scsi: target: tcmu: Fix a use after free in tcmu_check_expired_queue_cmd()

The pr_debug() dereferences "cmd" after we already freed it by calling
tcmu_free_cmd(cmd).  The debug printk needs to be done earlier.

Link: https://lore.kernel.org/r/20200523101129.GB98132@mwanda
Fixes: 61fb248 ("scsi: target: tcmu: Userspace must not complete queued commands")
Reviewed-by: Mike Christie <[email protected]>
Reviewed-by: David Disseldorp <[email protected]>
Signed-off-by: Dan Carpenter <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>

Author: Dan Carpenter

drivers/target/target_core_user.c

@@ -1292,13 +1292,13 @@ static void tcmu_check_expired_queue_cmd(struct tcmu_cmd *cmd)
 	if (!time_after(jiffies, cmd->deadline))
 		return;
 
+	pr_debug("Timing out queued cmd %p on dev %s.\n",
+		  cmd, cmd->tcmu_dev->name);
+
 	list_del_init(&cmd->queue_entry);
 	se_cmd = cmd->se_cmd;
 	tcmu_free_cmd(cmd);
 
-	pr_debug("Timing out queued cmd %p on dev %s.\n",
-		  cmd, cmd->tcmu_dev->name);
-
 	target_complete_cmd(se_cmd, SAM_STAT_TASK_SET_FULL);
 }