usb: gadget: f_fs: Use local copy of descriptors for userspace copy

usb: gadget: f_fs: Use local copy of descriptors for userspace copy

commit a4b98a7512f18534ce33a7e98e49115af59ffa00 upstream.

The function may be unbound causing the ffs_ep and its descriptors
to be freed while userspace is in the middle of an ioctl requesting
the same descriptors. Avoid dangling pointer reference by first
making a local copy of desctiptors before releasing the spinlock.

Fixes: c559a3534109 ("usb: gadget: f_fs: add ioctl returning ep descriptor")
Reviewed-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20201130203453.28154-1-jackp@codeaurora.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>