target: Add link_magic for fabric allow_link destination target_items

This patch adds [dev,lun]_link_magic value assignment + checks within generic
target_fabric_port_link() and target_fabric_mappedlun_link() code to ensure
destination config_item *target_item sent from configfs_symlink() ->
config_item_operations->allow_link() is the underlying se_device->dev_group
and se_lun->lun_group that we expect to symlink.
Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_device.c

@@ -1352,6 +1352,7 @@ struct se_device *target_alloc_device(struct se_hba *hba, const char *name)
 	if (!dev)
 		return NULL;
 
+	dev->dev_link_magic = SE_DEV_LINK_MAGIC;
 	dev->se_hba = hba;
 	dev->transport = hba->transport;
 

drivers/target/target_core_fabric_configfs.c

@@ -70,6 +70,12 @@ static int target_fabric_mappedlun_link(
 	struct se_portal_group *se_tpg;
 	struct config_item *nacl_ci, *tpg_ci, *tpg_ci_s, *wwn_ci, *wwn_ci_s;
 	int ret = 0, lun_access;
+
+	if (lun->lun_link_magic != SE_LUN_LINK_MAGIC) {
+		pr_err("Bad lun->lun_link_magic, not a valid lun_ci pointer:"
+			" %p to struct lun: %p\n", lun_ci, lun);
+		return -EFAULT;
+	}
 	/*
 	 * Ensure that the source port exists
 	 */
@@ -742,6 +748,12 @@ static int target_fabric_port_link(
 	struct target_fabric_configfs *tf;
 	int ret;
 
+	if (dev->dev_link_magic != SE_DEV_LINK_MAGIC) {
+		pr_err("Bad dev->dev_link_magic, not a valid se_dev_ci pointer:"
+			" %p to struct se_device: %p\n", se_dev_ci, dev);
+		return -EFAULT;
+	}
+
 	tpg_ci = &lun_ci->ci_parent->ci_group->cg_item;
 	se_tpg = container_of(to_config_group(tpg_ci),
 				struct se_portal_group, tpg_group);

drivers/target/target_core_tpg.c

@@ -669,6 +669,7 @@ int core_tpg_register(
 	for (i = 0; i < TRANSPORT_MAX_LUNS_PER_TPG; i++) {
 		lun = se_tpg->tpg_lun_list[i];
 		lun->unpacked_lun = i;
+		lun->lun_link_magic = SE_LUN_LINK_MAGIC;
 		lun->lun_status = TRANSPORT_LUN_STATUS_FREE;
 		atomic_set(&lun->lun_acl_count, 0);
 		init_completion(&lun->lun_shutdown_comp);

include/target/target_core_base.h

@@ -624,6 +624,8 @@ struct se_dev_stat_grps {
 };
 
 struct se_device {
+#define SE_DEV_LINK_MAGIC			0xfeeddeef
+	u32			dev_link_magic;
 	/* RELATIVE TARGET PORT IDENTIFER Counter */
 	u16			dev_rpti_counter;
 	/* Used for SAM Task Attribute ordering */
@@ -722,6 +724,8 @@ struct se_port_stat_grps {
 };
 
 struct se_lun {
+#define SE_LUN_LINK_MAGIC			0xffff7771
+	u32			lun_link_magic;
 	/* See transport_lun_status_table */
 	enum transport_lun_status_table lun_status;
 	u32			lun_access;