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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        Yi Zhang <yi.zhang@redhat.com>, Ming Lei <ming.lei@redhat.com>,
        Jens Axboe <axboe@kernel.dk>,
        Sasha Levin <alexander.levin@microsoft.com>
Subject: [PATCH 4.9 142/310] blk-mq: fix race between updating nr_hw_queues and switching io sched
Date:   Wed, 11 Apr 2018 20:34:41 +0200
Message-Id: <20180411183628.557715299@linuxfoundation.org>
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4.9-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
@@ -1907,6 +1907,9 @@ 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;
 
@@ -1956,6 +1959,7 @@ static void blk_mq_realloc_hw_ctxs(struc
 		}
 	}
 	q->nr_hw_queues = i;
+	mutex_unlock(&q->sysfs_lock);
 	blk_mq_sysfs_register(q);
 }