blk-mq: fix race between timeout and CPU hotplug

Firstly during CPU hotplug, even queue is freezed, timeout
handler still may come and access hctx->tags, which may cause
use after free, so this patch deactivates timeout handler
inside CPU hotplug notifier.

Secondly, tags can be shared by more than one queues, so we
have to check if the hctx has been unmapped, otherwise
still use-after-free on tags can be triggered.

Cc: <stable@vger.kernel.org>
Reported-by: Dongsu Park <dongsu.park@profitbricks.com>
Tested-by: Dongsu Park <dongsu.park@profitbricks.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -677,8 +677,11 @@ static void blk_mq_rq_timer(unsigned long priv)
 		data.next = blk_rq_timeout(round_jiffies_up(data.next));
 		mod_timer(&q->timeout, data.next);
 	} else {
-		queue_for_each_hw_ctx(q, hctx, i)
-			blk_mq_tag_idle(hctx);
+		queue_for_each_hw_ctx(q, hctx, i) {
+			/* the hctx may be unmapped, so check it here */
+			if (blk_mq_hw_queue_mapped(hctx))
+				blk_mq_tag_idle(hctx);
+		}
 	}
 }
 
@@ -2090,9 +2093,16 @@ static int blk_mq_queue_reinit_notify(struct notifier_block *nb,
 	 */
 	list_for_each_entry(q, &all_q_list, all_q_node)
 		blk_mq_freeze_queue_start(q);
-	list_for_each_entry(q, &all_q_list, all_q_node)
+	list_for_each_entry(q, &all_q_list, all_q_node) {
 		blk_mq_freeze_queue_wait(q);
 
+		/*
+		 * timeout handler can't touch hw queue during the
+		 * reinitialization
+		 */
+		del_timer_sync(&q->timeout);
+	}
+
 	list_for_each_entry(q, &all_q_list, all_q_node)
 		blk_mq_queue_reinit(q);