Re: [PATCH] USB: rio500: Fix lockdep violation

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Aug 15, 2019 at 10:47:45AM -0400, Alan Stern wrote:
> On Thu, 15 Aug 2019, Greg KH wrote:
> 
> > On Thu, Aug 08, 2019 at 02:23:00PM -0400, Alan Stern wrote:
> > > 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@syzkaller.appspotmail.com
> > > > > Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> > > > > Fixes: d710734b0677 ("USB: rio500: simplify locking")
> > > > > CC: Oliver Neukum <oneukum@suse.com>
> > > > > CC: <stable@vger.kernel.org>
> > > > > 
> > > > > ---
> > > > > 
> > > > > 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.
> > 
> > Ok, I applied the revert, and that's in 5.3-rc4.  So of course this does
> > not apply :)
> > 
> > Shoudl I revert the revert and then apply this?  I will if I can get an
> > ack from Oliver...
> 
> Either that or else Oliver and I can squash the two patches into one.

I've now merged both, thanks.

greg k-h