usb: gadget: f_fs: change ep->ep safe in ffs_epfile_io()

In ffs_epfile_io(), when read/write data in blocking mode, it will wait
the completion in interruptible mode, if task receive a signal, it will
terminate the wait, at same time, if function unbind occurs,
ffs_func_unbind() will kfree all eps, ffs_epfile_io() still try to
dequeue request by dereferencing ep which may become invalid.

Fix it by add ep spinlock and will not dereference ep if it is not valid.

Cc: <stable@vger.kernel.org> # 5.15
Reported-by: Michael Wu <michael@allwinnertech.com>
Tested-by: Michael Wu <michael@allwinnertech.com>
Reviewed-by: John Keeping <john@metanate.com>
Signed-off-by: Linyu Yuan <quic_linyyuan@quicinc.com>
Link: https://lore.kernel.org/r/1654863478-26228-3-git-send-email-quic_linyyuan@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Linyu Yuan

drivers/usb/gadget/function/f_fs.c

@@ -1080,13 +1080,19 @@ static ssize_t ffs_epfile_io(struct file *file, struct ffs_io_data *io_data)
 		spin_unlock_irq(&epfile->ffs->eps_lock);
 
 		if (wait_for_completion_interruptible(&io_data->done)) {
+			spin_lock_irq(&epfile->ffs->eps_lock);
+			if (epfile->ep != ep) {
+				ret = -ESHUTDOWN;
+				goto error_lock;
+			}
 			/*
 			 * To avoid race condition with ffs_epfile_io_complete,
 			 * dequeue the request first then check
 			 * status. usb_ep_dequeue API should guarantee no race
 			 * condition with req->complete callback.
 			 */
 			usb_ep_dequeue(ep->ep, req);
+			spin_unlock_irq(&epfile->ffs->eps_lock);
 			wait_for_completion(&io_data->done);
 			interrupted = io_data->status < 0;
 		}