blk-mq: fix race between updating nr_hw_queues and switching io sched

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 <yi.zhang@redhat.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index dd21051fb251351507fb1b2a1313a7eef1a66cfd..111e1aa5562fe84a217fefc40f0fe62b31e9def0 100644 (file)
--- 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);
 }