target: fix potential race window in target_sess_cmd_list_waiting()

target_sess_cmd_list_waiting() might hit on a condition where
the kref for the command is already 0, but the destructor has
not been called yet (or is stuck in waiting for a spin lock).
Rather than leaving the command on the list we should explicitly
remove it to avoid race issues later on.
Signed-off-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_transport.c

@@ -2535,7 +2535,9 @@ int target_get_sess_cmd(struct se_cmd *se_cmd, bool ack_kref)
 	 * invocations before se_cmd descriptor release.
 	 */
 	if (ack_kref) {
-		kref_get(&se_cmd->cmd_kref);
+		if (!kref_get_unless_zero(&se_cmd->cmd_kref))
+			return -EINVAL;
+
 		se_cmd->se_cmd_flags |= SCF_ACK_KREF;
 	}
 
@@ -2616,7 +2618,7 @@ EXPORT_SYMBOL(target_put_sess_cmd);
  */
 void target_sess_cmd_list_set_waiting(struct se_session *se_sess)
 {
-	struct se_cmd *se_cmd;
+	struct se_cmd *se_cmd, *tmp_cmd;
 	unsigned long flags;
 	int rc;
 
@@ -2628,14 +2630,16 @@ void target_sess_cmd_list_set_waiting(struct se_session *se_sess)
 	se_sess->sess_tearing_down = 1;
 	list_splice_init(&se_sess->sess_cmd_list, &se_sess->sess_wait_list);
 
-	list_for_each_entry(se_cmd, &se_sess->sess_wait_list, se_cmd_list) {
+	list_for_each_entry_safe(se_cmd, tmp_cmd,
+				 &se_sess->sess_wait_list, se_cmd_list) {
 		rc = kref_get_unless_zero(&se_cmd->cmd_kref);
 		if (rc) {
 			se_cmd->cmd_wait_set = 1;
 			spin_lock(&se_cmd->t_state_lock);
 			se_cmd->transport_state |= CMD_T_FABRIC_STOP;
 			spin_unlock(&se_cmd->t_state_lock);
-		}
+		} else
+			list_del_init(&se_cmd->se_cmd_list);
 	}
 
 	spin_unlock_irqrestore(&se_sess->sess_cmd_lock, flags);