In both elevator_switch_mq() and blk_mq_update_nr_hw_queues(), sched tags
can be allocated, and q->nr_hw_queue is used, and race is inevitable, for
example: blk_mq_init_sched() may trigger use-after-free on hctx, which is
freed in blk_mq_realloc_hw_ctxs() when nr_hw_queues is decreased.

This patch fixes the race be holding q->sysfs_lock.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reported-by: Yi Zhang <redacted>
Tested-by: Yi Zhang <redacted>
Signed-off-by: Ming Lei <redacted>
---
 block/blk-mq.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index dd21051fb251..111e1aa5562f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -2407,6 +2407,9 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
 	struct blk_mq_hw_ctx **hctxs = q->queue_hw_ctx;
 
 	blk_mq_sysfs_unregister(q);
+
+	/* protect against switching io scheduler  */
+	mutex_lock(&q->sysfs_lock);
 	for (i = 0; i < set->nr_hw_queues; i++) {
 		int node;
 

@@ -2451,6 +2454,7 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
 		}
 	}
 	q->nr_hw_queues = i;
+	mutex_unlock(&q->sysfs_lock);
 	blk_mq_sysfs_register(q);
 }
 

-- 
2.9.5