syzbot is reporting hung task at wdm_flush() [1], for there is a circular
dependency that wdm_flush() from flip_close() for /dev/cdc-wdm0 forever
waits for /dev/raw-gadget to be closed while close() for /dev/raw-gadget
cannot be called unless close() for /dev/cdc-wdm0 completes.

Tetsuo Handa considered that such circular dependency is an usage error [2]
which corresponds to an unresponding broken hardware [3]. But Alan Stern
responded that we should be prepared for such hardware [4]. Therefore,
this patch changes wdm_flush() to use wait_event_interruptible_timeout()
which gives up after 30 seconds, for hardware that remains silent must be
ignored. The 30 seconds are coming out of thin air.

Changing wait_event() to wait_event_interruptible_timeout() makes error
reporting from close() syscall less reliable. To compensate it, this patch
also implements wdm_fsync() which does not use timeout. Those who want to
be very sure that data has gone out to the device are now advised to call
fsync(), with a caveat that fsync() can return -EINVAL when running on
older kernels which do not implement wdm_fsync().

This patch also fixes three more problems (listed below) found during
exhaustive discussion and testing.

  Since multiple threads can concurrently call wdm_write()/wdm_flush(),
  we need to use wake_up_all() whenever clearing WDM_IN_USE in order to
  make sure that all waiters are woken up. Also, error reporting needs
  to use fetch-and-clear approach in order not to report same error for
  multiple times.

  Since wdm_flush() checks WDM_DISCONNECTING, wdm_write() should as well
  check WDM_DISCONNECTING.

  In wdm_flush(), since locks are not held, it is not safe to dereference
  desc->intf after checking that WDM_DISCONNECTING is not set [5]. Thus,
  remove dev_err() from wdm_flush().

[1] https://syzkaller.appspot.com/bug?id=e7b761593b23eb50855b9ea31e3be5472b711186
[2] https://lkml.kernel.org/r/27b7545e-8f41-10b8-7c02-e35a08eb1611@i-love.sakura.ne.jp
[3] https://lkml.kernel.org/r/79ba410f-e0ef-2465-b94f-6b9a4a82adf5@i-love.sakura.ne.jp
[4] https://lkml.kernel.org/r/20200530011040.GB12419@rowland.harvard.edu
[5] https://lkml.kernel.org/r/c85331fc-874c-6e46-a77f-0ef1dc075308@i-love.sakura.ne.jpReported-by: syzbot <syzbot+854768b99f19e89d7f81@syzkaller.appspotmail.com>
Cc: stable <stable@vger.kernel.org>
Co-developed-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20200928141755.3476-1-penguin-kernel@I-love.SAKURA.ne.jpSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>