usb: gadget: u_serial: Fix the issue that gs_start_io crashed due to accessing null pointer

Considering that in some extreme cases,
when u_serial driver is accessed by multiple threads,
Thread A is executing the open operation and calling the gs_open,
Thread B is executing the disconnect operation and calling the
gserial_disconnect function,The port->port_usb pointer will be set to NULL.

E.g.
    Thread A                                 Thread B
    gs_open()                                gadget_unbind_driver()
    gs_start_io()                            composite_disconnect()
    gs_start_rx()                            gserial_disconnect()
    ...                                      ...
    spin_unlock(&port->port_lock)
    status = usb_ep_queue()                  spin_lock(&port->port_lock)
    spin_lock(&port->port_lock)              port->port_usb = NULL
    gs_free_requests(port->port_usb->in)     spin_unlock(&port->port_lock)
    Crash

This causes thread A to access a null pointer (port->port_usb is null)
when calling the gs_free_requests function, causing a crash.

If port_usb is NULL, the release request will be skipped as it
will be done by gserial_disconnect.

So add a null pointer check to gs_start_io before attempting
to access the value of the pointer port->port_usb.

Call trace:
 gs_start_io+0x164/0x25c
 gs_open+0x108/0x13c
 tty_open+0x314/0x638
 chrdev_open+0x1b8/0x258
 do_dentry_open+0x2c4/0x700
 vfs_open+0x2c/0x3c
 path_openat+0xa64/0xc60
 do_filp_open+0xb8/0x164
 do_sys_openat2+0x84/0xf0
 __arm64_sys_openat+0x70/0x9c
 invoke_syscall+0x58/0x114
 el0_svc_common+0x80/0xe0
 do_el0_svc+0x1c/0x28
 el0_svc+0x38/0x68

Fixes: c1dca56 ("usb gadget: split out serial core")
Cc: [email protected]
Suggested-by: Prashanth K <[email protected]>
Signed-off-by: Lianqin Hu <[email protected]>
Acked-by: Prashanth K <[email protected]>
Link: https://lore.kernel.org/r/TYUPR06MB62178DC3473F9E1A537DCD02D2362@TYUPR06MB6217.apcprd06.prod.outlook.com
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Lianqin Hu

drivers/usb/gadget/function/u_serial.c

@@ -579,9 +579,12 @@ static int gs_start_io(struct gs_port *port)
 		 * we didn't in gs_start_tx() */
 		tty_wakeup(port->port.tty);
 	} else {
-		gs_free_requests(ep, head, &port->read_allocated);
-		gs_free_requests(port->port_usb->in, &port->write_pool,
-			&port->write_allocated);
+		/* Free reqs only if we are still connected */
+		if (port->port_usb) {
+			gs_free_requests(ep, head, &port->read_allocated);
+			gs_free_requests(port->port_usb->in, &port->write_pool,
+				&port->write_allocated);
+		}
 		status = -EIO;
 	}