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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
CC:     Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.15 057/124] blk-mq: fix race between updating
 nr_hw_queues and switching io sched
Thread-Topic: [PATCH AUTOSEL for 4.15 057/124] blk-mq: fix race between
 updating nr_hw_queues and switching io sched
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Date:   Mon, 19 Mar 2018 15:47:59 +0000
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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>
---
 block/blk-mq.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 4385c5cbf57b..1372ad3a21d3 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2388,6 +2388,9 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_=
set *set,
 	struct blk_mq_hw_ctx **hctxs =3D q->queue_hw_ctx;
=20
 	blk_mq_sysfs_unregister(q);
+
+	/* protect against switching io scheduler  */
+	mutex_lock(&q->sysfs_lock);
 	for (i =3D 0; i < set->nr_hw_queues; i++) {
 		int node;
=20
@@ -2432,6 +2435,7 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_=
set *set,
 		}
 	}
 	q->nr_hw_queues =3D i;
+	mutex_unlock(&q->sysfs_lock);
 	blk_mq_sysfs_register(q);
 }
=20
--=20
2.14.1