On Thu, 8 Aug 2019, Greg KH wrote:
> On Thu, Aug 08, 2019 at 01:34:08PM -0400, Alan Stern wrote:
> > The syzbot fuzzer found a lockdep violation in the rio500 driver:
> >
> > ======================================================
> > WARNING: possible circular locking dependency detected
> > 5.3.0-rc2+ #23 Not tainted
> > ------------------------------------------------------
> > syz-executor.2/20386 is trying to acquire lock:
> > 00000000772249c6 (rio500_mutex){+.+.}, at: open_rio+0x16/0xc0
> > drivers/usb/misc/rio500.c:64
> >
> > but task is already holding lock:
> > 00000000d3e8f4b9 (minor_rwsem){++++}, at: usb_open+0x23/0x270
> > drivers/usb/core/file.c:39
> >
> > which lock already depends on the new lock.
> >
> > The problem is that the driver's open_rio() routine is called while
> > the usbcore's minor_rwsem is locked for reading, and it acquires the
> > rio500_mutex; whereas conversely, probe_rio() and disconnect_rio()
> > first acquire the rio500_mutex and then call usb_register_dev() or
> > usb_deregister_dev(), which lock minor_rwsem for writing.
> >
> > The correct ordering of acquisition should be: minor_rwsem first, then
> > rio500_mutex (since the locking in open_rio() cannot be changed).
> > Thus, the probe and disconnect routines should avoid holding
> > rio500_mutex while doing their registration and deregistration.
> >
> > This patch adjusts the code in those two routines to do just that. It
> > also relies on the fact that the probe and disconnect routines are
> > protected by the device mutex, so the initial test of rio->present
> > needs no extra locking.
> >
> > Reported-by: syzbot+7bbcbe9c9ff0cd49592a@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > Fixes: d710734b0677 ("USB: rio500: simplify locking")
> > CC: Oliver Neukum <oneukum@xxxxxxxx>
> > CC: <stable@xxxxxxxxxxxxxxx>
> >
> > ---
> >
> > This patch is different from the one I posted earlier. I realized that
> > we don't want to register the device's char file until after the
> > buffers have been allocated.
>
> Should I revert Oliver's patch?
Sorry, I should have explained more clearly: This goes on top of
Oliver's patch. In fact, Oliver's patch is the one listed in the
Fixes: tag.
You do not need to apply Oliver's reversion. Assuming he agrees that
this patch is correct, of course.
Alan Stern
