target: Fix kref->refcount underflow in transport_cmd_finish_abort

commit 73d4e580 upstream.

This patch fixes a se_cmd->cmd_kref underflow during CMD_T_ABORTED
when a fabric driver drops it's second reference from below the
target_core_tmr.c based callers of transport_cmd_finish_abort().

Recently with the conversion of kref to refcount_t, this bug was
manifesting itself as:

[705519.601034] refcount_t: underflow; use-after-free.
[705519.604034] INFO: NMI handler (kgdb_nmi_handler) took too long to run: 20116.512 msecs
[705539.719111] ------------[ cut here ]------------
[705539.719117] WARNING: CPU: 3 PID: 26510 at lib/refcount.c:184 refcount_sub_and_test+0x33/0x51

Since the original kref atomic_t based kref_put() didn't check for
underflow and only invoked the final callback when zero was reached,
this bug did not manifest in practice since all se_cmd memory is
using preallocated tags.

To address this, go ahead and propigate the existing return from
transport_put_cmd() up via transport_cmd_finish_abort(), and
change transport_cmd_finish_abort() + core_tmr_handle_tas_abort()
callers to only do their local target_put_sess_cmd() if necessary.
Reported-by: Bart Van Assche <bart.vanassche@sandisk.com>
Tested-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Mike Christie <mchristi@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Himanshu Madhani <himanshu.madhani@qlogic.com>
Cc: Sagi Grimberg <sagig@mellanox.com>
Tested-by: Gary Guo <ghg@datera.io>
Tested-by: Chu Yuan Lin <cyl@datera.io>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/target/target_core_internal.h

@@ -92,7 +92,7 @@ int	init_se_kmem_caches(void);
 void	release_se_kmem_caches(void);
 u32	scsi_get_new_index(scsi_index_t);
 void	transport_subsystem_check_init(void);
-void	transport_cmd_finish_abort(struct se_cmd *, int);
+int	transport_cmd_finish_abort(struct se_cmd *, int);
 unsigned char *transport_dump_cmd_direction(struct se_cmd *);
 void	transport_dump_dev_state(struct se_device *, char *, int *);
 void	transport_dump_dev_info(struct se_device *, struct se_lun *,

drivers/target/target_core_tmr.c

@@ -82,7 +82,7 @@ void core_tmr_release_req(
 	kfree(tmr);
 }
 
-static void core_tmr_handle_tas_abort(struct se_cmd *cmd, int tas)
+static int core_tmr_handle_tas_abort(struct se_cmd *cmd, int tas)
 {
 	unsigned long flags;
 	bool remove = true, send_tas;
@@ -98,7 +98,7 @@ static void core_tmr_handle_tas_abort(struct se_cmd *cmd, int tas)
 		transport_send_task_abort(cmd);
 	}
 
-	transport_cmd_finish_abort(cmd, remove);
+	return transport_cmd_finish_abort(cmd, remove);
 }
 
 static int target_check_cdb_and_preempt(struct list_head *list,
@@ -195,8 +195,8 @@ void core_tmr_abort_task(
 		cancel_work_sync(&se_cmd->work);
 		transport_wait_for_tasks(se_cmd);
 
-		transport_cmd_finish_abort(se_cmd, true);
-		target_put_sess_cmd(se_sess, se_cmd);
+		if (!transport_cmd_finish_abort(se_cmd, true))
+			target_put_sess_cmd(se_sess, se_cmd);
 
 		printk("ABORT_TASK: Sending TMR_FUNCTION_COMPLETE for"
 				" ref_tag: %d\n", ref_tag);
@@ -296,8 +296,8 @@ static void core_tmr_drain_tmr_list(
 		cancel_work_sync(&cmd->work);
 		transport_wait_for_tasks(cmd);
 
-		transport_cmd_finish_abort(cmd, 1);
-		target_put_sess_cmd(cmd->se_sess, cmd);
+		if (!transport_cmd_finish_abort(cmd, 1))
+			target_put_sess_cmd(cmd->se_sess, cmd);
 	}
 }
 
@@ -395,8 +395,8 @@ static void core_tmr_drain_state_list(
 		cancel_work_sync(&cmd->work);
 		transport_wait_for_tasks(cmd);
 
-		core_tmr_handle_tas_abort(cmd, tas);
-		target_put_sess_cmd(cmd->se_sess, cmd);
+		if (!core_tmr_handle_tas_abort(cmd, tas))
+			target_put_sess_cmd(cmd->se_sess, cmd);
 	}
 }
 

drivers/target/target_core_transport.c

@@ -600,9 +600,10 @@ static void transport_lun_remove_cmd(struct se_cmd *cmd)
 		percpu_ref_put(&lun->lun_ref);
 }
 
-void transport_cmd_finish_abort(struct se_cmd *cmd, int remove)
+int transport_cmd_finish_abort(struct se_cmd *cmd, int remove)
 {
 	bool ack_kref = (cmd->se_cmd_flags & SCF_ACK_KREF);
+	int ret = 0;
 
 	if (cmd->se_cmd_flags & SCF_SE_LUN_CMD)
 		transport_lun_remove_cmd(cmd);
@@ -614,9 +615,11 @@ void transport_cmd_finish_abort(struct se_cmd *cmd, int remove)
 		cmd->se_tfo->aborted_task(cmd);
 
 	if (transport_cmd_check_stop_to_fabric(cmd))
-		return;
+		return 1;
 	if (remove && ack_kref)
-		transport_put_cmd(cmd);
+		ret = transport_put_cmd(cmd);
+
+	return ret;
 }
 
 static void target_complete_failure_work(struct work_struct *work)