to fix the crash caused by port being null

Considering that in some extreme cases, when performing the
unbinding operation, gserial_disconnect has cleared gser->ioport,
which triggers gadget reconfiguration, and then calls gs_read_complete,
resulting in access to a null pointer. Therefore, ep is disabled before
gserial_disconnect sets port to null to prevent this from happening.

Call trace:
 gs_read_complete+0x58/0x240
 usb_gadget_giveback_request+0x40/0x160
 dwc3_remove_requests+0x170/0x484
 dwc3_ep0_out_start+0xb0/0x1d4
 __dwc3_gadget_start+0x25c/0x720
 kretprobe_trampoline.cfi_jt+0x0/0x8
 kretprobe_trampoline.cfi_jt+0x0/0x8
 udc_bind_to_driver+0x1d8/0x300
 usb_gadget_probe_driver+0xa8/0x1dc
 gadget_dev_desc_UDC_store+0x13c/0x188
 configfs_write_iter+0x160/0x1f4
 vfs_write+0x2d0/0x40c
 ksys_write+0x7c/0xf0
 __arm64_sys_write+0x20/0x30
 invoke_syscall+0x60/0x150
 el0_svc_common+0x8c/0xf8
 do_el0_svc+0x28/0xa0
 el0_svc+0x24/0x84

Fixes: c1dca56 ("usb gadget: split out serial core")
Cc: stable <[email protected]>
Suggested-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Lianqin Hu <[email protected]>
Link: https://lore.kernel.org/r/TYUPR06MB621733B5AC690DBDF80A0DCCD2042@TYUPR06MB6217.apcprd06.prod.outlook.com
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Lianqin Hu

drivers/usb/gadget/function/u_serial.c

@@ -1420,6 +1420,10 @@ void gserial_disconnect(struct gserial *gser)
 	/* REVISIT as above: how best to track this? */
 	port->port_line_coding = gser->port_line_coding;
 
+	/* disable endpoints, aborting down any active I/O */
+	usb_ep_disable(gser->out);
+	usb_ep_disable(gser->in);
+
 	port->port_usb = NULL;
 	gser->ioport = NULL;
 	if (port->port.count > 0) {
@@ -1431,10 +1435,6 @@ void gserial_disconnect(struct gserial *gser)
 	spin_unlock(&port->port_lock);
 	spin_unlock_irqrestore(&serial_port_lock, flags);
 
-	/* disable endpoints, aborting down any active I/O */
-	usb_ep_disable(gser->out);
-	usb_ep_disable(gser->in);
-
 	/* finally, free any unused/unusable I/O buffers */
 	spin_lock_irqsave(&port->port_lock, flags);
 	if (port->port.count == 0)