From: Ming Lei <ming.lei@redhat.com> mainline inclusion from mainline-5.4-rc1 commit b89f625e28d44552083f43752f62d8621ded0a04 category: bugfix bugzilla: 23392 CVE: NA --------------------------- 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@infradead.org> Cc: Hannes Reinecke <hare@suse.com> Cc: Greg KH <gregkh@linuxfoundation.org> Cc: Mike Snitzer <snitzer@redhat.com> Reviewed-by: Bart Van Assche <bvanassche@acm.org> Signed-off-by: Ming Lei <ming.lei@redhat.com> Signed-off-by: Jens Axboe <axboe@kernel.dk> Conflicts: block/blk-sysfs.c block/blk-elevator.c Signed-off-by: Yu Kuai <yukuai3@huawei.com> Reviewed-by: Yufen Yu <yuyufen@huawei.com> Signed-off-by: Yang Yingliang <yangyingliang@huawei.com> --- block/blk-sysfs.c | 5 ++++- block/elevator.c | 31 +------------------------------ 2 files changed, 5 insertions(+), 31 deletions(-) diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c index ef727e0fc06e..15ad67c3c56b 100644 --- a/block/blk-sysfs.c +++ b/block/blk-sysfs.c @@ -934,9 +934,11 @@ int blk_register_queue(struct gendisk *disk) blk_mq_debugfs_register(q); } + mutex_lock(&q->sysfs_lock); 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); @@ -946,7 +948,6 @@ 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); @@ -1004,8 +1005,10 @@ void blk_unregister_queue(struct gendisk *disk) kobject_del(&q->kobj); blk_trace_remove_sysfs(disk_to_dev(disk)); + mutex_lock(&q->sysfs_lock); if (q->request_fn || (q->mq_ops && q->elevator)) elv_unregister_queue(q); + mutex_unlock(&q->sysfs_lock); mutex_unlock(&q->sysfs_dir_lock); kobject_put(&disk_to_dev(disk)->kobj); diff --git a/block/elevator.c b/block/elevator.c index cf10352e9154..f81592543dff 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; } @@ -875,11 +873,9 @@ 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); } } @@ -950,32 +946,11 @@ 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); @@ -983,11 +958,7 @@ 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