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    Subject[PATCH 4.14 040/138] blk-mq: fix race between updating nr_hw_queues and switching io sched
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Ming Lei <ming.lei@redhat.com>


    [ Upstream commit fb350e0ad99359768e1e80b4784692031ec340e4 ]

    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>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    block/blk-mq.c | 4 ++++
    1 file changed, 4 insertions(+)

    --- a/block/blk-mq.c
    +++ b/block/blk-mq.c
    @@ -2314,6 +2314,9 @@ static void blk_mq_realloc_hw_ctxs(struc
    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;

    @@ -2358,6 +2361,7 @@ static void blk_mq_realloc_hw_ctxs(struc
    }
    }
    q->nr_hw_queues = i;
    + mutex_unlock(&q->sysfs_lock);
    blk_mq_sysfs_register(q);
    }


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