USB: ldusb: fix ring-buffer locking

The custom ring-buffer implementation was merged without any locking or
explicit memory barriers, but a spinlock was later added by commit
9d33efd9 ("USB: ldusb bugfix").

The lock did not cover the update of the tail index once the entry had
been processed, something which could lead to memory corruption on
weakly ordered architectures or due to compiler optimisations.

Specifically, a completion handler running on another CPU might observe
the incremented tail index and update the entry before ld_usb_read() is
done with it.

Fixes: 2824bd25 ("[PATCH] USB: add ldusb driver")
Fixes: 9d33efd9 ("USB: ldusb bugfix")
Cc: stable <stable@vger.kernel.org>     # 2.6.13
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191022143203.5260-2-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/misc/ldusb.c

@@ -495,11 +495,11 @@ static ssize_t ld_usb_read(struct file *file, char __user *buffer, size_t count,
 		retval = -EFAULT;
 		goto unlock_exit;
 	}
-	dev->ring_tail = (dev->ring_tail+1) % ring_buffer_size;
-
 	retval = bytes_to_read;
 
 	spin_lock_irq(&dev->rbsl);
+	dev->ring_tail = (dev->ring_tail + 1) % ring_buffer_size;
+
 	if (dev->buffer_overflow) {
 		dev->buffer_overflow = 0;
 		spin_unlock_irq(&dev->rbsl);