vfio-pci: Fix buffer overfill

A read from a range hidden from the user (ex. MSI-X vector table)
attempts to fill the user buffer up to the end of the excluded range
instead of up to the requested count.  Fix it.
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: stable@vger.kernel.org

drivers/vfio/pci/vfio_pci_rdwr.c

@@ -240,17 +240,17 @@ ssize_t vfio_pci_mem_readwrite(struct vfio_pci_device *vdev, char __user *buf,
 			filled = 1;
 		} else {
 			/* Drop writes, fill reads with FF */
+			filled = min((size_t)(x_end - pos), count);
 			if (!iswrite) {
 				char val = 0xFF;
 				size_t i;
 
-				for (i = 0; i < x_end - pos; i++) {
+				for (i = 0; i < filled; i++) {
 					if (put_user(val, buf + i))
 						goto out;
 				}
 			}
 
-			filled = x_end - pos;
 		}
 
 		count -= filled;