USB: cdc-wdm: fix race between write and disconnect due to flag abuse

USB: cdc-wdm: fix race between write and disconnect due to flag abuse

commit 1426bd2c9f7e3126e2678e7469dca9fd9fc6dd3e upstream.

In case of a disconnect an ongoing flush() has to be made fail.
Nevertheless we cannot be sure that any pending URB has already
finished, so although they will never succeed, they still must
not be touched.
The clean solution for this is to check for WDM_IN_USE
and WDM_DISCONNECTED in flush(). There is no point in ever
clearing WDM_IN_USE, as no further writes make sense.

The issue is as old as the driver.

Fixes: afba937e540c9 ("USB: CDC WDM driver")
Reported-by: syzbot+d232cca6ec42c2edb3fc@syzkaller.appspotmail.com
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20190827103436.21143-1-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>