target: Fix divide by zero bug in fabric_max_sectors for unconfigured devices

commit 7a3cf6ca upstream

This patch fixes a possible divide by zero bug when the fabric_max_sectors
device attribute is written and backend se_device failed to be successfully
configured -> enabled.

Go ahead and use block_size=512 within se_dev_set_fabric_max_sectors()
in the event of a target_configure_device() failure case, as no valid
dev->dev_attrib.block_size value will have been setup yet.
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/target/target_core_device.c

@@ -1230,6 +1230,8 @@ int se_dev_set_max_sectors(struct se_device *dev, u32 max_sectors)
 
 int se_dev_set_fabric_max_sectors(struct se_device *dev, u32 fabric_max_sectors)
 {
+	int block_size = dev->se_sub_dev->se_dev_attrib.block_size;
+
 	if (atomic_read(&dev->dev_export_obj.obj_access_count)) {
 		pr_err("dev[%p]: Unable to change SE Device"
 			" fabric_max_sectors while dev_export_obj: %d count exists\n",
@@ -1267,8 +1269,12 @@ int se_dev_set_fabric_max_sectors(struct se_device *dev, u32 fabric_max_sectors)
 	/*
 	 * Align max_sectors down to PAGE_SIZE to follow transport_allocate_data_tasks()
 	 */
+	if (!block_size) {
+		block_size = 512;
+		pr_warn("Defaulting to 512 for zero block_size\n");
+	}
 	fabric_max_sectors = se_dev_align_max_sectors(fabric_max_sectors,
-						      dev->se_sub_dev->se_dev_attrib.block_size);
+						      block_size);
 
 	dev->se_sub_dev->se_dev_attrib.fabric_max_sectors = fabric_max_sectors;
 	pr_debug("dev[%p]: SE Device max_sectors changed to %u\n",