[PATCH V2 15/20] block: fail to show/store elevator sysfs attribute if elevator is dying

[Ming Lei <ming.lei@xxxxxxxxxx>]

Prepare for moving elv_register[unregister]_queue out of elevator_lock
& queue freezing, so we may have to call elv_unregister_queue() after
elevator ->exit() is called, then there is small window for user to
run into ->show()/store(), and user-after-free can be caused.

Fail to show/store elevator sysfs attribute if elevator is dying by
adding one new flag of ELEVATOR_FLAG_DYNG, which is protected by
elevator ->sysfs_lock.

Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
 block/blk-mq-sched.c |  1 +
 block/elevator.c     | 10 ++++++++--
 block/elevator.h     |  1 +
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 336a15ffecfa..55a0fd105147 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -551,5 +551,6 @@ void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
 	if (e->type->ops.exit_sched)
 		e->type->ops.exit_sched(e);
 	blk_mq_sched_tags_teardown(q, flags);
+	set_bit(ELEVATOR_FLAG_DYING, &q->elevator->flags);
 	q->elevator = NULL;
 }
diff --git a/block/elevator.c b/block/elevator.c
index 568457e01d28..16171ea92f80 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -422,7 +422,10 @@ elv_attr_show(struct kobject *kobj, struct attribute *attr, char *page)
 
 	e = container_of(kobj, struct elevator_queue, kobj);
 	mutex_lock(&e->sysfs_lock);
-	error = e->type ? entry->show(e, page) : -ENOENT;
+	if (test_bit(ELEVATOR_FLAG_DYING, &e->flags))
+		error = -ENODEV;
+	else
+		error = e->type ? entry->show(e, page) : -ENOENT;
 	mutex_unlock(&e->sysfs_lock);
 	return error;
 }
@@ -440,7 +443,10 @@ elv_attr_store(struct kobject *kobj, struct attribute *attr,
 
 	e = container_of(kobj, struct elevator_queue, kobj);
 	mutex_lock(&e->sysfs_lock);
-	error = e->type ? entry->store(e, page, length) : -ENOENT;
+	if (test_bit(ELEVATOR_FLAG_DYING, &e->flags))
+		error = -ENODEV;
+	else
+		error = e->type ? entry->store(e, page, length) : -ENOENT;
 	mutex_unlock(&e->sysfs_lock);
 	return error;
 }
diff --git a/block/elevator.h b/block/elevator.h
index e74e27dd6586..16d8888fa2b2 100644
--- a/block/elevator.h
+++ b/block/elevator.h
@@ -121,6 +121,7 @@ struct elevator_queue
 };
 
 #define ELEVATOR_FLAG_REGISTERED	0
+#define ELEVATOR_FLAG_DYING		1
 
 /* Holding context data for changing elevator */
 struct elv_change_ctx {
-- 
2.47.0