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

[ Upstream commit 9d7464b1 ]

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: 61fb2482 ("scsi: target: tcmu: Userspace must not complete queued commands")
Reviewed-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/target/target_core_user.c

@@ -1303,13 +1303,13 @@ 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);
 }