usb: gadget: ffs: race between ffs_epfile_io() and ffs_func_eps_disable()

ffs_epfile_io() is called from userspace, while ffs_func_eps_disable() might be
called from USB disconnect interrupt, the two functions would run in parallel
but they are not well protected, that epfile->ep would be removed by
ffs_func_eps_disable() during ffs_epfile_io() is referring this pointer, then
it leads to kernel PANIC.

The scenario is as below:

Thread 1                                 Thread 2
   |                                        |
SyS_read                             dwc3_gadget_disconnect_interrupt
   |                                        |
ffs_epfile_read                         reset_config
   |                                        |
ffs_epfile_io                       ffs_func_eps_disable
   |                                        |
 -----                      usb_ep_disable():  epfile->ep->ep->desc = NULL
   |                                        |
usb_ep_align_maybe():                     -----
it refers ep->desc->wMaxPacketSize        -----

Signed-off-by: Chao Bi <chao.bi@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/f_fs.c

@@ -745,6 +745,12 @@ static ssize_t ffs_epfile_io(struct file *file, struct ffs_io_data *io_data)
 		 */
 		struct usb_gadget *gadget = epfile->ffs->gadget;
 
+		spin_lock_irq(&epfile->ffs->eps_lock);
+		/* In the meantime, endpoint got disabled or changed. */
+		if (epfile->ep != ep) {
+			spin_unlock_irq(&epfile->ffs->eps_lock);
+			return -ESHUTDOWN;
+		}
 		/*
 		 * Controller may require buffer size to be aligned to
 		 * maxpacketsize of an out endpoint.
@@ -752,6 +758,7 @@ static ssize_t ffs_epfile_io(struct file *file, struct ffs_io_data *io_data)
 		data_len = io_data->read ?
 			   usb_ep_align_maybe(gadget, ep->ep, io_data->len) :
 			   io_data->len;
+		spin_unlock_irq(&epfile->ffs->eps_lock);
 
 		data = kmalloc(data_len, GFP_KERNEL);
 		if (unlikely(!data))