target: Avoid mappedlun symlink creation during lun shutdown

commit 49cb77e2 upstream.

This patch closes a race between se_lun deletion during configfs
unlink in target_fabric_port_unlink() -> core_dev_del_lun()
-> core_tpg_remove_lun(), when transport_clear_lun_ref() blocks
waiting for percpu_ref RCU grace period to finish, but a new
NodeACL mappedlun is added before the RCU grace period has
completed.

This can happen in target_fabric_mappedlun_link() because it
only checks for se_lun->lun_se_dev, which is not cleared until
after transport_clear_lun_ref() percpu_ref RCU grace period
finishes.

This bug originally manifested as NULL pointer dereference
OOPsen in target_stat_scsi_att_intr_port_show_attr_dev() on
v4.1.y code, because it dereferences lun->lun_se_dev without
a explicit NULL pointer check.

In post v4.1 code with target-core RCU conversion, the code
in target_stat_scsi_att_intr_port_show_attr_dev() no longer
uses se_lun->lun_se_dev, but the same race still exists.

To address the bug, go ahead and set se_lun>lun_shutdown as
early as possible in core_tpg_remove_lun(), and ensure new
NodeACL mappedlun creation in target_fabric_mappedlun_link()
fails during se_lun shutdown.
Reported-by: James Shen <jcs@datera.io>
Cc: James Shen <jcs@datera.io>
Tested-by: James Shen <jcs@datera.io>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/target/target_core_fabric_configfs.c

@@ -84,6 +84,11 @@ static int target_fabric_mappedlun_link(
 				"_tpg does not exist\n");
 		return -EINVAL;
 	}
+	if (lun->lun_shutdown) {
+		pr_err("Unable to create mappedlun symlink because"
+			" lun->lun_shutdown=true\n");
+		return -EINVAL;
+	}
 	se_tpg = lun->lun_sep->sep_tpg;
 
 	nacl_ci = &lun_acl_ci->ci_parent->ci_group->cg_item;

drivers/target/target_core_tpg.c

@@ -843,6 +843,8 @@ void core_tpg_remove_lun(
 	struct se_portal_group *tpg,
 	struct se_lun *lun)
 {
+	lun->lun_shutdown = true;
+
 	core_clear_lun_from_tpg(lun, tpg);
 	transport_clear_lun_ref(lun);
 
@@ -850,6 +852,7 @@ void core_tpg_remove_lun(
 
 	spin_lock(&tpg->tpg_lun_lock);
 	lun->lun_status = TRANSPORT_LUN_STATUS_FREE;
+	lun->lun_shutdown = false;
 	spin_unlock(&tpg->tpg_lun_lock);
 
 	percpu_ref_exit(&lun->lun_ref);

include/target/target_core_base.h

@@ -719,6 +719,7 @@ struct se_lun {
 	u32			lun_access;
 	u32			lun_flags;
 	u32			unpacked_lun;
+	bool			lun_shutdown;
 	atomic_t		lun_acl_count;
 	spinlock_t		lun_acl_lock;
 	spinlock_t		lun_sep_lock;