HID: hiddev: protect against disconnect/NULL-dereference race

The patch 7032269e87a "HID: hiddev: protect against disconnect/NULL-dereference race" added a check to make sure that "hid" was non-NULL. The problem is that it added the check after "hid" had already been dereferenced.

drivers/hid/usbhid/hiddev.c +606 hiddev_ioctl(19)
warn: variable dereferenced before check 'hid'

   599          struct usbhid_device *usbhid = hid->driver_data;
                                               ^^^^^^^^^^^^^^^^
	dereferenced here.

600 void __user *user_arg = (void __user *)arg;
601 int i, r;
602
603 /* Called without BKL by compat methods so no BKL taken */
604
605 /* FIXME: Who or what stop this racing with a disconnect ?? */
606 if (!hiddev->exist || !hid)
^^^^
new check added here.

607 return -EIO;

So obviously in hind-sight this patch didn't fix the user's problem. Also the user reported a NULL dereference, yes, but the stack dump which
says: "BUG: unable to handle kernel paging request at 6b6b6b6b" is a use after free bug and that isn't addressed by adding a check for NULL.

I am just looking at this because it showed up with my static checker and I'm not very familiar with this code. I think we need to take "hiddev->existancelock" and make sure that exist is true before we dereference hiddev in the ioctl function? Also it seems like there should probably be some locking in hiddev_release().

regards,
dan carpenter

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