From: Ming Lei <ming.lei@xxxxxxxxxx>
cecf5d87ff20 ("block: split .sysfs_lock into two locks") starts to
release & acquire sysfs_lock before registering/un-registering elevator
queue during switching elevator for avoiding potential deadlock from
showing & storing 'queue/iosched' attributes and removing elevator's
kobject.
Turns out there isn't such deadlock because 'q->sysfs_lock' isn't
required in .show & .store of queue/iosched's attributes, and just
elevator's sysfs lock is acquired in elv_iosched_store() and
elv_iosched_show(). So it is safe to hold queue's sysfs lock when
registering/un-registering elevator queue.
The biggest issue is that commit cecf5d87ff20 assumes that concurrent
write on 'queue/scheduler' can't happen. However, this assumption isn't
true, because kernfs_fop_write() only guarantees that concurrent write
aren't called on the same open file, but the write could be from
different open on the file. So we can't release & re-acquire queue's
sysfs lock during switching elevator, otherwise use-after-free on
elevator could be triggered.
Fixes the issue by not releasing queue's sysfs lock during switching
elevator.
Fixes: cecf5d87ff20 ("block: split .sysfs_lock into two locks")
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
Reviewed-by: Bart Van Assche <bvanassche@xxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
(jwang:cherry picked from commit b89f625e28d44552083f43752f62d8621ded0a04
adjust ctx for 4.19)
Signed-off-by: Jack Wang <jinpu.wang@xxxxxxxxxxxxxxx>
---
block/blk-sysfs.c | 3 ++-
block/elevator.c | 31 +------------------------------
2 files changed, 3 insertions(+), 31 deletions(-)
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 899987152701..07494deb1a26 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -933,6 +933,7 @@ int blk_register_queue(struct gendisk *disk)
blk_mq_debugfs_register(q);
}
+ mutex_lock(&q->sysfs_lock);
/*
* The flag of QUEUE_FLAG_REGISTERED isn't set yet, so elevator
* switch won't happen at all.
@@ -940,6 +941,7 @@ int blk_register_queue(struct gendisk *disk)
if (q->request_fn || (q->mq_ops && q->elevator)) {
ret = elv_register_queue(q, false);
if (ret) {
+ mutex_unlock(&q->sysfs_lock);
mutex_unlock(&q->sysfs_dir_lock);
kobject_del(&q->kobj);
blk_trace_remove_sysfs(dev);
@@ -949,7 +951,6 @@ int blk_register_queue(struct gendisk *disk)
has_elevator = true;
}
- mutex_lock(&q->sysfs_lock);
blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);
wbt_enable_default(q);
blk_throtl_register_queue(q);
diff --git a/block/elevator.c b/block/elevator.c
index 2ff0859e8b35..5b51bc5fad9f 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -856,11 +856,9 @@ int elv_register_queue(struct request_queue *q, bool uevent)
if (uevent)
kobject_uevent(&e->kobj, KOBJ_ADD);
- mutex_lock(&q->sysfs_lock);
e->registered = 1;
if (!e->uses_mq && e->type->ops.sq.elevator_registered_fn)
e->type->ops.sq.elevator_registered_fn(q);
- mutex_unlock(&q->sysfs_lock);
}
return error;
}
@@ -878,11 +876,9 @@ void elv_unregister_queue(struct request_queue *q)
kobject_uevent(&e->kobj, KOBJ_REMOVE);
kobject_del(&e->kobj);
- mutex_lock(&q->sysfs_lock);
e->registered = 0;
/* Re-enable throttling in case elevator disabled it */
wbt_enable_default(q);
- mutex_unlock(&q->sysfs_lock);
}
}
@@ -953,32 +949,11 @@ int elevator_switch_mq(struct request_queue *q,
lockdep_assert_held(&q->sysfs_lock);
if (q->elevator) {
- if (q->elevator->registered) {
- mutex_unlock(&q->sysfs_lock);
-
- /*
- * Concurrent elevator switch can't happen becasue
- * sysfs write is always exclusively on same file.
- *
- * Also the elevator queue won't be freed after
- * sysfs_lock is released becasue kobject_del() in
- * blk_unregister_queue() waits for completion of
- * .store & .show on its attributes.
- */
+ if (q->elevator->registered)
elv_unregister_queue(q);
- mutex_lock(&q->sysfs_lock);
- }
ioc_clear_queue(q);
elevator_exit(q, q->elevator);
-
- /*
- * sysfs_lock may be dropped, so re-check if queue is
- * unregistered. If yes, don't switch to new elevator
- * any more
- */
- if (!blk_queue_registered(q))
- return 0;
}
ret = blk_mq_init_sched(q, new_e);
@@ -986,11 +961,7 @@ int elevator_switch_mq(struct request_queue *q,
goto out;
if (new_e) {
- mutex_unlock(&q->sysfs_lock);
-
ret = elv_register_queue(q, true);
-
- mutex_lock(&q->sysfs_lock);
if (ret) {
elevator_exit(q, q->elevator);
goto out;
--
2.25.1
